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1. Soil samples were obtained using a standard split spoon sampler and the Standard Penetration Test SPT in general accordance with the procedures described in American Society for Testing and Materials ASTM D1586 90 Driving resistances from the SPT tests i e blow counts were recorded on the boring logs The SPT utilizes a 140 lb hammer falling 30 inches to drive a 2 inch outside diameter O D split spoon barrel sampler for 18 inches At the completion of drilling test borings were backfilled with drill cuttings However drilling was done using only soil drilling equipment and rock coring was not done as part of the site investigations Soil samples were obtained using a standard split spoon sampler and the Standard Penetration Test SPT The Standard Penetration Tests aims to determine the SPT N value which gives an indication of the soil stiffness and can be empirically related to many engineering properties The test is conducted inside a borehole A split spoon 83 sampler is attached to the bottom of a core barrel and lowered into position at the bottom of the borehole Split spoon samplers are used for taking samples for lithological descriptions geotechnical analyses that do not require undisturbed samples and chemical analysis Spit spoons consist of a barrel that is attached to a drive tip Sampling is accomplished by attaching the spit spoon to the end lowering it to the bottom of a borehole then driv
2. Figure 4 19 Comparison of 2 D and 3 D resistivity values at Point B Resistivity Ohm m 0 200 400 600 800 Z 0 5 1 2D 5 Feet 1 5 3D 5 Feet 2 EO _ 3D 20 Feet Depth m Ww gi 4 5 5 Figure 4 20 Comparison of 2 D and 3 D resistivity values at Point C 102 The resistivity results at point A Figure 4 17 for a 2 D survey with 5 feet and 3 D survey with 5 feet spacing are very similar almost overlapping However the results for 3 D survey with 20 feet spacing are not matching with 2 D survey for 5 feet spacing Similar trend was observed at point B Figure 4 19 and point C Figure 4 20 Based on these limited data the resistivity results for 2 D and 3 D data using same electrode spacing provide very close results Therefore using 2 D with 5 feet spacing or 3 D with 5 feet spacing should provide us the same results 4 5 Effect of electrode spacing Resistivity imaging was conducted at the same location using different electrode spacing to know effect of spacing between the electrodes on the quality and resolution of resistivity results The effect of spacing was investigated for both 2 D and 3 D 2 D imaging was conducted for 2 feet and 5 feet electrode spacing whereas 3 D imaging was conducted for 5 feet and 20 feet electrode Figure 4 21 presents the resistivity imaging results for 3 D survey with 5 feet and 3 D s
3. Light brown colored LIMESTONE material generally hard N 30 1 105 4 7 Relation between Resistivity and Water Content Figure 4 23 4 24 and 4 25 presents the measured electrical resistivity and moisture content versus depth at each borehole location At location B 1 Figure 4 23 the moisture content decreases within the elevation and then it increases In the same elevation range the resistivity starts to increase and then it decreases The inverse relationship between electrical resistivity and moisture is evident from these results At location B 2 Figure 2 24 moisture content is low close to the surface then it increases to an elevation and finally shows decreasing trend at the bottom of the borehole The decrease in resistivity with increase in moisture is also observed at this location The trend for the moisture content of location B 3 is also the same as shown in Figure 4 25 A general trend of slightly decrease of the moisture with the depth is observed This is correlated with the slight increase of resistivity with depth 106 Moisture content a resistivity oOo 10 20 30 40 50 60 70 Resistivity Ohm m and Moisture Content Figure 4 23 Resistivity and Moisture content graph for 3d 5feet and B 1 Moisture content a resistivity 0 10 20 30 40 50 60 Resistivity Ohm m and Moisture Content Figure 4 24
4. The goal of resistivity survey is to image a subsurface resistivity distribution which is closely correlated with subsurface geology The subsurface resistivity distribution or its reciprocal electrical conductivity is the model parameter in the inversion The model is the partial differential equation that governs the relationship between data and model parameters Forward modeling is defined as the process of predicting the data on the basis of the known distribution of model parameter electrode configuration and model It is a mapping from the model space to the data space Forward modeling creates synthetic data sets Forward modeling is also known as forward simulation forward problem and forward solution Inversion is defined as the process of determining the estimates of the model parameter on the basis of the data and the model Inversion is a mapping from data space to model space and it reconstructs the subsurface resistivity distribution from measured voltage and current data Inversion is also known as inverse modeling inverse simulation and inverse problem The resistivity data inversion proceeds as follows 1 A starting resistivity model is constructed based on either the average apparent resistivity or apparent resistivity distribution or user assumption or a priori knowledge of subsurface resistivity distribution 2 A virtual survey forward modeling is carried out for a predicated data set over the starting model Th
5. 2000 sz nseni sins eveeteaveueg saan sees sane ued vaav ecec eden larg nana ved a e 11 Point source of current at the surface of a homogeneous medium where C and C are the locations of the current Sources Telford et al 1990 12 The sensitivity patterns for a pole pole b Wenner c Schlumberger and d dipole dipole arrays Loke 1999 eois ccsoceccecssptevecsascaeagitasseagatentes getahavecsnase 17 Common arrays used in resistivity surveys and their geometric factors 21 Two different arrangements for a dipole dipole array measurement with the same array length but with different a and n factors resulting in very different signal strengths Loke 1999 0 2 lt b oe ust sascastienereunedecn terest saan 23 Data measurement sequence using dipole dipole array Zhou 1999 24 Forward and reverse pole dipole arrays Loke 1999 oo eeeeceeseeeeesteeeeneees 25 The arrangement of electrodes for 3 D SUIVEY ceeeeeseeeseeeeeeeereeeeeeeeseeeees 29 Two possible measurement sequences for a 3 D survey The location of potential electrodes corresponding to a single current electrode in the arrangement used by a a survey to measure the complete data set and b a cross diagonal survey sesssesseeesssessessseetssersseressessereseeessressresseesseeesee 29 Outline sketch of computer controlled data collection system Dahlin 2001 osea e s E AE e E A eie 31 2 12 2 13 2 14 2 15 2 16
6. 29 56 Normalized L2 88 38 Figure 3 28 Data misfit crossplot 3 6 2 7 EarthImager 3D New Resistivity Inversion Software This software inverts reisistivity data acquired with electrodes arranged in boreholes and or on the surface and presents a 3D volume of inverted resistivity data with advanced volume rendering technique The final resistivity or IP image volume can be rotated in any orientation zoomed in and out and translated to anywhere inside the image window in order to see the volume of interest in detail Colors representing areas of less interest can be made transparent so that the shape of a pollution plume for example can be visible Similar steps as in Earth Imager 2D are followed in Earth imager 3D also Main Window of EarthImager 3D is shown in the Figure 3 29 79 Fa aci Larthimager 3D C Program Files GI Earthimager 3D demoWoreholeS ste Ble Edt Settings Inversion wew Took Help 2lOlalsinl BIBAL E Electrode Scatter Plot kat Mescured Apparent Reasteity ohm m 5 6 199 646 Elapsed Time 0 4 55 066 Figure 3 29 Main window of Earth Imager 3D The left image in the Figure 3 29 shows the arrangement of the electrodes The electrodes can be arranged both in boreholes and or on the surface The right picture shows the scatter plot of measured apparent resistivity By left clicking on one data point in this picture the corresponding four active electrodes are showed as red electrodes current and blue el
7. Resistivity and Moisture content graph for 2d 2feet and B 2 107 0 a 0 5 1 1 5 R E 2 gt Moisture content 25 o 2 a resistivity o 3 o 3 5 rs 4 i 4 5 5 TO T E E dd 0 20 40 60 80 Resistivity Ohm m and Moisture Content Figure 4 25 Resistivity and Moisture content graph for 2d 2 feet and B 3 4 8 Comparison between Resistivity and Drilling Based on the 3D resistivity imaging for Sections A A and A B UTA observed an anomalous location along the E Center Street Initially the anomaly was observed while resistivity imaging was conducted using electrode spacing 20 ft Section A A However based on the results from Section A A another 3D resistivity imaging was conducted Section A B electrode spacing 5 ft within Section A A area to get a better resolution and to identify the possible anomaly The new section also showed the same anomaly Therefore to further verify the site condition a 2D resistivity imaging Line A was conducted along the anomalous location and it was found that the anomaly was mainly due to possible presence of relatively dense weathered Limestone only 2 ft below 108 the ground surface and presence of hard Limestone at about 15 ft below the ground surface The other three 2D imaging showed the similar trend 65ft E Cherry Street r 3 Section C C Section A B Section A
8. delineate groundwater aquifer ees eeeeeseeeeseeesseeeseeeeeeeeee 2 6 4 Contaminant delineation at Lernacken Sweden 2 6 5 Application of 3D electrical resistivity imaging in an underground potash Mine eee eeeeeeeeeeeeeeee 3 FIELD STUDIES SAS IS WES CHIP OOM 2a ga a sccaisthsy ee varse nae a aeS pga EEEE se EEA 3 2 History of the Site and Existing Site Conditions eee eeeeeeeeeeeeee vi 3J Site CHONG y ooe hs won ie aga ea Une EN es ecw a WONG ean aaa ed owe a He eee ea 50 34 Field Investigations a ai ei r e gaat a eas ES ER 51 3 5 High Resolution Resistivity scncdn ntsc nitiontiletininiataninsiaias 51 3 5 1 SuperSting R8 IP Equipment Description ssesssessesssesesseressressee 52 DOS UAL Rey Benefitszscnent nt satan nea Er ATE A A 54 3 5 1 2 Multi channel Resistivity Imaging System eee 54 3 5 1 3 Power CONNE CON 2s ssessaedaceisagedeeavedeontensyenitesdemeaidoneewiiersnids 55 3S kA PUse HO det s sis chercle Rios i eee E eve E iR a 56 Des ON OFF sp epe o ois ee eras 56 SD O The Rey boar A e e a ed ese cola eee 57 3 5 1 7 The power supply ssssssssesssessssssssesssrsseresseesseresseesseressressee 58 3 5 1 8 SuperSting electrode switch bOX sssssesesseessessseresseresseesseee 58 Se EO O 1o IS NE A E ene eS 60 3 5 1 10 Stainless steel electrode stakes 0 0 eee eeseeseeeeseeeseeeeeeees 61 3 5 1 11 Electrode cable design scgisiaciieacuiaisrveiesneees 61
9. difficult to interpret GPR data HRR is used for enhanced mapping of lateral and vertical variations in subsurface moisture content environmental contamination and near surface geology HRR can accurately map ground water flow within earthen embankments and dams It provides finer details and greater depths up to 50 meters or more of investigation that traditional Electromagnetic Induction EMI method A typical HRR survey utilizes a multi channel multi electrode automatic sequencing data acquisition system Figure 1 2 High Resolution Resistivity Equipment 2 Electrical resistivity imaging ERI is a modified direct current method Khesin et al 1996 and is based on the dependence of electrical resistivity of different materials on the content of moisture and high conductive elements porosity temperature etc Aristodemou and Thomas Betts 2000 Gawande et al 2003 Guerin et al 2004 Electrical resistivity surveying is a popular geophysical exploration technique because of its simple physical principle and efficient data acquisition Traditional resistivity measurements are carried out on earth s surface with a specified array in order to obtain apparent resistivity sounding curves apparent resistivity profiling or apparent resistivity pseudosections All of which qualitatively reflect vertical or horizontal variations in subsurface resistivity This technique is widely used in groundwater civil engineering and environmental
10. especially more competent rock has higher resistivity anywhere from orange to red low to mid 100 s ohm meters Thus higher resistivity values would be expected over a greater portion of the map where competent rock was present at depth 99 4 4 Comparison between 2 D and 3 D resistivity values with depth The Figure 4 18 shows the top view of the site with a 2 D Survey Shown by the red line and two 3 D surveys at 5 feet and 20 feet electrode spacing shown by the green and blue lines respectively Since the distance between the Line A Section A A and Section A B are negligible as can be seen in the Figure 4 18 three points are taken and resisitivity depth relationship graphs are plotted for 2 D 5 feet electrode spacing 3 D 5 feet electrode spacing and 3 D 20 feet electrode spacing for each point Resistivity Ohm m 0 200 400 600 800 0 05 2D 5 Feet 1 5 2 3D 5 Feet 25 H EE EE EE te n 3D S 20 Feet Depth m 3 5 Figure 4 17 Comparison of 2 D and 3 D resistivity values at point A 100 N Merrill Ave Section A E Cherrv Street Section A BE Center Line A Figure 4 18 locations of Point A Point B and Point C 101 Resistivity Ohm m 0 200 400 600 800 0 0 5 15 2 25 SS ieee 3D 20 Feet 2D 5 Feet 3D 5 Feet Depth m 3 5 45
11. 0 1 5 1 0 0 5 o o o s 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 a s Fas f n a co 2 00 abo ooo so mm ip m Pseudo section Array parameters a 1 8 n 3 Sensitivitu value 100 PPS ee rag PERNE Dipole dipole array 2D sensitivity function plot c2 ca Pa P2 Deptn 1 0 a a 1 0 2 0 3 0 4 0 5 0 6 8 aa ba e 1 54 Fais ze a o Em SS ee ee e it 128 C220 8 00 3 00 2 00 Boo 2 0 2s Pseudosection Array peremeters 1 8 wea Sensitivity value x 100 Plotting Point d Figure 2 4 The sensitivity patterns for a pole pole b Wenner c Schlumberger and d dipole dipole arrays Loke 1999 17 The effective depth of a measurement is sequentially increased by increasing the distance between the source electrodes and the receiving electrodes The farther is distance the greater the vertical interval in which the bulk of the current flows To acquire data from a new lateral position the lateral location of these electrode pairs is sequentially moved down the array after each measurement and a new measurement is taken at a new lateral location As the electrode separation is incrementally changed or moved and a new resistor or different geoelectric property is encountered differences or contrasts of resistivity are noted As stated above the depth of investigation the sensitivity of the array to vertical and horizontal changes in the subsurface resistivity the horizontal data coverage and the signal strength are some of the characteristic
12. 18 3 27 4 36 6 45 7 54 8 64 0 73 1 82 3 Ohm m 0 0 e EEE EEE EEE EEPE EEE EE a 118 394 ms 46 7 118 5 29 3 157 18 4 Calculated Apparent Resistivity Pscudosection 0 0 9 1 18 3 274 36 6 45 7 54 8 64 0 Ba 82 3 Ohm m 0 0 100 43 45 2 8 6 20 4 13 0 9 2 W738 4 2 Inverted Resistivity Section Iteration 5 RMS 2 77 1L2 0 80 Electrode Spacing 1 52 m Figure 4 2 Apparent resistivity pseudosection and inverted resistivity section for LINE A 88 At the end of inversion the data misfit histogram is displayed to remove the poorly fit data The horizontal axis shows the absolute value of the relative data misfit that is defined as the ratio of the difference between calculated and measured data to the measured data In Figure 4 3 6 is used as removal threshold that is represented by blue line and click remove button and start inversion again Data Misfit Histogram Earthinaggs 2D cz Data Misfit Histogram for Removal of Poorly Fit Data 884 Number of Data 12 15 18 Relative Data Misfit Number of Data Removed 55 3 7 Total Number of Data 1500 Figure 4 3 Data misfit histogram for Line A Figure 4 4 shows an XY curve of root mean squared RMS error in percentage versus iteration number which is called the convergence curve As seen RMS error for Line A in Figure 4 4 drops from 14 4 to 2 77 i e less than 3 implying a good fit between them and hence reasonable accur
13. 3 2 E VV Where e J is the current density in ampere m2 e Eis electric field in volt m e Vis the gradient e Vis the scalar electric potential in volt and e Qis the charge in coulomb Combining equation 2 1 1 with equation 2 1 2 gives the following equation Equation 2 3 3 J ovV Since for a steady current in a homogeneous medium V J 0 from equation 2 3 3 Therefore Equation 2 3 4 V oVV 0 The electric current flows in the subsurface by three different means which are electronic electrolytic and dielectric conduction Since most of the mineral grains that we encounter in geophysical surveying are insulators and electrical resistivity operates at low frequency it is the second type of conduction that we observe i e current flow by ions such as in water and clay Telford et al 1990 According to Ohm s Law resistance R Equation 2 3 5 and resistivity p Equation 2 3 6 are defined by the following equations Reynolds 1997 Equation 2 3 5 R x Ohm s Law For a single current electrode at the surface of a homogeneous medium delivering J amperes and assuming that the air above has zero conductivity the voltage measured at a distance r will be Telford et al 1990 10 I Equation 2 3 6 V 5 1 r Where R is the resistance in ohms V is the potential voltage difference in volts I is the electric current passing through the material in amps p is the resistivity in ohm meters A
14. Convergence CUNT YS eea e tee cap shea E E SREE AE OE EEEE EES 78 Data misfit crossplotssceeneenin a i E E E N RNE 79 Main window of Earth Imager 3D ssssssesesessssesesserssersssresseesseeessersssresseesseresee 80 Apparent resistivity cross plot and 3D resistivity image 81 Dynamic slices of resistivity ocacsascesasocsectacedicsaayansnssacdsssevacacettensodiesse acabaseescrene 82 Boring locations Plane ciccavesciessscsonessvs senncdaguvensevvdecusvasuvevstaeseonavagnncavisieontaceeds 85 Apparent resistivity data for line A Line B Line C and Line D 87 Apparent resistivity pseudosections and inverted resistivity SECHON DOE Lame Asis a bea ee al octes dy Mae A ea ne AE AA EA eia 88 Data misfit histogram for Line A sicesevacssocaensashvaseossacee Aga engetestagevagaeovanedyetoess 89 Convergence curve TOR Line A sets scovacstntons teadscenehlatenapabcuavie tat scedatcieeaentatenenenal 90 Data misfit Crossplot for Line A i avsssicsicssisaceceiaviedeceevssescastatieswssiceteaweelecvnngelt 90 Resistivity Image along Line sede oa li cians titers led Rel sind noe atid seals 91 Resistivity Image along Line B ceseini couse ces steven ocvu sess shgenstnt ans ateraavardeaaaeeahe 91 Resistivity Image along Line Ds kiss asc casey spses au oewaa assesses msacaeecmeees 92 Resistivity Image along Line C oscicaste decsant heces ieee teeavendeadetarilantesaes eel als 92 Electrode geometry for Section A B isis jseenacistantapastndvarseagavclevtedawwhomast
15. The SuperSting used is an 8 channel instrument and can therefore takes up to eight readings for each current injection Therefore the equipment is up to eight times faster than the one channel instruments Figure 3 4 SuperSting R8 IP Multi channel Resistivity Imaging System 52 The SuperSting also incorporates productivity features designed to enhance efficiency in the field for doing resistivity and IP imaging surveys Such features are selectable electrode spacing in the instrument built in roll along possibility to shut off electrodes in case the profile ends up against an obstacle re addressing of the electrodes in the field and much more The SuperSting is a multi electrode system A multi electrode system is a system where a large number of electrodes are attached to the instrument The SuperSting consists of single channel and eight channel instruments Oo Figure 3 5 Single channel SuperSting R1 The single channel instrument has one receiver Thus for each current injection the potential can only be measured between two electrodes Therefore when using a single channel instrument only four electrodes two for current and two for potential are used for each measurement The 8 channel instrument has eight receivers Therefore for each current injection the potential between nine electrodes can be measured simultaneously thus speeding up the measurement process We used SuperSting R8 as a multi channel instrument wh
16. as Dipole Dipole wenner Schlumberger Pole Pole arrays Each type of combination has advantages and limitations in terms of lateral resolution and vertical penetration for instance as summarized in table Table 2 3 Properties of electrode arrays Bernard 2004 DIPOLE DIPOLE WENNER SCHLUMB POLE POLE Resolution best regular Main Depth weak regular criteria Field set up regular regular Amplitude weak regular Other Natural noise regular regular criteria Coupling noise best regular CONFIGURATION 1 1 ESTIMATED about 0 9 x L about 0 2 x L about 0 2 x L INVESTIGATION DEPTH 2 4 2 Three Dimensional Imaging Methods The pole pole pole dipole and dipole dipole arrays are frequently used for 3 D surveys Figure 2 9 This is because other arrays have poorer data coverage near the edges of the survey grid Figure 2 10 The advantages and disadvantages of the pole pole pole dipole and dipole dipole arrays that were discussed in 2 D surveys are also valid for 3 D surveys Two possible measurements for a 3 D survey are shown in Figure 2 10 27 2 4 2 1 Pole pole array Electrodes are usually arranged in a square grid with the same unit electrode spacing in the x and y directions To map slightly elongated bodies a rectangular grid with different numbers of electrodes and spacing in the x and y directions could be used The pole pole electrode configuration is commonly used for 3 D surveys such as the E SCAN metho
17. clip soldered to the cable as seen in Figure 3 11 When the electrode distance is short at shallow surveys the electrodes should not be pushed too deep into the ground in order to avoid changing the geometry the electrode is supposed to be a point source In these cases electrodes should not be kept deeper into the ground than about 5 of the distance between the electrodes Figure 3 11 Electrode stake and electrode switch 3 5 1 11 Electrode cable design The Swift cable used for automatic resistivity or resistivity IP survey The cable has a number of electrode switches smart electrodes mounted most commonly at equal intervals along the cable The electrode switches are numbered consecutively along the 61 cable The number of each electrode switch is marked on the cable beside the switch Each electrode switch also has this number or address stored in its memory Figure 3 12 Electrode Switch 3 5 2 Resistivity Survey Design The Electrical Resistivity Imaging ERI technique was selected as a primary sampling method in this investigation because of its portable economy and practicable advantages in an urban area The resistivity imaging was conducted between august 16 2007 and December 31 2007 in the vicinity of the City of Duncanville Texas USA The locations of the ERI survey lines were chosen based on the locations of previous and or present details of the site as presented by the city of Duncanville Both two dim
18. evaluation of a LNAPL site using electrical resistivity imaging Journal of Environmental Monitoring JEM 7 4 283 287 Schwartz F W and Zhang H 2003 Fundamentals of Ground Water John Wiley amp Sons New York 592 pp Reynolds J M 1997 An introduction to applied and environmental geophysics John Wiley amp Sons New York 796 pp 114 16 17 18 19 20 21 22 23 24 Robinson E S and Coruh C 1988 Basic Exploration Geophysics Wiley New York 562pp Telford W M Geldart L P and Sheriff R E 1990 Applied Geophysics Second Edition Cambridge University Press Reynolds John M 2000 An introduction to applied and environmental geophysics John Wiley amp Sons Ltd Borcea L 2002 Electrical Impedance tomography Institute of physics publishing Inverse Problems 18 p 99 136 Klein K A and Santamarina J C 1997 Methods for broad band dielectric permittivity measurements soil water mixtures 5 Hz to 1 3 GHz Geotechnical Testing Journal 20 2 P 168 178 Griffiths D H Turnbull J Olayinka A I 1990 T wodimensional resistivity mappingwith a computer controlled array First Break 8 4 121 129 Griffiths D H Barker R D 1993 Two dimensional resistivity imaging and modelling in areas of complex geology Journal of Applied Geophysics 29 211 226 Dahlin T Loke M H 1998 Resolution of 2D Wenner
19. has relatively poor resolution as spacing between the electrodes is increased This method cannot take advantage of multi channel system as only single channel is used during the testing al Wenner Alpha b Wenner Beta C1 P1 P2 C2 C2 C1 P1 P2 ec a _ gt ee a _50 lt _a _ 0 e lt a _ gt ese aose aoe k 2x a k 6xa c Wenner Gamma d Pole Pole C1 P1 C2 P2 C1 P1 e a _ gt ee a 5 0 lt _a __9 lt a gt k 3xa k 2x a e Dipole Dipole f Pole Dipole C2 C1 P1 P2 C1 P1 P2 e a eec na _5e lt a _ 30 k xn nt 1l n 2 a g Wenner Schlumberger C1 P1 P2 C2 s6 na gt e lt lt a gt e lt _ na e k xn nt l a k Geometric Factor eqc___ N a______ e eca e k 2xn n l a Equatorial Dipole Dipole h C2 P2 b a nn a 4 t C1 P1 na k 2x bL L b L asa b b 9 5 Figure 2 5 Common arrays used in resistivity surveys and their geometric factors Note The dipole dipole pole dipole and Wenner Schlumberger arrays have two 6699 parameters the dipole length a and the dipole separation factor n While the n factor is commonly an integer value non integer values can also be used 2 4 1 2 Wenner Schlumberger Method One of the first arrays used in the 1920 s and still popular today is the Schlumberger array The most commonly used array for resistivity sounding surveys is Schlumberger array This array can be used on a system by arrangi
20. in 2 D resistivity surveys are fairly well developed Quite a number of international organizations provide the necessary field equipment commercially Approximately this equipment costs about US 15 000 A few institutions have even constructed manually operated switching units at a nominal cost by using a seismic cable Normally a constant spacing between adjacent electrodes is used which is attached to an electronic switching unit which in turn is connected to a laptop computer A laptop is used to record the sequence of measurements to take the type of array to use and other survey parameters As different resistivity meters use different formats for the control file there is a need to refer to the manual for the system After reading the control file the computer program then automatically selects the appropriate electrodes for each measurement In a typical survey most of the fieldwork is in laying out the cable and electrodes After that the measurements are taken automatically and stored in the computer Most of the survey time is spent waiting for the resistivity meter to complete the set of measurements In a typical 1 D resistivity sounding survey usually involves about 10 to 20 readings while 2 D imaging surveys involve about 100 to 1000 measurements There is also a vast difference of cost between a typical 2 D survey and 1 D sounding survey 2 D electrical imaging surveys can give useful results that are complementary to the informatio
21. investigations In the last decade there have been great improvements in computerized data acquisition systems and 2D and 3D inversion software Therefore resistivity imaging has become an increasingly attractive exploration technique Electrical surveys may be a useful tool for the fast investigation of the geometry water content and fluid movement of subsurface The resistivity method is one of the standard methods of the geophysical prospecting for solution of shallow geological problems Resistivity images are created by inverting hundreds to thousands of individual resistivity measurements e g Loke and Barker 1996a b to produce an approximate model of the subsurface resistivity University of Texas at Arlington UTA has recently acquired a High Resolution Resistivity equipment Figure 1 2 The High Resolution Resistivity system is an 8 channel recording instrument to carry out surface measurements for electric resistivity or induced polarization imaging of the shallow subsurface of the earth The system includes a maximum use of 56 electrodes in a single layout The maximum allowable distance between two adjacent electrodes is 20 feet Both line and grid acquisition configurations are possible for 2D and 3D imaging Penetration depths vary depending on average soil resistivity and the number of electrodes in a single layout Electrical resistivity imaging was performed at E Center Development in Duncanville Texas The site was previo
22. is the cross sectional area of the material in meters square L is the length of the resistive material in meters Figure 2 2 Basic definition of resistivity across a homogeneous block of side L and area A with an applied current and potential drop V Reynolds 2000 ERI survey is performed to measure the resistivity distribution of the subsurface Figure 2 3 shows a schematic diagram of an ERI survey Telford et al 1990 Electrical resistivity method considers the subsurface geologic setting as a series of electrical resistors that naturally inhibit an electrical current The success of electrical resistivity surveys lies in their ability to detect changes in the electrical field caused by these 11 resistors and subsequently to determine their locations depths and thicknesses An electrical current is applied to the ground through two source electrodes along this array and the potential difference voltage drop created at the surface is measured between two receiving electrodes located at a known location and distance The potential difference produced as the current encounters earthen material a resistor is measured and used to determine the resistivity of the material between the electrode pairs Power C Surface y r 4 Cai a 5 an U E P miorm resisuivity p f d i q Current flow a Equipotentials Figure 2 3 Point source of current at the surface of a homogene
23. of boreholes on lines A B and D wo c cc cceeeeeeeeeccccsceeeeeeee Comparing results from Line A and B 1o eee eeseesreceseeeseeeeseeeneeenseesees Comparing results from Line B and B 2 00 00 eeeeeeeseeeeeneeeeseeeeeseeeeeneeeeeneeeee Comparing results from Line C and B23 ss sais eissunnsdysncivnsncavasstal cncveeavoaveecsnads xiv LIST OF TABLES Table 2 1 Electrical resistivity range of minerals rocks and fluids Telford et al 1990 Klein and santamarina 1997 Borcea 2002 2 2 The median depth of investigation z e for the different arrays L is the total length of the array Edwards 1977 2 3 Properties of electrode arrays Bernard 2004 3 1 Site Investigations Program 4 1 General descriptions of each stratum XV CHAPTER 1 INTRODUCTION 1 1 Background and Importance The use of geophysical methods for the evaluation of geohazard potential of a site is increasingly becoming popular all over the world During failure analyses several parameters are investigated by geologists and geotechnical engineers However they can only obtain information at certain points not a general view of site conditions There are some major drawbacks of using only drilling and sampling methods for site investigations which are described as follows e Costly Investigations e Few useful data points low data density e Too much interpretation between data points e No continuous picture of the subsurface e Site
24. resistivity imaging as assessed by numerical modelling Journal of Applied Geophysics 38 4 237 249 Overmeeren R A van Ritsema I L 1988 Continuous vertical electrical sounding First Break 6 10 313 324 115 23 26 27 28 29 30 31 D2 33 34 Loke M H 1999 Time lapse resistivity imaging inversion Proceedings of the 5th Meeting of the Environmental and Engineering Geophysical Society in press Van Schoor M 2002 Detection of sinkholes using 2D electrical resistivity imaging Elsevier Science B V Kemna A Engels O G Martin U I 1996 RESITOMO Resistivity Tomography v 2 0 User Manual Software Package for 2 5D Modelling and 2D Inversion of DC Resistivity Measurements Harbour Dom GmbH Ko ln Germany Coggon JH 1971 Electromagnetic and electrical modelling by the finite element method Geophysics 36 132 155 Dey A Morrison HF 1979 Resistivity modelling for arbitrarily shaped two dimensional structures Geophys Prospect 27 106 136 Drahor M G 2006 Application of electrical resistivity tomography technique for investigation of landslides a case from Turkey Environmental Geol 2006 50 147 155 R A Eso D W Oldenburg 2006 Application of 3D electrical resistivity imaging in an underground potash mine Golder Associates Ltd Dahlin T 1996 2D resistivity surveying for environmental and engineering appli
25. to extend the roll along technique used in 2 D surveys to 3 D surveys Dahlin and Bernstone 1997 Figure 2 14 shows an example of a survey using a multi electrode resistivity meter system with 50 electrodes to survey a 10 by 10 grid Initially the electrodes are arranged with the longer lines orientated in the x direction in a 10 by 5 grid Figure 2 15 a Measurements are made primary in the x direction With some possibilities some measurements are taken in diagonal directions Next the entire grid is moved in the y direction The 10 by 5 grid now covers the second half of the 10 by 10 grid area The 10 by 5 grid of electrodes is next orientated in the y direction Figure 2 15 b Now the measurements are made primary in y direction 36 a First position of resistivity meter x direction y direction First position of resistivity meter b Fourth position o resistivity meter Third position of resistivity meter Figure 2 15 Using the roll along method to survey a 10 by 10 grid with a resistivity meter system with 50 electrodes a Surveys using a 10 by 5 grid with the lines orientated in the xdirection b Surveys with the lines orientated in the y direction Loke 1999 37 2 6 Previous Practical Applications 2 6 1 Detection of sink holes using electrical resistivity imaging The site is located near Centurion south of Pretoria South Africa the surface expression with a diameter of about
26. 0 0 30 0 40 0 ohm m Clay Sand Metasediment D __ _ _ gt Figure 2 21 Inverse model section for Olak 1 a Olak 2 b and Olak 3 c Hamzah 2005 2 6 4 Contaminant delineation at Lernacken Sweden Next example is Lernacken sludge deposit in southern Sweden at the abutment of the Oresund bridge under construction It consists of fill material of limestone quarry waste and till which rests on top of glacial till and bedrock consisting of tertiary limestone There is also municipal waste and industrial sludge has been deposited in ponds dugout in the fill Heavily contaminated groundwater is present with organic compounds and heavy metals as major contaminants Bernstone and Dahlin 1997 43 Linhamn Lernacken Sludge Deposit y 140m 1995 10 04 WENNER 2D INVERTED MODEL r m s residuals 7 9 Distance Im 100 100 300 20 zo 10 Limestone with 10 fresh water 20 20 Limestone with 30 4 saltwater 30 50 Levelzim 10 14 19 27 37 52 72 6100 140 190 270 Resistivity fm Figure 2 22 Lernacken sludge deposit Example of inverted depth section where this profile was measured through sludge deposit modified from Bernstone and Dahlin 1997 Resistivity investigations were conducted along 10 lines separated by 20m over the sludge deposit An example of the resulting inverted resistivity sections is shown in Figure 2 22 The sections that are outside the sludge deposit are rather ho
27. 0 Ebschode Spacing 1 52 Figure 4 6 Resistivity Image along Line A 2D Line B o l Possible Hard Material Depih im 4 vested Remi Seika leran 3 RMS 10 L2 0 75 Electrode Spacing 0 61 m Figure 4 7 Resistivity Image along Line B 91 2D Line D Possible Hard Material Ohm m Deepal inm imid RS 27 LE 08S Electrode Spacing 0 61 m Figure 4 8 Resistivity Image along Line D Possible Hard Material 2D Line C 10 6 Ohm m 0 0 1000 18 178 354 31 6 a oO 4 Baye 5 6 10 L0 Inverted Resistivity Section Iteration 7 RMS 4 80 12 2 56 Electrode Spacing 0 62 m Figure 4 9 Resistivity Image along Line C 4 3 Inverted resistivity section results for 3 D Electrode geometry of Section A B is shown in Figure 4 10 As in Earthimager 2D the raw field data or measured apparent resistivity data were processed using EarthImager 3D This computer program uses a leastsquares inversion to convert measured apparent resistivity values to true resistivity values and plots them in cross section The Figure 4 11 shows the inverted resistivity section 92 Electrode Geometry 0 0 13 2 26 4 39 6 Figure 4 10 Electrode geometry of section A B Inverted Resistivity Image Resistivity ohm m 4 cA 26 65 160 Figure 4 11 Inverted resistivity section for Section A B Figure 4 12 shows the crossplot of Section A B which shows the data misfit The horizontal axis
28. 005 Wenner electrode configuration was used because it provides a good vertical resolution and gives clear image for groundwater and sand clay boundaries as horizontal structures The 2 D resistivity data of subsurface material for each survey line was calculated through inverse modeling and then compared with borehole data Resistivity measurements were carried out along several survey lines in SW NE EW and NW SE directions Figure 2 20 The results of this study show the delineation of the aquiferous sand and gravel zone lying below the marine clay in the study area Additionally clay lenses were also detected Bedrock surface was successfully mapped quite precisely at about 40 70 m depth The resistivity interpretation clearly shows the thickness of the aquifer zone about 10 30 m The results of this study further demonstrate that the 42 resistivity imaging is an effective tool for defining the thickness of groundwater aquifers and mapping of bedrocks with relatively shallow depth Distance m 1120 1760 Metasediment 2 a a a a l 0 ee es OUO en en ee ee ee ee o1 2 0 4 0 8 0 13 20 30 40 Ohm m e Cay __ _sand _ Metasectment Distance m 160 640 sand amp Gravel b Metasediment GG 0 We We Whee ee 0 1 2 0 40 8 0 13 0 20 0 30 40 0 Ohm m X 1 0 Clay Sand Metasediment gt t Distance m 20 640 800 g Sand amp Graver Z ES Metasediment a een en 2 ee ee ee 0 1 2 0 4 0 8 0 13 0 2
29. 2 17 2 18 2 19 2 20 2 21 2 22 2 23 2 24 3 1 3 2 3 3 The use of the roll along method to extend the area covered by a survey 34 Slid along LECH GUE onen renren cheandes ae oE E A oE daa EAEE 34 Sketch of 3 D multi electrode survey layout Dahlin and Bernstone 1997 35 Using the roll along method to survey a 10 by 10 grid with a resistivity meter system with 50 electrodes a Surveys using a 10 by 5 grid with the lines orientated in the x direction b Surveys with the lines orientated in the y direction Loke 1999 yc sescts ssc tete aces setsiacatdegctateatetac sates 3 Gadadynass seimeeuseat 37 Schematic showing the basic principle of conducting a conventional dipole dipole resistivity survey Van Schoor 2002 eee eeseeseesteeeseeeseeeeeeeeeeens 38 RESTOM image resulting from the survey at Site Van Schoor 2002 39 Study area after the landslide Drahor 2006 ecceeeseceeseeceeseeeeeeeeeeeneeeees 40 3 D fence diagram of the resistivity sections Drahor 2006 ceeeeeee 41 Location of survey lines Hamzah 2005 cre scxccucs wersszesansencaeataconsvars pote ant cena 42 Inverse model section for Olak 1 a Olak 2 b and Olak 3 c Hamzah Lernacken sludge deposit Example of inverted depth section where this profile was measured through sludge deposit modified from Bernstone and Dahling OO a e aaa as ees A eee ee 44 Geometry of the underground mine drif
30. 3 5 2 Resistivity SUTVEY DESIGN in bisa Wea ademas 62 5 52 1 Eme Ao 2D SURV eyar enone n eee aan 63 3 39 22 Eime B 2D SUV CY sscceyaectiede ee Se aed te aA 63 3 5 2 3 Line C 2 D Survey to the North West Corner eee 64 3 5 2 4 Line D 2 D Survey to North East Corner 0 00 0 eeeeeeseeeeees 66 3 5 2 5 Section A A 3 D Survey 5 iiuasoect sel ceraied ga tnaeg Sat seoae la pedbe tes 66 3 5 2 6 Section A B 3 D SUL VOY voc dace preecth aaact cand peadestinara tena pened artes othe 67 vii 3 3 2 Secon C C 3 D SURVEY gy sacconse wis iaasa a a a as 67 3 6 Data Acquisition and Data Inversion essseeesseseseresssessesseresseresersssse 68 3 0 1 Data ACQUISITION senesine i e E eeuaanuea tee 68 3 0 2 Data INVeErsiOnascs siysctesdecasd nate eee co a Ae ea oe 69 3 6 2 1 Earth Imager 2D inversion and modeling software 72 J G 2 2 26 Gata Mle 1s Cea 4 coc huts ea suede a aR EE 13 3 6 2 3 Data file is edited to remove noisy data eeeeseeeeseeeeees 74 5 G22 A Inversion ISTUN pine ri i E E Tevet tide eae ae 75 3 6 2 5 Removal of poorly misfit data eee ee seeeeseeceeseeeeeneeeeeaes 75 3 6 2 QO ny rsion results aE AR Aa aes 76 3 6 2 7 EarthImager 3D New Resistivity Inversion Software 79 3 7 Subsurface Investigation by Soil Drilling eee eeeeeeteeeeneeeeeneeeeaes 82 4 RESULTS AND DISCUSSION sissssessesnseeshazaesnactnocntvhsae dennenebeqatonod egesescecutouse 86 4 1 Apparent Resis
31. 30 m of a known sinkhole could be seen Van Schoor 2002 Conducted 2 d electrical resistivity imaging RESTOM technique for solving the sinkhole problem Using the dipole dipole electrode configuration RESTOM survey was conducted In RESTOM survey two surface electrodes A and B spaced 8 m apart and positioned at one end of the profile Figure 2 16 are involved in introducing a direct current field into earth For several adjacent potential electrode pairs Mi and Ni by incrementally increasing the distance from the current electrode positions potential difference measurements were acquired The Potential difference profile is acquired with AB positions advanced by 8 m with MN electrode spacing of 4 m This process is repeated until the AB positions are advanced to about 30 m past the area of interest perceived sinkhole edge A total profile length of approximately 100 m was surveyed CURRENT 1 POTENTIAL i POTENTIAL DIPOLE DIPOLE DIPOLE i SURFACE A B M N M N Figure 2 16 Schematic showing the basic principle of conducting a conventional dipole dipole resistivity survey Van Schoor 2002 For the iterative inversion scheme the data set is used as input A commercially available software package RESITOMO developed by DMT Germany Kemna et al 1996 was used for the inversion The image in Figure 2 17 clearly maps the sinkhole 38 structure that affected the road at this site Within the basin shaped Zone t
32. 41 5 View Data Misfit Histogram View Misfit Pseudosection View Misfit Crossplot m 7481 2668 951 Depth m 339 0 Inverted Resistivity Section Iteration 3 RMS 2 80 Normalized L2 0 87 Advanced Geosciences Inc Survey Date Oct 29 1994 Instrument AGI SuperSting RS Software AGI EarthImager 2D Elapsed Time 0 0 1 422 Length in meters Resistivity in ohm m Figure 3 21 Earth Imager 2D main window 72 The order of main menus is basically that of inversion steps That is the inversion with EarthImager in general consists of these steps e A data file is read e Data file is edited to remove noisy data e Inversion is run e Inversion results are viewed and e The inversion results are saved and printed 3 6 2 2 A data file is read Read Data menu item from the File menu should be chosen Then the data file is allowed to process After reading data successfully EarthImager will display either an apparent resistivity pseudosection for surface survey For apparent resistivity pseudosections shown in Figure 3 21 the color scale is logarithmic There is a vertical exaggeration of the depth scale The text font number of colors and Figure title can be changed The grid dots showing the plotting location of measurements may be turned on and off and the grid dot size and color are changeable The larger black squares on the surface show the electrode locations and are also editable This is
33. 9 2008 ii ABSTRACT SUBSURFACE INVESTIGATIONS USING HIGH RESOLUTION RESISTIVITY DHARMATEJA MAGANTI MLS The University of Texas at Arlington 2008 Supervising Professor MD Sahadat Hossain Electrical resistivity imaging has been proved to be well suited to produce images of subsurface conditions because of the ability of the technique for detecting resistive features and discriminating subtle resistivity variations in the soil media Electrical resistivity imaging survey was performed at E Center Development in Duncanville Texas The site was previously used as both commercial and residential property The majority of the land was used as residential property City officials were concerned about the presence of any contaminants at the site Therefore the objective of the research is to investigate and find any possible contaminants at the site The survey was conducted using SuperSting R8 IP Multi channel Resistivity Imaging System to image the subsurface structure Both 2 D and 3 D electrical resistivity imaging tests were conducted using dipole dipole array at 2 feet 5 feet and 20 feet electrode spacing Earth Imager 2D and Earth Imager 3D software s were then ill used to invert the apparent resistivity data Based on the resistivity a subsurface profile was developed A subsurface exploration program was conducted with a total of three test borings labeled BH 1 through BH 3 and were drilled by Apex Geoscience Inc u
34. A I Figure 4 26 Location of boreholes on lines A B and D 109 However to confirm and verify the actual subsurface information a soil test boring program was completed and three soil test borings were drilled as shown in Figure 4 26 All three soil test borings confirmed the trend that was observed during the resistivity imaging as shown in Figures 4 27 4 28 and 4 29 However no contaminants or environmental conditions were found in any soil samples that were collected during drilling Depth m Maximum boring depth 15 feet 6 45 7 MS 640 mM 823 Depth m 27 freration RMS Lag 00 3 73 6 336 ede Spacing 0 61 m Presence of hard material Figure 4 28 Comparing results from Line B and B 2 lenerted Rossu Secu ergxe 3 EMS 2 110 Depth tm Berea 4 RMS 279 Lo 086 Eoowode Spang 061 m i Presence of hard material Figure 4 29 Comparing results from Line D and B 3 111 CHAPTER 5 CONCLUSIONS Based on comprehensive analysis of the test results following conclusions can be drawn from the present thesis work e Based on the resistivity imaging and soil test boring no contaminants were found at the site e The resistivity imaging shows the presence of weathered Limestone was present only 2 ft below the ground surface and hard Limestone was present at about 15 ft below the ground surface e Based on the res
35. Resistivity Pseudosection LINE B 0 0 3 7 T3 11 0 14 6 18 3 21 9 25 6 29 3 32 9 Obm m 0 0 E O O O O O O O O O 99 16 34 6 p 12 1 4 8 4 2 re z 15 Measured Apparent Resistivity Pseudosection LINE C 0 0 37 T3 11 0 14 6 18 3 21 9 25 6 29 3 32 9 Ohm m 0 0 ee eee eee eb ee 74 51 0 35 3 24 4 16 9 Measured Apparent Resistivity Pseudosection LINE D Figure 4 1 Apparent resistivity data for Line A Line B Line C and Line D 87 4 2 Inverted resistivity section results for 2 D The raw field data or measured apparent resistivity data were processed using EarthImager 2D This computer program uses a leastsquares inversion to convert measured apparent resistivity values to true resistivity values and plots them in cross section The program creates a resistivity cross section calculates the apparent resistivities for that cross section and compares the calculated apparent resistivities to the measured apparent resistivities The iteration continues until a combined smoothness constrained objective function is minimized The measured apparent resistivity pseudosection calculated apparent resistivity pseudosection and inverted resistivity section for LINE A are shown in Figure 4 2 Depth m Depth m Depth m Dun 3V 2D1_trial4 stg 0 0 9 1 18 3 27 4 36 6 45 7 54 8 64 0 73 1 82 3 Ohm m 0 0 pe E G A O A E E 118 294 or 118 4 15 7 Measured Apparent Resistivity Pseudosection 0 0 9 1
36. SUBSURFACE INVESTIGATIONS USING HIGH RESOLUTION RESISTIVITY by DHARMATEJA MAGANTI Presented to the Faculty of the Graduate School of The University of Texas at Arlington in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN CIVIL ENGINEERING THE UNIVERSITY OF TEXAS AT ARLINGTON December 2008 ACKNOWLEDGEMENTS The author would like to thank his supervising professor Dr Sahadat Hossain for all his guidance and unconditional support throughout the course of this research effort Thanks are also extended to the other members of his thesis committee Dr Laureano R Hoyos and Dr Mohammad Najafi for their valuable advice and review of this manuscript In addition the author would like to thank the faculty and staff of the Department of Civil and Environmental Engineering at The University of Texas at Arlington for their valuable assistance during his graduate studies The author also would like to thank all the geotechnical engineering graduate students in this institution for all their help and support Special thanks are also extended to the Indian group for their worthy friendship and the good times The author would also like to thank his dearest friends for their worthy friendship and the good times Finally and most of all the author would like to thank his parents for all their love encouragement and great support It is the best thing in his life to be a part of their family July 2
37. acy of inverted resistivity section 89 Convergence Curve of Resistivity Inversion 14 4 10 8 1 2 RMS Error 3 6 Tteration Number Figure 4 4 Convergence curve for Line A Figure 4 5 shows the crossplot of Line A which is another good way to show data misfit The horizontal axis is the logarithm of measured apparent resistivty and vertical axis is the calculated apparent resistivty As most of the data points lie on the green line the calculated apparent resitivity fits corresponding measured apparent resistivty Crossplot of Measured vs Predicted Apparent Res Data 1 2E 02 E 7 4E 01 a g 4 g 2 r irar a E E 4 7E 01 oak ae T a 3 S 3 a 29E 01 4 1 8E 01 T T T 1 8E 01 2 9E 01 4 7E 01 7 4E 01 1 2E 02 Measured Apparent Resistivity ohm m Iteration 5 RMS 2 77 L2 0 80 Electrode Spacing 1 52 m Figure 4 5 Data misfit Crossplot for Line A 90 A similar trend is followed for data misfit histogram crossplot and convergence curve in all the 2D Lines The resistivity cross sections Figure 4 6 4 7 4 8 and 4 9 are modeled from apparent resistivity data collected in the dipole dipole array mode Since electrical current will follow the path of least resistance distortions may be created by low resistivity features outside the plane of data collection Possible Hard Material 2D Line A 00 Depth im 114 ieas RMS 2TA 1290 8
38. al in the Fourier transform domain and I is the electric current source k is the wavenumber in the transform domain o is electric 71 conductivity as a function of x z Both finite difference and finite element methods were implemented 3 6 2 1 Earth Imager 2D inversion and modeling software EarthImager 2D is the new standard for affordable Resistivity Imaging software With this software data collected with the AGI SuperSting earth imaging resistivity instruments can be interpreted into easy to read 2D earth sections The processed data can be converted to various types of files and can be processed into reports Main Window of EarthImager 2D is shown in the Figure 3 21 EarthImager is based on Windows 32 bit platforms Its user friendly graphical user interface B aci EarthImager 2D C Program Files AGI EarthImager2D demo cave stg File Edit Settings Inversion View Tools Help eHe SHl e gt inea z SiGe eh defis Hm ms ap a Sec F m Cave Detection in Texas Ohm m 1548 859 476 Depth m M d Ay t Resistivity Pseudosecti easured Apparent Resistivity Pseudosection v Show Graphics ToolBar 0 14 37 41 Color and Contour Properties Input Graphics Title hm 3 Ohms 1548 Input Project Information Show Animation 859 Copy Image To Clipboard 476 Save Data in XYZ Format 264 Save Image 3 Ohm m a 2 a 146 Calculated Apparent Resistivity Pseudosection Print Image View Convergence Curve 0 14 27
39. an intrinsic electrical property of a material to oppose the flow of the electric current Schwartz 2003 meanwhile resistance is a ratio of the voltage potential difference and the electrical current retardation which depends on inherent property of material and its physical geometry Reynolds 1997 The measured quantity is called apparent resistivity ia Current meter Current i Lr iraramitter a gt ne JEG pe a con Fi Brr f ae a we poupee Sal F 7 ao a oe Al Kaa AEN ee oe A VN HES py ee GS a ee ee eel lt 2 wie oe oe Pee corn Saeeee ee an I f a x Te ae x jx x Bedrock NS s 3 a 4 anA Ak j ta h of eleciric curent Fi A x rs Pi lt a x d x Z S Pa x p Figure 2 1 Principle of resistivity measurement modified from Robinson and Coruh 1988 Electrical resistivity technology has recently evolved with the increased sophistication of electrical hardware and software Formerly two methods of subsurface imaging using electrical resistivity were available electrical sounding for investigating depth and electrical trenching for assessing lateral Variations Electrical soundings presented a one dimensional vertical profile of limited lateral control whereas electrical trenching generated a lateral profile but was limited to a constant depth and the bulk resistivity of only that depth interval Telford et al 1990 However with multi electrode systems
40. array measurements are made with the forward and reverse arrangements of the electrodes The problem of low signal strength for large values of the n factor exceeding 8 to 10 can be avoided by increasing the a spacing between the P1 P2 dipole pair to get a deeper depth of investigation with a smaller n factor The use of redundant measurements with overlapping data levels to increase the data density can in some cases help to improve the resolution of the resulting inversion model 2 4 2 3 Dipole dipole array It is used for grids which are larger than 12 by 12 due because the horizontal data coverage at the sides is poor The primary concern is the low signal strength This problem can be overcome with the procedure followed for the 2 D surveys i e increasing the a spacing between the P1 P2 dipole to get a deeper depth of investigation as the distance between the C1 C2 and P1 P2 dipoles is increased Data can also be overlapped for the same In most cases a number of parallel 2 D survey lines are used for the construction of 3 D data sets for the pole dipole and dipole dipole arrays 30 2 5 2 D and 3 D Imaging Methods 2 D and even 3 D electrical surveys are now practical commercial techniques with the relatively recent development of multi electrode resistivity surveying instruments Griffiths et al 1990 and fast computer inversion software Loke 1994 2 5 1 2 D Method 2D technique separates elec
41. atic shut off function will cause no loss of data or instrument setting The automatic shut off function can be disabled or the time setting can be changed on menu 6 4 1 The shut off time can be set for any time between 1 60 minutes 3 5 1 6 The Keyboard The keyboard has 20 tactile keys Alphanumerical keys are used to enter letters and numbers and to select options on the different menus To enter letters select the first second or third letter on the key by first pressing F1 F2 or F3 For example to enter the word STING press F1 1 F2 1 F3 9 F2 5 and F1 9 The menu key is used to go up one step in the menu system The function keys F1 F2 and F3 are used to select certain functions The measure key marked MEA is used to start the measurements The combination contrast adjustment key and back space key is located in the top right corner of the keyboard The LCD is sensitive for temperature and may turn pale or too dark with change of ambient temperature To adjust the contrast to a suitable level the contrast key should be pressed repeatedly The enter key is used to enter or select data 57 3 5 1 7 The power supply The power supply is used when the SuperSting is used in an office environment for example connected to a PC for data download command file upload or flash memory upload It can handle 100 250 V AC at 50 60 Hz input power The power supply has an ON OFF switch which is used to turn on and off the SuperSting when th
42. both types of acquisition modes can be carried out simultaneously It is easier using today s instruments to acquire the data as a set of soundings comprising a two dimensional cross section or profile of the subsurface On the basis of the distribution of resistivity within a profile an accurate interpretation of the subsurface geologic setting can be made Electrical Resistivity Imaging ERI is a noninvasive geophysical technique that is commonly used in near surface exploration characterization and monitoring When compared with the best geophysical methods it provides inferior resolution but its merit is the fact that it provides resistivity which is an important physical property 2 3 Theory Electrical resistivity is defined as an intrinsic electrical property of a material to oppose the flow of the electric current Schwartz 2003 it is the opposite or reciprocal of electrical conductivity Resistance is a ratio of the voltage potential difference and the electrical current retardation which depends on inherent property of material and its physical geometry Reynolds 1997 The electrical resistivity is described by Ohm s law shown in equation 2 3 1 that shows that the electric field is the electrical resistivity times the current density The electric field can also be described by equation 2 3 2 that says that the electric field is the gradient of a scalar potential Telford et al 1990 Equation 2 3 1 J 0E Equation 2
43. bottom of the section 3 4 Figure 2 13 Slide along technique 34 2 5 2 3 D Method Now a days 3 D surveys are a subject of active research yet not reached the level of 2 D surveys which are used in routine This is because the survey cost of 3 D survey in an area is very high then that of 2 D survey In near future two developments can be made to 3 D surveys which might cut down the cost effectively One is to reduce the survey time by the development of multi channel resistivity meters that enables taking more than one reading at a time Secondly to use faster microcomputers and enable the inversion of very large data sets to be completed within a reasonable time where the dataset can be with more than 8 000 data points and survey grids of greater than 30 by 30 In theory a fully 3 D resistivity survey using a 3 D interpretation model should give the most accurate results as all geological structures are 3 D in nature 43 48 53 58 63 Coble 3 To 22 27 32 37 42 Coble 2 1 6 t1 16 2 Coble 1 Figure 2 14 Sketch of 3 D multi electrode survey layout Dahlin and Bernstone 1997 2 5 2 1 3 D Roll Along Technique Mostly to cover a reasonably large area the commercial 3 D surveys will probably involve grids of at least 16 by 16 A 16 by 16 grid will require 256 electrodes which is more than that available on many multi electrode resistivity meter systems One 35 method to survey such large grids with a limited number of electrodes is
44. cations First Break 14 275 284 Loke M H 1999 2 D and 3 D electrical imaging surveys Edwards L S 1977 A modified pseudosection for resistivity and induced polarization Geophysics 42 1020 1036 116 35 Griffiths D Turnbull J Olayinka A 1990 Two dimensional resistivity mapping with a computer controlled array First Break 8 121 129 36 Li Y Oldenburg D 1992 Approximate inverse mappings in DC resistivity problems Geophysical Journal International 109 343 362 37 J Bernard O Leite and F Vermeersch 2004 Multi electrode resistivity imaging for environmental and mining applications IRIS Instruments 117 BIOGRAPHICAL INFORMATION Dharmateja Maganti was born on January 9th 1984 at the City of Hyderabad India He received her bachelor degree in Civil Engineering from Jawaharlal Nehru University India in June 2005 With the great motivation and enthusiasm for developing higher level skills and knowledge in the area of civil engineering he decided to pursue M S graduate studies majoring in geotechnical engineering at The University of Texas at Arlington In January 2006 he was admitted to the Department of Civil Engineering at The University of Texas at Arlington as a Masters candidate During his studies he had the opportunity to work as a graduate teaching assistant under the supervision of Dr Shahadat Hossain Mr Dharmateja Maganti has successfully completed all r
45. ces of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image 96 Section A B Electrode Spacing 5ft Possible Anomaly 0 004 57 000 4 57 Resistivity ohm m 5 Inverted Resistivity ohm m 15 85 473 a b Possible Anomaly Inverted Resistivity ohm m Inverted Resistivity ohm m 85 473 i 85 473 2623 c d Figure 4 15 a Resistivity Image of Section A B b X Slices of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image 97 Section C C Electrode Spacing 10ft Possible Hard material 39 6 91 ee Inverted Resistivity ohm m asa amaaga u 26 65 160 4 11 26 65 39 6 0 0 a6 Inverted Resistivity ohm m Inverted Resistivity ohm m 11 26 65 ii 26 6 c d Figure 4 16 a Resistivity Image of Section C C b X Slices of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image 98 The resistivity cross sections and lines are modeled from apparent resistivity data collected in the dipole dipole array mode Since electrical current will follow the path of least resistance distortions may be created by low resistivity features outside the plane of data collection From the results lowest resistivity values less than 50 ohm meters are interpreted as thick soil layers which may also be higher in moisture content Conversely shallow bedrock
46. cted to electrode 1 pin 59 2 to electrode 2 and so on The connector marked High address cable is used to address electrodes 28 54 Pin 1 of the connector should be connected to electrode 28 pin 2 to electrode 29 and so on An adapter is delivered with the switch box for connecting the female connector of a passive cable to the female connector of the switch box 3 5 1 9 Cables It is not advisable to use any unshielded cable to connect to the electrode stakes Crosstalk between members of the cable may be a serious problem causing coherent noise in the data Since the noise is coherent it is impossible to distinguish from real data Figure 3 10 Cable s It is not advisable to use seismic cables to connect to the electrode stakes The reason is that a seismic cable is not designed to transmit electrical current but to transmit a small voltage signal in the millivolt range Since the signal voltage in the seismic application is low there is no reason to protect the wires from cross talk between members In the electrical resistivity application current are driven into the ground with up to 400 V and at the same time the response is measured on other cable members in the millivolt range 60 3 5 1 10 Stainless steel electrode stakes For manual resistivity work using four cable reels and four stakes the stainless steel electrode stakes are sufficient The cables are connected to the electrodes with a solid copper Muller
47. ctrode is approximately proportional to the square of ratio of the C1 P1 distance to the C2 P1 distance Hence the pole dipole array is less affected by the C2 remote electrode when compared to the pole pole array If the distance of the C2 electrode is more than 5 times the largest C1 P1 distance used the error caused by neglecting the effect of the C2 electrode is less than 5 The exact error depends on two factors one being the location of the P2 electrode for the particular measurement and the other is subsurface resistivity distribution This is an attractive array for multi electrode resistivity meter systems with relatively small number of nodes because of its good horizontal coverage The signal strength lines between the Wenner and Wenner Schlumberger arrays and Dipole dipole arrays where dipole dipole has the less signal strength For IP surveys this array is an attractive alternative as the signal strength of this array is higher then dipole dipole array and also the EM coupling is lower when compared with the Wenner and Wenner Schlumberger arrays due to the separation of the circuitry of the current and potential electrodes 25 2 4 1 5 Pole Pole Method This array is not as commonly used as the Wenner dipole dipole and Schlumberger arrays In practice the ideal pole pole array with only one current and one potential electrode Figure 2 5 does not exist To approximate the pole pole array the second current and potential electrode
48. d The maximum number of independent measurements nmax that can be made with ne electrodes is given by Equation 2 3 10 nmax ne ne 1 2 For instance each electrode can be used as a current electrode and a potential where all the other electrodes are measured The reciprocity makes it necessary to measure the potentials at the electrodes with a higher index number than the current electrode There are 300 possible measurements for a 5 by 5 electrodes grid and 1176 and 4500 for a 7 by 7 and 10 by 10 electrodes grids respectively For commercial surveys grids of more than 10 by 10 would be more practical due to the wide coverage It can take several hours to make such a large number of measurements particularly with typical single channel resistivity meters commonly used for 2 D surveys To reduce the number of measurements required without seriously degrading the quality of the model obtained an alternative measurement cross diagonal survey method can be introduced Here the potential measurements are only made at the electrodes along the x direction the y direction and the 45 degrees diagonal lines passing through the current electrode 28 Laptop Computer a Resistivity Meter x direction y direction e Electrode Figure 2 9 The arrangement of electrodes for 3 D survey The pole pole array has two main disadvantages First it has a poor resolution Subsurface structures tend to b
49. dry to moist Sandstones Unconsolidated wet clay Clays Kaolinite dry to moist Bentonite dry to moist Granite Bauxite Chromite Hematite Magnetite Calcite Diamond Mica Biotite Sea water Resistivity Qcm 1 10 15 50 10 200 50 5000 0 16 12 4x 10 2x10 5 9 x 10 2 9 x 10 1 6 4 x 10 2x10 10 10 10 2 1 x 10 10 1 5 x 10 500 2000 2x 10 6x 10 1x 10 3 5 10 5x 10 5 7x 10 2x 10 10 10 9x 10 10 2x 10 10 20 2 4 Resistivity imaging methods For a few decades now various electrode arrays like pole pole PP pole dipole PD half Wenner HW Wenner a WN Wenner Schlumberger SC Wenner B WB dipole dipole DD and y array Figure 2 5 have been used for electrical exploration Some of them are now frequently employed in 2 D and 3 D resistivity imaging applications namely PP PD SC WN and DD Dahlin 1996 Chambers et al 1999 Storz Storz and Jacobs 2000 The choice of the best array for a field survey depends on the type of structure to be mapped the sensitivity of the resistivity meter and the background noise level Loke 2004 The depth of investigation the sensitivity of the array to vertical and horizontal changes in the subsurface resistivity the horizontal data coverage and the signal strength are some of the characteristics of an array that are to be considered Depth of investigation is an important parameter in any resistivity s
50. e asia alas 9 2 4 Resistivity imaging IMCUIOUS 5c0r sadsacatceadersegedecesesbehogcteadetatdveneesaseatevtes 16 2 4 1 Two Dimensional Imaging Methods eeeceeeseeeeeeeeeeeeeeeneeeees 20 2 4 1 1 Wenner Method ccsceieccideaenaGcertetiih oetardeeeeites Heres aeeebtenteeks 20 2 4 1 2 Wenner Schlumberger Method 1 0 0 0 eeeeccesseeeeeseeeeeneeeeenes 22 2 4 1 3 Dipole dipole Method s1sjsscrccassecgsans conde cdseteetiastepuaatdeerseaneeede 23 24 VA Pole dipole Method cnra anei 24 2 4 1 5 Pole Pole Method aene nierien natia aaia 2 4 2 Three Dimensional Imaging Methods ceeeeceeeseeeeeneeeees 242 1 Pole pole array onien ited ak anaes DAD 2 Pol dipole AIT AY oei a E aS 2 4 2 3 Dipole dipole array sy sarse Ae iseavdeivaios cae vbecvavdeatasdcconencuewaye 2 5 2 D and 3 D Imaging Methods sissies scazhacecsasatasccscespacecenstecitompctceaeices BSD 2D MICRO oi arenie e ee a Eaa 2 5 1 1 2 D Slide along and Roll along Technique DYDD Bald Methodes naun nes timp senate a Satan Ste 2 5 2 1 3 D Roll Along Technique 00 0 0 eee eeeeeeeeseeeeeneeeeeees 2 6 PreVi0US WOLK 8 essice eshte bicinepacleednacldeous thie pivkipepesice boawldges ia 2 6 1 Detection of sink holes using electrical resistivity imaging 2 6 2 Electrical resistivity imaging technique for investigation of Landslides isscissiecepessvavacatadesededervacsantinesate 2 6 3 Electrical resistivity imaging technique to
51. e initial root mean squared RMS error at the zero th iteration may be calculated at this step 70 3 4 5 6 7 A linearized inverse problem is solved based on the current model and data misfit for a model update Am The resistivity model is updated using a formula like this mj m Am The model parameter m consists of electrical conductivity of all model blocks in the finite difference or finite element mesh The symbol I is the iteration number A forward modeling virtual survey is run based on the updated model for a updated predicted data set A new RMS error is calculated between the predicated data and the measured data If any of version stop criteria is satisfied the inversion is stopped Otherwise repeat the steps 3 to 7 Forward modeling is the process which undergoes in the software In forward modeling we are dealing with a 2D 3D problem here that is a 2D earth model but a 3D electrical field due to point source It has become a standard practice to solve 2D 3D resistivity forward modeling problem using numerical methods by discretization of the domain of investigation The governing 3D partial differential equation is Fourier transformed into a 2D equation to reduce computing time The forward solution can be obtained by solving the 2D partial differential equation in the Fourier transform domain 2 6 4 2 o K oV 1 6 x 6 z Where V is the scalar electrical potenti
52. e instrument is powered by the power supply Note that the ON OFF switch on the SuperSting front panel does not work when the instrument is powered in this manner 3 5 1 8 SuperSting electrode switch box In some applications it is not economically feasible to use the intelligent smart electrodes in such cases switch box is used with passive cables and electrodes to form a central switching system When using the switch box any cable can be used to connect to the electrode stakes Figure 3 8 The Switch Box and Amphenol connector 58 The switch box Figure 3 8 available is 56 electrode switching capability for the SuperSting R8 IP SuperSting R8 IP is available with built in switch box for 28 and 56 electrodes The switch box has two Amphenol male and female connectors black plastic type and two female MS3476L18 32S connectors metal type as in Figure 3 8 On the SuperSting models with built in switch capacity the MS3476L18 32S connectors are placed on the right side of the instrument enclosure The Amphenol connectors are used to connect the switch box to the SuperSting instrument or daisy chaining to other switch boxes in order to control more electrodes Figure 3 9 SuperSting Switch box connection The MS3476L18 32S are used to connect the electrodes to the instrument use mating connector MS3476L18 32P The connector marked Low address cable is used to address electrodes 1 28 Pin 1 of the connector should be conne
53. e of magnesia is found in it 3 4 Field Investigations Field investigations in the site were conducted using both High Resolution Resistivity HRR imaging and Soil drilling 2 D resistivity imaging along four profiles were carried out over the site using Dipole Dipole configuration In addition three 3 D resistivity imaging were carried out Three boreholes were drilled in the site to confirm the site conditions and verify HRR imaging results 3 5 High Resolution Resistivty The High Resolution Resistivity system is an 8 channel recording instrument to carry out surface measurements for electric resistivity or induced polarization imaging of the shallow subsurface of the earth The system includes a maximum use of 56 electrodes 51 in a single layout The maximum allowable distance between two adjacent electrodes is 20 feet Both line and grid acquisition configurations are possible for 2D and 3D imaging Penetration depths vary depending on average soil resistivity and the number of electrodes in a single layout 3 5 1 SuperSting RS IP Equipment Description The new SuperSting R8 IP 8 Figure 3 4 channel multi electrode resistivity and IP imaging system released at SAGEEP 2000 continues to sell all over the world The SuperSting is a breakthrough product from the market leader Advanced Geosciences Most multi electrode resistivity instruments are one channel instruments and therefore take one reading for each current injection
54. e resistivity variations Griffiths and Barker 1993 Figure 2 6 shows the data collection sequence for the dipole dipole array in an ERT investigation The symbol a denotes the unit spacing of electrodes which is selected based on the desired depth of penetration the required resolution and the type of array The electrode spacing and dipole separation are constant for each traverse n and increases with each successive traverse Larger electrode spacing provides data from greater depths but with poor resolution b C C1 P P N ae 3 gt Figure 2 6 Two different arrangements for a dipole dipole array measurement with the same array length but with different a and n factors resulting in very different signal strengths Loke 1999 53 0 mo W The sensitivity pattern in Figure 2 4 d shows that the array is sensitive to resistivity changes between the current electrodes pair as well as the potential electrodes pair As seen in the Figure 2 4 d the sensitivity pattern is almost vertical Therefore the dipole dipole array is very sensitive to horizontal changes in resistivity Thus the dipole dipole array gets good resolution for vertical structures but relatively poor mapping for horizontal structures 23 Several measurements for each current location nef se tangssiied yeneena reieme pans H v2 V3 V4 i Surface San a a 45 Position m n e E a i Serene reer
55. e smeared out in the final inversion model Second particularly for large electrode spacing the second current and potential electrodes must be placed at a sufficiently large distance from the survey grid a Survey to measure the complete data set b Cross diagonal survey 1 2 3 4 5 1 2 3 4 5 6 7 8 9 10 6 7 8 9 10 LARS f T cai come es VERNOS ye S 8 E Oe 1 11 12 13 lt 14 15 E x if j i RA x Xx z x lt x lt Ko e a i aj a A a Y 5 E PaE i Re ed P Sa 167 47 f B A e S20 16 17 18 19 20 lt Pa KOEK K x X x P t j y S Fi f 1 S 21 22 23 24 25 21 22 23 24 25 x x Xx X X current potential electrode x lt electrode Figure 2 10 Two possible measurement sequences for a 3 D survey The location of potential electrodes corresponding to a single current electrode in the arrangement used by a a survey to measure the complete data set and b a cross diagonal survey 29 2 4 2 2 Pole dipole array An alternate to the pole pole array for surveys grids of 12 by 12 and above is the Pole dipole array It has a better resolving power and less susceptible to telluric noise compared to the pole pole array Also when compared to the dipole dipole array it has significantly stronger signal strength Although it has one remote electrode the effect of this electrode on the measurements is much smaller compared to the pole pole array The pole dipole array being an asymmetrical
56. ectrodes potential and the picked data point as a black dot The electrode arrangement on the left side can be rotated or turned in any way so that electrodes further back which are blocked from view by electrodes closer can be seen The inversion is started by clicking on the green button with the arrow When the inversion is started a crossplot of measured versus calculated apparent resistivities Figure 3 30 is shown to the left Ideally all of the dots should end up on a 45 80 degree line for noise free data and error free numerical modeling The right image shows a 3D volume of inverted resistivity Resistivity inversion is an iterative process These two images will update at the end of each iteration PJaci Earthimager 3D C Program Files AGI Earthimager 3D demoWorehole5 stg Fle Eck Settings Inversion Yew Tools Help B0l8lslh araf 3 Apparent Resistivity Crossplot 3D Inverted Resistivity Image LoglolAppRes Meaz Iteration No 2 RMS 27 01 Inverted Reccevity ohmm 14 49 Iteration No 2 of 3 Vole rendering PTET TTT TTT Figure 3 30 Apparent resistivity cross plot and 3D resistivity image These dynamic slices Figure 3 31 are a powerful tool for interactive data interpretation Any of three slices may be turned on or off Any slice can be moved along the axis to sweep through the entire volume in search for anomalies A left mouse clicking on a slice would reveal the x y z coordinate and corresponding res
57. eg i za D TEEEIEETEETELETLIETELE eres a Fa 3 E AAT TE cee S K 4 series eee iveisikicceatdsbatusraaarar Piotting points Figure 2 7 Data measurement sequence using dipole dipole array Zhou 1999 2 4 1 4 Pole dipole Method The pole dipole array is an asymmetrical array Figure 2 5 unlike the other common arrays and over symmetrical structures the apparent resistivity anomalies are asymmetrical in the pseudosection In some cases the model obtained after inversion can be influenced by the asymmetry in the measured apparent resistivity values The effect of the asymmetry can be eliminated by repeating the measurements by arranging the electrodes in the reverse order Figure 2 8 With the combination of forward and reverse pole dipole arrays any bias in the model caused by the asymmetrical nature of the array can be removed The signal strength of pole dipole array is significantly higher when compared to the dipole dipole array and also the pole dipole array is not as sensitive as pole pole array to telluric noise but the pole dipole array has relatively good horizontal coverage 24 Cy RB 5 a me Pole dipole P x R C1 Reverse ee Pole dipole Figure 2 8 Forward and reverse pole dipole arrays Loke 1999 The pole dipole array requires a remote electrode the C2 electrode The distance between this electrode and the survey line should be sufficiently far For the pole dipole array the effect of the C2 ele
58. ensional 2D and three dimensional 3D investigations were conducted for the site Three 3 3D resistivity imaging surveys and four 4 2D resistivity imaging surveys Figure 3 16 were conducted in the site The 3D imaging areas are identified and presented in Figure 3 16 as Section A A Section A B and Section C C The 2D imaging sections are identified and presented in Figure 3 13 to 3 15 as as Line A Line B Line C and Line D 62 3 5 2 1 Line A 2 D Survey The Line A survey is close to E Center Street The resistivity imaging was conducted with 5 ft electrode spacing and using the Dipole Dipole Array The survey was conducted West to East as shown in the Figure 3 13 The total length of the survey line was 275 feet E Cherry Street Figure 3 13 Line A Survey from West to East 3 5 2 2 Line B 2 D Survey Line B is close to N Merrill Ave The resistivity imaging was conducted with 2 ft electrode spacing and using the Dipole Dipole Array The survey was conducted South to North as shown in the Figure 3 14 The total length of the survey line was 112 feet 63 E Cherry Street Figure 3 14 Line B Survey from South to North 3 5 2 3 Line C 2 D Survey to the North West Corner Line C is close to N Merrill Ave The resistivity imaging was conducted with 2 ft electrode spacing and using the Dipole Dipole Array The survey was conducted Southeast to Northwest as shown in the Figure 3 15 The total length of the survey l
59. eoes 61 El ctrode Sy 0G Hise c ros a asec aed ps ed e a a es Ge sae he 62 Line A Survey from West to East sii einlecaitede inex oy Gees weceeeehl ods eta oe oes 63 Line B Survey from South to North eeeseessceceeseceeneeeeeeeeeeeneeeeeeeeeenaeeees 64 Line survey alone he channel octane sean seis a a ai aca eats 64 Resistivity Imaging LOCauOns ys iscessccadeasacavehasesteesd veda cntstassceasadesesesetarsecebieeioonk 65 Line D Survey from West to East scousiveccsaseneeguts arasdeenarseca ee otaes arate 66 Section A A Survey from West to East 0 00 ceeseccesseeeeseeeeseeeeeseeeeeneeeeeneeeees 67 Section A B Survey from West to Bast Jasco ssa ngeoanade ac anaaad amp 67 Section C C Survey from West to East cceescccssscccssncceeseecsseeeesseeeeeeneeees 68 Earth Imager 2D main window sci scs coycsevede se puteoee tea eakspurctucassteeden Steerer erears AZ RAW r sistivity Lata sissien pends achcsaad e e iir sata veccnensease sans Sa 74 Progress OF AIDVERSIONY fetes ietatie ulin eee ate ras UN aaa a adn abt 75 Data misfit Histogtam 5 csevercecnnvsessedgussuadan ctsesagece goisa wes bene tacaadetesds EE E 76 Inverted resistivity sectionis srren ienee i enctaresee sae gona EEEE ERST 77 xii 3 26 3 27 3 28 3 29 3 30 3 31 3 32 4 1 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9 4 10 4 11 4 12 4 13 4 14 Measured and apparent resistivity pSeUdOSECTIONS ecceeeeeceeseeeeneeeeeneers 11
60. equirements for the Degree of Masters of Science in Civil Engineering 118
61. erels 93 Inverted resistivity section for Section A B sseeseseeseserierererseresrersrrsresressesee 93 Apparent resistivity crossplot for Section A B seessseessseseesreerersrrrereererrrsee 94 Convergence curve for Section A B esssesssssssseresssesseessseessereseressessereseressees 95 a Resistivity Image of Section A A b X Slices of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image 96 xiii 4 15 4 16 4 17 4 18 4 19 4 20 4 21 4 22 4 23 4 24 4 25 4 26 4 27 4 28 4 29 a Resistivity Image of Section A B b X Slices of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image a Resistivity Image of Section C C b X Slices of Resistivity Image c Y Slices of Resistivity Image and d Z Slices of Resistivity Image Comparison of 2 D and 3 D resistivity values at Point A Locations of Point A Point B Point C oo ceecccccccccsessseeeesescscsseeeeeees Comparison of 2 D and 3 D resistivity values at Point B Comparison of 2 D and 3 D resistivity values at Point C Effective spacing for 3 D at Point Aveiccsc 5d tide ee es Effective spacing for 2 De sisie ionni e i a i ena tute ages i Resistivity and Moisture content graph for 3D 5 feet and B 1 oe Resistivity and Moisture content graph for 2D 2 feet and B 2 seese Resistivity and Moisture content graph for 2D 2 feet and B 3 oe Locations
62. he two cavities can be seen Thesr are the two smaller locally resistive zones at electrode positions 11 13 and 16 19 respectively Therefore this paper illustrates that in order to detect and map the known sinkholes in dolomitic areas RESTOM is a geophysical tool well suited NORTH g E s S RO SOUTH o oy 3 19 SF 2 Om 3 0 RELATIVELY RESISTIVE ZONES WITHIN GREATER CONDUCTIVE ZONE KNOWN SURFACE CAVINGS HIGHLY RESISTIVE ZONE SUSPECTED SUBSURFACE CAVITY RELATIVELY CONDUCTIVE WEATHERED ZONE DEVELOPING SINKHOLE SYSTEM 20 50m log Pa ELECTRODE POSITIONS 4 m INTERVALS Figure 2 17 RESTOM image resulting from the survey at Site Van Schoor 2002 2 6 2 Electrical resistivity imaging technique for investigation of Landslide In 2003 in the Soke district of Aydin located in the Aegean Region of Turkey a landslide occurred after a heavy rainfall To determine the landslide geometry a 2 D resistivity survey was carried out along three lines using a Wenner electrode configuration By using Wenner configuration data is acquired using a 30 electrode cable which is 5 m apart To process the data using RES2DINV a 2 D tomographic inversion technique was employed which is based on the smoothness constrained least squares inversion of pseudo section data Tripp et al 1984 deGroot Hedlin and Constable 1990 Sasaki 1992 Loke and Barker 1996 The smoothness constraint algorithm will result in a solution with smooth re
63. hiter color and greater induration Austin chalk formation is the most readily distinguishable of the several formations of the Upper Cretaceous which are mostly unconsolidated clays These facts make it a most important datum point in differentiating the strata of the Upper Cretaceous and are of special value in aiding in the determination of the depth of underground waters The formation consists of beds of impure chalk containing 85 per cent or more of carbonate of lime interstratified with softer beds of Marl It is usually of an earthy texture free from grit and when freshly exposed easily cut with a handsaw Under the microscope the material shows calcite crystals particles of amorphous calcite and the 50 shells of foraminifers mollusks echinoids and other marine organic debris such as usually constitute chalk formations The saturated subterranean portions of the rocks have a bluish color but the air dried indurated surfaces are glaring chalky white The air dried rock usually weathers in large conchoidal flakes with an earthy fracture Occasionally small nests of pyrites of iron occur in the fresh rock On weathered surfaces these are usually altered into round balls of marcasite an inch or less in diameter Streams of rust from these locally discolor the face of the rocks in places The Austin chalk was called magnesian limestone by some of the earlier writers and is still so called by some persons Only a small percentag
64. ich is much faster in the field than single channel instrument 53 Figure 3 6 Eight Channel SuperSting R8 3 5 1 1 Key Benefits The major benefits of SuperSting R8 Multi channel resistivity imaging system are e 8 channel simultaneous measure capability cuts field time dramatically e High power transmitter Can use both 12V and 24V batteries for added power e Field adapted rugged construction Built to last in real conditions e Easy to use menu driven system e The best accuracy and noise performance in the industry e Large capacity internal memory for storage of measurement results e User programmed measure cycles can be loaded into memory from a PC and later executed in the field e Directly controls the Multi Channel Swift Dual Mode Automatic Multi electrode system e Induced Polarization mode records 6 individual IP chargeability windows e Manual measurements are available via four banana pole screws on the top of the instrument for connecting current and potential electrodes Manual measurement array types include Resistance Schlumberger Wenner Dipole dipole Pole dipole and Pole pole 3 5 1 2 Multi channel Resistivity Imaging System The SuperSting is a memory earth resistivity instrument The controls the display and the connectors are all located on the front panel 54 Figure 3 7 SuperSting R8 IP front panel 1 Power connector 2 Fuse holder 3 O I OFF ON 4 Night light used to i
65. idential property Based on the drawings provided by the City presence of a backfilled swimming pool was located at the center of the property However city officials were concerned about the presence of possible contaminants at the site Currently the site located at 300 E Center Street is a vacant lot Figure 3 3 The City has recently completed demolition of all the old buildings and structures at the site The site is bounded by E Center Street on the south E Cherry Street on the north N Merrill Avenue on the east and a commercial property on the west Figure 3 3 Site location and view 49 The ground surface gradually slopes toward the northeast corner of the site where a water drain is located An inspection of the lined channel did not indicate evidence of surface contamination This is the broken portion of an underground pipe A pole mounted transformer is located on the site The transformer did not have a visible sign indicating the dielectric fluid was poly chlorinated biphenyl PCB free The transformer appeared in good condition without signs of leaking The northeast corner of the site is currently being used as fill soil and pipe line storage area 3 3 Site Geology The Duncanville E Center Development site is located within the Austin Chalk Formation In general the Austin Chalk Formation consists of chalk mostly micro granular calcite massive some interbeds and partings of calcareous clay Due to its w
66. ies with a geological approach had been conducted in the early eighteenth century the application of electrical resistivity methods came into wide use only after the advent of computing technology in the 1970 s Reynolds 1997 The modern survey technologies provide faster data collection and better data quality One of the advanced method examples is the Oklahoma State University proprietary Halihan Fenstemaker technique which can provide high resolution subsurface Electrical resistivity imaging images Halihan et al 2005 2 2 Electrical Resistivity Method Electrical resistivity is one of the oldest and most popular geophysical techniques in electrical exploration due to its ability to produce images of the subsurface efficiently and effectively as a result of the availability of automated data acquisition systems and efficient user friendly inversion software The first electrical resistivity survey was performed in 1920 s by the Schlumberger brothers Since then electrical resisitivity is one of the methods of primary interest in measuring the electrical resistive properties of earthen materials Electrical resistivity measurements are made by placing four electrodes in contact with the soil or rock A current is caused to flow in the earth between one pair of electrodes current electrodes while the voltage across the other pair of electrodes potential electrodes is measured Figure 2 1 Electrical resistivity is defined as
67. impacts with no known source 1 Drilling and sampling method provides a discontinuous subsurface picture and can be a very tedious and expensive procedure Zume et al 2006 That is the reason why an interest in applying nondestructive and noninvasive surface geophysical methods has increased Geophysical methods have the possibility to give an image of the subsurface as shown in Figure 1 1 Also with the development of new software for the interpretation of resistivity measurements 2D and 3D resistivity imaging or resistivity tomography is extensively used today in shallow geophysical investigation and especially for geohazard study thkhok Alr kd uo Ms Probabk caulty 200 soo 600 2000 20000 92000 Figure 1 1 High Resolution Resistivity Image Source www agiusa com Two commonly used geophysical methods are Ground Penetrating Radar GPR and High Resolution Resistivity HRR Figure 1 2 GPR is a powerful tool which makes possible to find the groundwater level distinguish different layers in a slope material and even the rupture surface and find cavities GPR uses high frequency electromagnetic waves between 10 to 1200 MHz which gives a resolution between 0 1 to 3 m Thus higher frequencies result in a higher resolution In optimum cases the penetration is about 40 60 m and can reach 300 m in some special cases like dry unweathered granite or ice However presence of clayey soils in DFW area makes it
68. ine was 112 feet E Cherry Street Figure 3 15 Line C survey along the channel 64 N Merrill Ave W v uono s g y UON S 391 S 19 U49 9 J v y F a Ll E a z a ee ee 1 pr 9 9 U0N S S Au YI J Figure 3 16 Resistivity Imaging Locations 65 3 5 2 4 Line D 2 D Survey to North East Corner Line D is close to S Hastings Street The resistivity imaging was conducted with 2 ft electrode spacing and using the Dipole Dipole Array The survey was conducted North to South as shown in the Figure 3 17 The total length of the survey line was 112 feet n E Cherry Street Figure 3 17 Line D Survey from West to East 3 5 2 5 Section A A 3 D Survey Section A A is close to E Center Street The Electrical Resistivity Imaging was conducted with 20 ft electrode spacing and using the Dipole Dipole Array The data recorded during the imaging was transferred to a computer Finally the Earth Imager 3D software was used to analyze the obtained data from the field 66 ECTION A A E Cherry E Center Figure 3 18 Section A A Survey from West to East 3 5 2 6 Section A B 3 D Survey Section A B is close to E Center Street and within section A A The Electrical Resistivity Imaging was conducted with 5 ft electrode spacing and using the Dipole Dipole Array so eee SECTION A B E Cherry Street E Center Street Figure 3 19 Section A B Survey from West to East 3 5 2 7 Sec
69. ing it into the soil with a geotechnical hammer 84 LNAWNIOTHAAT LAPALS WHLNAD F Figure 3 32 Boring locations Plan 85 CHAPTER 4 RESULTS AND DISCUSSION 4 1 Apparent Resistivity Imaging Results Figure 4 1 shows the apparent resistivity data for all the lines down to a maximum depth of about 15 7 meters When data is transferred to a computer from the SuperSting the results of resistivity measurement is transferred and stored in as sting files stg file The files stored contain the apparent resistivity data Reading the data successfully the software will display a measured apparent resistivity pseudosection for surface survey as shown in Figure 4 1 The apparent resistivity data emphasizes nearer surface features in its measurements Therefore this map is a reflection of the shallower subsurface materials generally residual soils fill or weathered bedrock It has been assumed that apparent resistivity data reflects primarily variations in the depth to bedrock but also variations in soil porosity moisture content and clay content The larger black squares on the surface show the electrode locations and are also editable 86 Depth m Depth m Depth m Depth m Ohm m 118 T4 46 7 29 3 18 4 Measured Apparent Resistivity Pseudosection LINEA 25 6 oy Ohm m ne K EPEE APE ee See RS TOE AERE ee ES OE 101 1 6 63 3 1 39 3 47 24 6 63 15 3 Measured Apparent
70. is the logarithm of measured apparent resistivty and vertical axis is the calculated apparent resistivty As most of the data points lie on the 45 degree line the calculated apparent resitivity fits corresponding measured apparent resistivty If all the points fall on the 45 degree line a perfect data fit with zero RMS error is achieved Apparent Resistivity Crossplot Calc 0 45 11 1 8 2 5 Measured Log10 AppRes Iteration No 4 RMS 8 8 L2 8 6 Figure 4 12 Apparent resistivity Crossplot for Section A B 94 Convergence graph is shown in Figure 4 13 The graph shows XY curve of root mean squared RMS error in percentage versus iteration number As seen RMS error for section A B in Figure 4 4 drops from 43 7 to 8 83 i e less than 9 implying a good fit Convergence Curve 2 iteration Number Figure 4 13 Convergence curve for Section A B All the 3D sections follow a similar trend The Inverted resistivity sections are modified from the measured apparent resistivity with dipole dipole array mode and inverted resistivity sections are shown below 95 Section A A Electrode Spacing 20 ft Possible Anomaly ae 14 2 0 0 0 0 Resistivity ohm m Inverted Resistivity ohm m 1 8 49 323 2113 i ch 49 323 a b Inverted Resistivity ohm m Inverted Resistivity ohm m 49 1 8 49 323 1 8 323 2113 c d Figure 4 14 a Resistivity Image of Section A A b X Sli
71. istivity results 5 feet electrode spacing in 3D gives better resolution when compared to 3 D 20 feet In case of 2D there is not much of difference between 2 feet and 5 feet electrode spacing Any one of them should produce good results e The resistivity results for 2 D and 3 D data using same electrode spacing provide very close results Therefore using 2 D with 5 feet spacing or 3 D 5 feet spacing should provide us similar results e A trend is observed between moisture content and resistivity results As the moisture content decreases the resistivity increases and vice versa e The three soil test borings confirmed the site profile that was observed during the resistivity imaging 112 REFERENCES Dahlin T 1993 On the automation of 2D resistivity surveying for engineering and environmental applications Doctoral Thesis ISRN LUTVDG TVDG 1007 SE ISBN 91 628 1032 4 Lund University 187pp Dahlin T Bernstone C 1997 A roll alongtechnique for 3D resistivity data acquisition with multi electrode arrays Proceedings SAGEEP 97 Reno Nevada Vol 2 March 23 26 1997 pp 927 935 Zume JT Tarhule A Christenson S 2006 Subsurface imaging of an abandoned solid waste landfill site in Norman Oklahoma Ground Water Monit Remediat 26 2 62 69 Khesin B Alexeyev V Eppelbaum L 1996 Interpretation of geophysical fields in complicated environments bookseries Modern approaches in geophysics
72. istivity value In addition EarthImager also shows a stack of four static slices in either X Y or Z orientation 81 Dynamic Slices of Inverted Resistivity 7 5 7 12 6 13 Y aa 64 0 32 x outed Recitinty ohn lo 42 ing 316 io Figure 3 31 Dynamic slices of resistivity 3 7 Subsurface Investigation by Soil Drilling A subsurface exploration program was conducted on October 22 2007 A total of three test borings labeled BH 1 through BH 3 were drilled by Apex Geoscience Inc under the supervision of a UTA field representative The drilling was performed using a truck mounted rig Test boring locations were approximately located in the field by the UTA representative relative to existing site features and are indicated on Figure 3 32 and Table 3 1 The locations were selected based on the resistivity imaging The resistivity imaging showed few locations with very high resistivity and was anomalous compared to the other sections of the sites Also resistivity imaging showed 82 possibility of presence of hard soil or rock materials at 2 ft to 15 ft below the ground surface Therefore to confirm materials present at the anomalous locations and site geology the soil test boring locations were decided and soil samples were collected from those locations Table 3 1 Site Investigations Program Ground Surface Penetration Boring Elevation ft Depth ft BH 1 0 0 15 BH 2 0 0 15 BH 3 0 0 15
73. ivity imaging technique to delineate groundwater aquifer Next example is in the Banting area of Malaysia to delineate groundwater aquifer and marine clay layer The study area consists of Quaternary deposits of Beruas Gula and Simpang Formations which overlie the sedimentary bedrock of Kenny Hill Formation The uppermost layer is Beruas Formation which mainly consists of clay sandy clay and peat of Holocene fluviatile estuarine deposits The underlying Gula Formation represents Holocene marine to estuarine sediments which mostly consist of clay and minor sand The Simpang Formation is a Pleistocene deposit comprised of gravel sand clay and silt The Kenny Hill Formation consists of a monotonous sequence of interbedded shales mudstones and sandstones A 2 D geoelectrical resistivity technique was used In this survey 61 electrodes were arranged in a straight line with constant spacing 10 m and connected to a multicore cable 41 101 27 101 28 101 29 101 30 101 31 101 32 101 33 E I I I I CT 2 49 x 2 49 _ x yo 2 48 JUGRA ga _ e TELUK DATUK 2 48 D Bh 5m Olak 2 Bh 2 Bh7 2 47 Poraa 24T Bh 8 E l 7 Olak 3 2 46 2 46 2 45 w 2 45 3 Km oa Km j ia I J T 101 27 101 28 101 29 101 30 101 31 101 32 101 33 Key E Borehole Imaging survey lines Main road Town Figure 2 20 Location of survey lines Hamzah 2
74. ll progress is shown by the first progress bar to the left and the progress within one iteration is shown by the second one to the right Iteration No 308 DAANAN Iiii Figure 3 23 Progress of inversion 3 6 2 5 Removal of poorly misfit data At the end of inversion a data misfit histogram can be displayed by choosing View Convergence and Data Misfit Data Misfit Histogram Figure 3 24 The horizontal axis shows the absolute value of the relative data misfit that is defined as the ratio the difference between the calculated and measured data to the measured data apparent resistivity In general any data with a relative data misfit greater than 50 may be considered as a poorly fit data and should be removed The user should also take the error distribution into consideration In the Figure 3 24 below 36 can be used as the removal 75 threshold that is represented with a vertical blue line The data with a misfit larger than the threshold will be removed after clicking the Remove button To set a removal threshold any of the arrow keys Left Right Up and Down may be used The Left and Down arrow keys will decrease the threshold The status bar at the bottom shows the number of data to be removed and total number of data points To remove poorly fit data an appropriate misfit threshold should be set and Remove button is clicked and then Inversion is started again If the Cancel button is clicked the threshold selection
75. lluminate the display during poor light conditions 5 Connector for optional external transmitter 6 Indicator light for main and booster mode 7 Connectors for communication with optional external transmitter also used for firmware upload 8 Liquid crystal display LCD window with 16 text lines of 30 characters each 9 Keyboard 10 Connectors for Swift cable 1 and 2 or to connect one or two switch boxes 11 Positive and negative current terminals A and B For use with banana connectors or stripped wire 12 Positive and negative potential terminals M and N For use with banana connectors or stripped wire 13 Test terminal for use with banana connector or stripped wire 14 Connector used for serial communication with a PC used for data download and command file upload 15 On SuperSting R8 IP connector for future development 3 5 1 3 Power connector One or two external 12 V DC batteries power the instrument The batteries are connected to the instrument front panel connector by the special power cable delivered with the instrument The power cable has a pig tail connector for the boost battery 55 Main mode the instrument is powered by one 12 V battery and the instrument operates in the range 0 100 Watt the Main indicator light is on Boost mode the instrument is powered by two 12 V batteries and the instrument operates in the range 0 200 Watt the Main and Boost indicator light is on Office mode when the inst
76. maintained Soil samples were generally taken at 2 foot depth intervals in each boring All samples were transported to the laboratory for further review and laboratory testing The subsurface conditions as interpreted from the field investigation program indicate a subsurface profile consisting of a layer of Miscellaneous Fill underlain by weathered and disintegrated rock formation The Miscellaneous Fill layer as described below overlies the entire site and extends as much as 5 ft in borehole BH 3 below the ground surface Weathered rock consists mainly of dense sands The weathered rock is underlain by very hard Limestone The recent test boring data indicates the following generalized soils strata underlie the site to the depths investigated at soil boring locations The strata designations do not imply continuity of materials described but reflect the general description of the subsurface materials at the site table 4 1 Table 4 1 General descriptions of each stratum Stratum A From the ground surface to Brown clay and sand FILL Fill Soil the depths of 2 5 ft to 5 ft generally loose to dense sand N 5 to 15 3 Stratum B Below Stratum A to depths Dense to very dense brown Disintegrated Weathered Rock of about 14 5 feet and light brown colored LIMESTONE material N 19 to 30 2 Stratum C Hard Soil or Rock Below Stratum A and Stratum B to the maximum depth of boring at 15 feet
77. mogeneous in the upper part not shown here The low resistivity zone coming in from the west seen at lower depths in all the sections is due to the saltwater front from the near by Oresund A number of low resistive zones corresponding to sludge ponds are clearly visible in the sections within the sludge deposit exemplified in Figure 2 22 2 6 5 Application of 3D electrical resistivity imaging in an underground potash mine The study area is located at the mine level within the Prairie Evaporite and is shown in Figure 2 23 The area had previously experienced a water inflow and mitigation efforts had been undertaken However mine personnel were interested in ascertaining whether the water entering the evaporate formation in the area is descending 44 down from the Second Red Bed and if so the location of descent or if the water is traveling horizontally through the evaporite prior to arriving in the area An active underground mine presents a difficult environment to collect high quality electrical resistivity measurements In a 2D survey a pseudo section can be created which encapsulates the spatial relationship between the data and which can also be used to visually gauge the coherency of the data The 3D acquisition geometry here does not lend itself to the creation of pseudo sections Electrical measurements were collected using an Iris Syscal resistivity meter with 96 electrodes The often complex yet limited distribution of drifts i
78. n an underground mine poses a difficulty for designing a geophysical survey which has the ability to both detect and resolve resistivity variations in 3D There is often very little geometry available for deploying electrodes and the available geometry is deficient Applying standard suites of resistivity arrays has proven to be insufficient for resolving 3D models in these environments and a more involved survey design process is required Legend electrode location raise drift sub drift 50m Figure 2 23 Geometry of the underground mine drifts of the study area Eso and Oldenburg 2006 45 3D ERI survey techniques in a challenging underground mining environment were employed The resulting resistivity models show the distribution of conductive wet salt in the area and a known void was successfully imaged O D 0 5 5 0 50 0 Resistivity Qm Figure 2 24 Planview resistivity section of the recovered model taken at an elevation of 10 meters above the back of the mine The detected void is circled in red Eso and Oldenburg 2006 46 CHAPTER 3 FIELD STUDIES 3 1 Site Description Duncanville is a city in Dallas County Texas USA Duncanville is a suburb of Dallas and is part of the Best Southwest area The city of Duncanville Figure 3 1 is located just minutes from downtown Dallas and 20 minutes from Fort Worth The site is located at 300 E Center Street Duncanville Texa
79. n obtained by other geophysical method For instance seismic methods can map undulating interfaces effectively but would have difficulty in mapping discrete bodies such as boulders cavities and pollution plumes Ground radar surveys can provide more detailed pictures but have very limited depth penetration in areas with conductive 33 unconsolidated sediments such as clayey soils A conjunction of two dimensional electrical surveys and seismic or GPR surveys provide comprehensive and complementary information about the subsurface 2 5 1 1 2 D Slide along and Roll along Technique One technique used to extend horizontally the area covered by the survey particularly for a system with a limited number of electrodes is the roll along method After completing the sequence of measurements the cable is moved past one end of the line by several unit electrode spacings All the measurements which involve the electrodes on part of the cable which do not overlap the original end of the survey line are repeated Figure 2 12 Figure 2 12 The use of the roll along method to extend the area covered by a survey In slide along technique Figure 2 13 the entire cables are moved forward Subsections B and C are sampled with the same command file as that of subsection A zones and 2 are sampled unnecessarily Zones 3 and 4 are missing a large number of data points A roll along survey is faster than side along survey and has better model resolution at the
80. n the horizontal extent of the intended survey Conducting roll along surveys extend the length of these surveys A roll along survey begins by collecting a standard data set A number of electrodes for example 14 are then moved from the beginning of the survey line to the end of the survey line without moving any of the other electrodes Data is again collected in what would be a spatially overlapping data set however a special command file avoids collecting redundant data This hop scotching of electrodes from the front of the survey line to the end can be repeated any number of times and therefore extend a survey line to any length 3 6 2 Data Inverion Computer program Earth Imager 2D and Earth Imager 3D Advanced Geosciences Inc 2006 was used to invert the apparent resistivity data Earth Imager 2D and Earth Imager 3D are large and complex programs with many user modifiable inversion parameters The software manual provides a detailed explanation of each parameter and its influence on their inversion process Resistivity surveys measure injected current I through transmitting electrodes and potential difference voltage V between two receiving electrodes Measured current and voltage together with electrode geometry K may be converted into apparent resistivity Normalized voltage by current V I and p a data are equivalent quantities than can be transformed back and forth with the help of a geometric factor K 69
81. nder the supervision of a UTA field representative Three soil test borings were drilled in the site 1 To determine the presence of any contaminants 2 To confirm and verify the actual subsurface information The results obtained from resistivity imaging and three soil test borings were compared Based on the resistivity imaging and soil test boring no contaminants were found at the site The subsurface profile developed from resistivity imaging was verified and confirmed by all the three soil test borings iv TABLE OF CONTENTS ACKNOWLEDGEMENTS asscssisszivssseitccedsdnaquepeatsvensabesssny an tendvan teu vaaneees ii ABSTRACT aiis rann A aa Saab edt esteem aaa ume aie iii LIST OF ILLUS FRATIONS reeet oaea apaa e E rE S x LIST OORT ABLES oaran cons tawauies nae staan dessin coxa dase N sree coy A E EAA oA outs XV Chapter 1 INTRODUCTION orere che Se Rodel Pind dan Mn pee E vale shed trae SEG 1 1 1 Background and Importance soesseeesseeseeesseeesseeeseeessessereseeesseresseesseess 1 1 2 Objective and SCPE seroren i a dente peomsieoeeaet ovens 4 T3 COTO AINA e 85 Gxt aaa den E EEE E ica oes ese Se nena 5 2 EITERATURE REVIEW treenerite noniin ar E A e a A E ore 6 2 1 Electrical Resistivity Development s sesssessersesessrseseerersrrsresrersresreesessre 6 2 2 Electrical Resistivity Method siiciscigscisseccaes they sanescies cdot cans cabs vevevaea ressdenseces 7 Dit MIME ORY Scenes alice dota dere E Poets a E ae a
82. ng the electrodes with a constant spacing as shown in Figure 2 5 The n factor for this array is the ratio of the distance between the C1 P1 or P2 C2 electrodes and the spacing between the P1 P2 potential pair The sensitivity patterns of Schlumberger array and Wenner array vary slightly The Schlumberger array is moderately sensitive to both horizontal and vertical structures which has a slight vertical curvature below the centre of the array and also has slightly lower sensitivity values in the regions between the Cl and Pl C2 and P2 electrodes Hence the Schlumberger array might be a good compromise between the Wenner and the dipole dipole array in the areas where both horizontal and vertical structures are expected The signal strength of Wenner array is high and the signal strength of Schlumberger array is between Wenner array and dipole dipole array When compared to the Wenner array the median depth of investigation with the same distance between the outer C1 and C2 electrodes is about 10 larger for Schlumberger array 2 4 1 3 Dipole dipole Method The dipole dipole array is logistically the most convenient in the field especially for the work over large areas and larger spacing s between the electrodes Dipole dipole provides the highest resolution and is most sensitive to vertical resistivity boundaries Griffiths and Barker 1993 However the data collected from dipole dipole array are 22 easily affected by near surfac
83. ns of the data is often more diagnostic These spatial variations are boundaries from which inference can be made regarding the distribution of the stratigraphy or sediment faces These changes are inferred as geologic boundaries that may actually be caused by one or more of the following a change in lithology sediment grain size or chemical boundaries resulting from changes in water chemistry or moisture content In all instances the user must determine if the results are possible given the existing knowledge of site conditions 13 It is important therefore to have conceptual ideas of the geologic setting and the effects it may have on the geophysical signature for a more accurate interpretation The electric current is commonly conducted through rocks and sediments by the pore fluids clay minerals and metallic minerals in the rocks and sediments Reynolds 1997 Schwartz 2003 Because properties of same rock sediment types e g grain size porosity permeability and clay mineral contents can vary from locations to locations the resistivity range of different materials are shown in Table 2 1 14 Table 2 1 Electrical resistivity range of minerals rocks and fluids Telford et al 1990 Klein ans santamarina 1997 Borcea 2002 Rock or fluid Marine sand shale Terrestrical sands claystone Volcanic rocks basalt Limestone dolomite anhydrite Chloride water from oil fields Sulfate water from oil fields Quartz Quartz sand
84. ous medium where C and C are the locations of the current Source respectively Telford et al 1990 In field surveys the resistivity of geological properties is determined by measuring the electrical potential difference between electrodes thus apparent resistivity can be calculated from the measured voltage drop V the electric current induced into the ground J and the geometric factor K which depends on the electrode configuration type Schwartz 2003 Equation 2 3 8 12 Equation 2 3 8 pa a By the superposition of the influence of the two current electrodes at each voltage electrode location the apparent resistivity of the medium through which the current has traveled pa is calculated by the following equation where AV is the measured voltage 7 is the input current 27VV 1 pP ze Se ees ee rl r2 r3 r4 Equation 2 3 9 All the geological materials through which current travels alter the resistivity measurements Therefore the resistivity that we get is inaccurate hence the term apparent resistivity In most cases today multi channel systems that have many more than four electrodes are used so that a large dataset can be collected without moving any electrodes The resulting data a pseudosection of the apparent resistivities below the survey can be generated Although the magnitude of resistivity and cause is important the information obtained from the lateral and vertical changes 1 e spatial distributio
85. r which the data acquisition software automatically checks the electrode contact and scans through a pre defined measurement protocol Extension of the line is achieved through a roll along technique in which part of the layout is shifted for example by quarter of the total layout length and new measurements are added Resistivity sounding method has a major limitation because it does not take into account horizontal changes in the subsurface resistivity A relatively accurate model is a two dimensional 2 D model as it changes the resistivity in the vertical as well as in the horizontal direction In this case it is assumed that resistivity does not change in the direction that is perpendicular to the survey line This would be a reasonable assumption especially for surveys over elongated bodies Penetration model should be even more accurate but at the present time 2 D surveys are economically practical compromise between obtaining very accurate results and keeping the survey costs down One of the new innovations is the mapping of areas with moderately complex geology using 2 D electrical imaging tomography Griffiths and Barker 1993 Usually such surveys are usually carried out with 25 or more electrodes which are connected to a multi core cable A laptop microcomputer along with an electronic switching unit is used to automatically select the relevant four electrodes for each measurement 32 Currently field techniques and equipment used
86. rays for 2 D imaging surveys are i Wenner Schlumberger 11 Schlumberger iii pole pole iv pole dipole and v dipole dipole 2 4 1 1 Wenner Method This is a robust array type which was popularized by the pioneering work carried by the University of Birmingham research group Griffiths and Turnbull 1985 Griffiths Turnbull and Olayinka 1990 The Wenner array maintains a constant spacing between the electrodes as shown in Figure 2 5 It is best suited for profiling because only one electrode is moved for measurements Beard and Morgan 1991 found that the expanding electrodes of the Wenner array create false analomous zones that complicate interpretation Figure 2 4 b shows that the sensitivity plot for the Wenner array has almost horizontal contours beneath the center of the array So the Wenner array is 20 relatively more sensitive to vertical changes than to horizontal changes in subsurface resistivity Furthermore the Wenner array has a moderate depth of investigation compared to the other arrays As seen in Table 2 2 the median depth of investigation for Wenner array is approximately 0 5 times the spacing used The geometric factor for Wenner array is 2ra The geometric factor is smaller when compared to all other arrays Wenner array has the strongest signal strength among all the arrays because the potential electrodes are placed between the two current electrodes The disadvantage of this array for 2 D imaging surveys is it
87. rodes are placed at infinity this depth reaches 90 of the total length or 220m Table 2 2 gives the median depth of investigation for the different arrays In layman s terms the upper section of the earth which is above the median depth of investigation and the lower section of the earth have the same influence on the measured potential and this tells us roughly about the depth we can see with an array This depth is not dependent on the measured apparent resistivity or the resistivity of the homogeneous earth model The depth models are probably good enough for planning field surveys even it is strictly valid only for a homogeneous earth model The actual depth of investigation may be a bit different if there are large resistivity contrasts near the surface 19 Table 2 2 The median depth of investigation z e for the different arrays L is the total length of the array Edwards 1977 Array type Z e a Z e i Wenner alpha D STS 0 173 Dipole dipole lt 2 0 416 0 139 n 2 0 697 0 174 a 0 962 0 192 n 4 1 220 0 203 n 5 1 476 0 211 n 6 1 730 0 216 Equatorial dipole dipole n 0 451 0 319 n 2 0 809 0 362 m 1 180 O 373 n 4 1 556 O 377 Wenner Schlumberger n l 0 52 0 173 m 2 0 93 0 186 n 3 1 32 0 189 n 4 a Eas a 0 190 n s5 2 09 0 190 n 6 2 48 0 190 Pole dipole w i 0 52 m z2 0 93 m 1 32 n a4 1 71 n s5 2 09 n 6 2 48 Pole Pole 0 867 2 4 1 Two Dimensional Imaging Methods The most commonly used ar
88. rument is used in an office environment it is powered by a Power Supply delivered with the instrument The Power Supply is connected to the mains 100 250 V AC at 50 60 Hz input power and the front panel connector marked Power To turn the instrument on in this mode use the ON OFF switch on the Power Supply Note that the front panel ON OFF switch will not work when the instrument is powered by the Power Supply In this mode the processor is working but the transmitter is unable to transmit any current and the instrument is therefore not able to take any readings Downloading of data uploading of command files and instrument firmware is possible in this mode 3 5 1 4 Fuse holder The main fuse holder is located on the front panel of the instrument The fuse is a 20 Amp 32 V 1 x 4 fuse 3 5 1 5 ON OFF When the battery is attached turn the instrument on by turning the I O On Off switch to the I On position The following message will be displayed Advanced Geosciences Inc SuperSting Hardware initializing 56 After a short moment the main menu will replace the text The instrument can be turned off at any time and will keep its current settings until it is turned on again Data in the memory will not be lost when the instrument is shut off or disconnected from the battery Note that the instrument in order to save battery as default will automatically shut off if no keyboard action has been detected for 5 minutes The autom
89. s C2 and P2 must be placed at a distance that is more than 20 times the maximum separation between C1 and P1 electrodes used in the survey The effect of the C2 and similarly for the P2 electrode is approximately proportional to the ratio of the C1 P1 distance to the C2 P1 distance If the effects of the C2 and P2 electrodes are not taken into account the distance of these electrodes from the survey line must be at least 20 times the largest C1 P1 spacing used to ensure that the error is less than 5 In surveys where the inter electrode spacing along the survey line is more than a few meters there might be practical problems in finding suitable locations for the C2 and P2 electrodes to satisfy this requirement Another disadvantage of this array is that because of the large distance between the P1 and P2 electrodes it is can pick up a large amount of telluric noise that can severely degrade the quality of the measurements Thus this array is mainly used in surveys where relatively small electrode spacings less than a few meters are used It is popular in some applications such as archaeological surveys where small electrode spacings are used It has also been used for 3 D surveys Li and Oldenburg 1992 This array has the widest horizontal coverage and the deepest depth of investigation However it has the poorest resolution which is reflected by the comparatively large spacing 26 Various types of electrode combinations can be used such
90. s Figure 3 1 presents a general site vicinity map The site is located approximately three hundred fifty 350 feet east of the intersection of N Main Street and E Center Street JI City of Duncanville Figure 3 1 An overview aerial photo showing the city of Duncanville Courtesy of Google Earth 2006 47 The site is located in a low density residential and retail area The surrounding area is predominantly residential with retail shops located along E Center Street The site is comprised of a green field covered by small grasses There are several trees and a channel line The channel is dried out However broken drainage pipe passes under the site Figure 3 2 A General Site Vicinity Map The surrounding area is sparsely developed with retail and residential To the East N Merrill Avenue is located along the eastern side of the subject property The area is sparsely developed with single family homes surrounded by trees South Trees line the property with some single family homes adjoining the subject site E Center Street runs along this side West Two two story buildings are located beyond a parking lot 48 North E Cherry Street is at the north side Small residential subdivisions are located beyond the site 3 2 History of the Site and Existing Site Conditions The site was previously used as both commercial and residential property However the majority of the land was used as res
91. s of an array that are to be considered Depth of investigation is an important parameter in any resistivity survey Depth of investigation depends on the type of instrument electrode spacing and separation factor property contrast body geometry data coverage and signal to noise ratio Loke 2004 The sensitivity function basically tells us the degree to which a change in the resistivity of a section of the subsurface will influence the potential measured by the array The higher the value of the sensitivity function the greater is the influence of the subsurface region on the measurement Loke 2004 The sensitivity values are found higher near the electrodes for all the arrays In electrical methods the depth of penetration is linked to the distance between electrodes For a set of 48 electrodes spaced at 5m two segments with 24 electrodes each are usually used the resistivity meter being placed between both segments the total length of the cables is 240m In a first approximation for Dipole Dipole Schlumberger 18 and Wenner arrays the maximum depth of penetration is of the order of 20 of the total length of the cable or 50m in the present case This depth is reached for the combination of the two extreme left and the two extreme right electrodes of the profile and the measuring report plotting point corresponds to the bottom angle of the triangle of the pseudo section For a Pole Pole array where one current and one potential elect
92. sistivity changes In this algorithm the subsurface is divided into rectangular blocks and each block has constant resistivity 39 Using iterative Scheme by minimizing the difference between observed and calculated pseudo sections the resistivity of each block is evaluated The calculated pseudo sections can be obtained by either finite difference or finite element methods Coggon 1971 Dey and Morrison 1979 Figure 2 18 Study area after the landslide Drahor 2006 The slope forming material was saturated as the survey was performed right after a heavy rain This condition affected the resistivities in the subsurface and lower apparent dataset resistivities are obtained For the determination of the failure surface and presence of a fault in the Soke landslide area the resistivity data is inverted According to ERT result in Line 3 it is in rotational form and the depth to failure surface varies between 5 and 15 m The borehole results added to the confirmation of the presence of a fault in the site The water saturated zones which are indicated by low resistivities were identified Moreover the ERT was successful in detection of the consolidated and unconsolidated geologic units which might have potential for future landslides 40 LINE 3 oe m a l a a a C l a l FF Fl 7 10 15 22 32 47 69 100 Resistivity in Q m Figure 2 19 3 D fence diagram of the resistivity sections Drahor 2006 2 6 3 Electrical resist
93. surface investigation test results Chapter 5 includes a summary of accomplished work and the main conclusions CHAPTER 2 LITERATURE REVIEW 2 1 Electrical Resistivity Development Electrical resistivity studies have evolved and been employed to study electrical properties of rocks and materials in the subsurface One of the earliest recorded electrical conductivity studies in geology related fields have been performed by Gray and Wheeler in 1720 and Watson in 1746 Jakosky 1950 Van Nostrand and Cook 1966 Gray and Wheeler measured electrical conductivities of various rocks and Watson ascertained that the ground conducts electricity Van Nostrand and Cook 1966 The next recorded series of earth electrical property studies have been done by Robert W Fox in 1830 Van Nostrand and Cook 1966 He discovered the existence of natural electrical currents within sulfide ore deposits and successfully measured the electrical current between his copper plate electrodes Van Nostrand and Cook 1966 Several advanced experiments based on Fox s discovery had been performed by many scientists since Fox s first discovery However the early electrical conductivity and resistivity studies did not deal with quantitative approaches The first successful experiments of application of direct currents to measure earth resistivity were conducted by Conrad Schlumberger in 1920 Van Nostrand and Cook 1966 Although the electrical resistivity conductivity stud
94. the raw resistivity data 73 EJ AGI EarthImager 2D C Program Files AGI EarthImager 2D demo cave stg File Edit Settings Inversion View Tools Help elelo Slk m cave stg 55 69 Measured Apparent Resistivity Pseudosection Number of Data 172 Number of Electrodes 28 Figure 3 22 Raw resitivity data 3 6 2 3 Data file is edited to remove noisy data After loading the data file data editing statistics may be checked by choosing the menu item Edit Editing Statistics The number of data flagged for removal is counted based on the Criteria for Data Removal in the Initial Settings Window Any edits from the Electrode Editor or manual editing will be added to this number But any change in the Criteria for Data Removal will reset this number There are three ways to identify and remove noisy data In the first approach some possibly bad electrodes would be singled out for removal The second approach is based on the initial data trimming threshold on the Initial Settings window The noisy 74 data will be automatically removed based on the threshold In the last approach noisy data would be identified and removed by the user manually with the mouse clicks and Delete key 3 6 2 4 Inversion is run Inversion can be started by either clicking the green tool button or choosing the menu Inversion Start Inversion At the bottom of the window there are two progress bars showing the progress of the inversion Figure 3 23 The overa
95. tion C C 3 D Survey Section C C is close to E Cherry Street The Electrical Resistivity Imaging was conducted with 5 ft electrode spacing and using the Dipole Dipole Array 67 SECTION C C E Cherry E Center Figure 3 20 Section C C Survey from West to East 3 6 Data Acquisition and Data Inversion 3 6 1 Data Acquisition The research project used a 56 electrode AGI Sting Swift R8 Earth Resistivity Meter to collect the apparent resistivity data The 56 electrodes are on four inter connectable electrode cables with 14 electrodes each The electrodes are separated by 20 feet of cable For each surveys with smaller spacing s the excess cable is simply laid out to one side of the survey line The AGI system uses smart electrodes Each smart electrode can be passive or act as the A B M or N electrode for a resistivity measurement The smart electrodes are controlled by a user modifiable command file on the String The electrodes make electrical contact with the ground by being connected to a metal spike that has been driven into the ground Laying the electrode on a shelf a 10 cm length of angle iron welded across the stake provides the electrical connection 68 between the electrode and the stake The electrode is held on the shelf by an elastic band Sometimes the compromise between depth of investigation and resolution produced an overall electrode line length that was shorter tha
96. tivity Imaging Results 000 0 ee eee eeseeeeeeeereeeneeeneeenseeeees 86 4 2 Inverted resistivity section results for 2 D eee ee eeeeeeseeeeeeeeeeeeneeeees 88 4 3 Inverted resistivity section results for 3 D eee eeeeeeeseeeeeeeeeeeeeeeneeeees 92 4 4 Comparison between 2 D and 3 D resistivity values with depth 100 4 5 Effect of electrode spacing sssesssessessseeesseeeseessrtsseressetssersseresseesseeesee 103 4 6 Drilling Results s senene a E lig i eee tabi 105 4 7 Relation between Resistivity with Moisture Content sseeeeeeeeeeereeees 106 4 8 Comparison between resistivity and drilling sssssseseseessereseresssesseresee 108 Ds CONCLUSIONS eepe ea a tars a a Aa EIES 112 viii REFERENCES src accuses sa raira a aTe oct a edut vai Ta a see TE atone 113 BIOGRAPHICAL INFORMATION cciscsseccsscescsscees pcnsedssensesdeasonssscostsoubevnbsoensdecet ce 118 1X Figure 1 1 1 2 2 1 22 2 3 2 4 2 5 2 6 21 2 8 29 2 10 2 11 LIST OF ILLUSTRATIONS Page High Resolution Resistivity Image Source Www agiUSa COM ss cc 2 High Resolution Resistivity Equipment s seseessseseeseessssreereserssressrseresreserssresee 3 Principle of resistivity measurement modified from Robinson and Coruh 1988 ccccccsssscccccceeesesssnsseeeeeeees 8 Basic definition of resistivity across a homogeneous block of side L and area A with an applied current I and potential drop V Reynolds
97. trodes along a line and requires data to be recorded with various electrode separations along a line A dense data is important to cover laterally Griffiths and Barker 1993 Dahlin and Loke 1998 It means that in terms of electrode separations it is a complex recovery of structures in the ground To be practical it would demand the use of automated multi electrode data acquisition systems 1 Coble a A F ay 2 3 4 First spread AAAAARAAAA MALARAS j rr t 4 4 t Second spread HHHH tetere 3 4 at i Third spread elc Station Station 2 Station 3 Figure 2 11 Outline sketch of computer controlled data collection system Dahlin 2001 A Computer controlled data acquisition system Figure 2 11 in general consists of a resistivity instrument a relay switching unit a computer electrode cables various connectors and electrodes Overmeeren and Ritsema 1988 Griffiths et al 1990 Dahlin 1993 For example two or more components exist in the same box e g a computer 31 integrated with the instrument A few systems employ intelligent switches at each electrode take out instead of a central switching unit Usually the number of electrode channels of the switching device would range from around 25 to 64 In which the electrode take out spacing varies from less than 1 to 25m or more This depends on the application itself In field survey the electrode cables are rolled out and electrodes are connected to it afte
98. ts of the study area Eso and Oldenburg 2006 yc cciscceg dass eceyscasenadcdsn thas sasaacenadgoennesidbsrevevdsatenedeveeendea 45 Plan view resistivity section of the recovered model taken at an elevation of 10 meters above the back of the mine The detected void is circled in red Esoand Old nb rg 2000 ssnin ea e E EE EENES 46 An overview aerial photo showing the city of Duncanville Courtesy of G gle Earth 2000 prerese a r R E N AET 47 Fe General Site Vicinity Map aerea a E EESE 48 Site locaton and View aronet nra eiia e EEE A EE E Nieves cowie 48 xi 3 4 3 5 3 6 3 7 3 8 3 9 3 10 3 12 3 13 3 14 3 15 3 16 3 17 3 18 3 19 3 20 3 21 3 22 3 23 3 24 3 25 SuperSting R8 IP Multi channel Resistivity Imaging System ee 51 Single channel SuperSting RRL cc syesiscsgacs pues aeeeecedesceussovassedotaccueq aes cosanetecsueremes 52 Eight Channel SuperSting R8 ssssssesesesesesessesssseessersssressesseeesseesseresseesseeesee 53 SUperoune R3 IP front panel ennusa ena a a a 54 The Switch Box and Amphenol connector cceesceeesseeesteceeseeeeeneeeeenneeees 58 SuperSting Switch box connection ssesesesessssssesssseesseressresseesseressersseresseese 59 Cable Sons sb ose denis oh ose ates ee vai ca ayes ected eea aE i ae inserat oastei tiasa rikta si deis 60 Electrode stake and electrode switch 0 eee ese eseeceseeesceceeecsseceseetsseseseern
99. umber of Electrodes 28 Elapsed Time 0 0 3 957 Length in meters Resistivity Figure 3 26 Measured and apparent resistivity pseudosections 77 Convergence curve is submenu item under Convergence and Data Misfit and it shows an XY chart of root mean squared RMS error in percentage versus iteration number Convergence Curve of Inversion g 29 E 157 a 10 4 eee BaT I 2 3 4 5 Iteration Number Figure 3 27 Convergence curve Data misfit cross plot is a submenu item under Convergence and Data Misfit A crossplot is another good way to show data misfit If a calculated apparent resistivity fits corresponding measured apparent exactly the data point must lie on the green line The horizontal axis is the logarithm of measured apparent resistivity and the vertical axis is the logarithm of calculated or modeled apparent resistivity Because the logarithm of a negative number is undefined an absolute value is then used A square of four colors at the lower right corner of the image is the legend showing the signs of measured and calculated apparent resistivities For example if both measured and calculated data are positive the data point is black If both are negative the data point is green 78 Crossplot of Measured vs Predicted Apparent Res Data 7 SEH 3 3 9E 02 2 0E 01 Predicted Apparent Resistivity 1 0E 00 5 4E 02 5 4E 02 1 0E 00 2 0E 01 3 9E 2 7 5E 03 Measured Apparent Resistivity ohm m Iteration 8 RMS
100. urvey Depth of investigation depends on the type of instrument electrode spacing and separation factor property contrast body geometry data coverage and signal to noise ratio Loke 2004 The sensitivity function basically tells us the degree to which a change in the resistivity of a section of the subsurface will influence the potential measured by the array The higher the value of the sensitivity function the greater is the influence of the subsurface region on the measurement Loke 2004 The sensitivity values are found higher near the electrodes for all the arrays Figure 2 4 shows the sensitivity pattern for different arrays The contour patterns are different for the different arrays at larger distances from the electrodes 16 Pole pole array 2D sensit y function plot ca Pa Deptn 3 0 2 5 2 0 41 5 1 0 0 5 sti o s 1 0 1 5 2 0 2 5 3 0 1 04 1 0 1 54 1 5 z a 2 8 mre n e co 2 00 aca eoa Mone mm pm mm p PSP td ase otron Dipole array parameters a 1 8 Sensitivitu value x 188 Plotting Point Wenner array 2D sensitivity tunction plot Pa ca c2 P2 Dentn 3 0 2 5 2 0 1 5 1 0 0 5 o o 0 5 1 0 1 5 2 0 2 5 3 0 ee 1 0 2 0 2 5 eS SS ee E i 128 22 0 3 060 2 00 2 00 2 08 32 0 12e a eine tate igr AA ea Array parameters a Sensitivitu value x 188 a UM EMEN Wonnor Schlumberger array 2D sensitivity function plot pa fa cz p2 Depth 4 5 4 0 3 5 3 0 2 5 2
101. urvey with 20 feet electrode spacing at point A The results shows the quality of images of the known layer does show some degradation to practical significance as we increased the spacing by a factor of 4 and also the data collection time increases with spacing nearly exponentially Figure 4 22 shows the resistivity depth relationship for 2 D survey with 2 feet and 5 feet electrode spacing The hand results did not vary significantly when 2 D imaging was done using 5 feet and 2 feet spacing 103 Effective Spacing for 3d Resistivity Ohm m 0 20 40 60 80 0 t 05 1 p 3D 5 Feet l 1 5 3D 20 Feet 2 ft b a E25 a 3 NX N 3 5 N 4 N N 4 5 5 Figure 4 21 Effective spacing for 3 D at point A Feddiuity hm m 0 20 40 60 80 100 2 D5 fet l s 2 D2 fet a m Depth m 34 4 44 45 Figure 4 22 Effective spacing for 2 D Based on the results it can be summarized that closer the electrode spacing the better is the resolution Therefore 5 feet electrode spacing in 3D gives better resolution when compared to 20 feet In case of 2D there is not much of difference between 2 feet and 5 feet electrode spacing Any one of them should produce good results 104 4 6 Drilling Results During drilling collected soils were visually classified and soil borings were
102. usly used as both commercial and residential property The majority of the land was used as residential property City officials were concerned about the presence of any contaminants at the site 1 2 Objective and Scope The objective of the current research is to investigate the utilization of High Resolution Resistivity HRR in site investigations The Specific research objectives are 4 e To identify the presence of any possible contaminants at the site e To calibrate the equipment for site investigations e To investigate the applications of HRR for the site investigations e To compare 2 D and 3 D imaging methods e To determine the effective spacing between the electrodes e To investigate the site conditions using soil drilling method and finally to compare the resistivity and soil drilling results 1 3 Organization A brief summary of the chapters included in this thesis document is presented in the following paragraphs Chapter 2 presents the concept of electrical resistivity imaging and a brief review of available array methods in electrical resistivity imaging This chapter also includes a literature review of previous studies reported in the literature Chapter 3 is devoted to field studies which describe the SuperSting R8 IP Multi channel Resistivity Imaging System survey design and drilling Chapter 4 describes comprehensive analysis of all electrical resistivity test results This chapter also includes the sub
103. vol 14 Kluwer Dordrecht Loke MH Barker RD 1996a Practical techniques for 3D resistivity surveys and data inversion Geophys Prospect 44 499 523 Loke MH Barker RD 1996b Rapid least squares inversion of apparent resistivity pseudosections by a quasi Newton method Geophys Prospect 44 131 152 Dahlin T 2001 The development of DC resistivity imaging techniques Comput Geosci 27 1019 1029 113 10 11 12 13 14 15 Guerin R Munoz ML Christophe A Laperrelle C Hidra M Drouart E Grellier S 2004 Leachate recirculation moisture content assessment by means of a geophysical technique Waste Manag 24 785 794 Gawande NA Reinhart DR Thomas PA McCreanor PT Townsend TG 2003 Municipal solid waste in situ moisture content measurement using an electrical resistance sensor Waste Management 23 667 674 Advanced Geosciences Inc 2004 Instruction Manual for EarthImager 2D Version 1 7 4 Resistivity and IP Inversion Software P O Box 201087 Austin Texas 78720 Tel 512 335 3338 Fax 512 258 9958 www agiusa com Jakosky J J 1950 Exploration geophysics Trija Pub Co Los Angeles 1195pp Van Nostrand R and Cook K L 1966 Interpretation of resistivity data Geological Survey Professional paper 499 U S Govt Print Off Washington pp 310 Halihan T Paxton S Graham I Fenstemaker T and Riley M 2005 Postremediation
104. will have no effect in the following inversion EarthImager 2D Data Misfit Histogram Data Misfit Histogram for Removal of Poorly Fit Data 32 S a a S z E 24 30 36 Relative Data Misfit Cancel Remove Number of Data Removed 13 11 1 Total Number of Data 117 Iteration No 6 Figure 3 24 Data misfit Histogram 3 6 2 6 Inversion results Figure 3 25 shows a single inverted resistivity section at a user specified iteration for both surface and borehole data sets When the mouse pointer moves on the inverted 76 resistivity section its X location Depth and Resistivity value are shown on the bottom status bar of the main window Inverted one Section 0 0 549 86 123 Ohm m 00 7293 G 35 2039 70 570 p 105 159 14 0 44 5 Iteration 6 RMS 2 52 Normalized L2 0 71 Figure 3 25 Inverted resistivity section Figure 3 26 shows measured and calculated apparent resistivity pseudosections and inverted resistivity section at a user specified iteration EJ AGI Earthimager 2D C Program Files AGI EarthImager2D demo cave stg File Edit Settings Inversion Yiew Tools Help m arent Resistivity Pseudosection and Inverted Resistivity Section 123 Ohm m 0 0 2 6 5 1 FF 10 2 Measured oe arent Re ee Pseudosection 0 0 2 6 S1 77 Calculated Apparent Resistivity Pseudosection 14 27 41 55 69 82 Inverted Resistivity Section Iteration 5 RMS 2 57 Normalized L2 0 73 Number of Data 172 N

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