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Gravity corrections (T54)

1. Radius in cells of Ring 5 1024 Calculate scalar tensor terrain correction using sloping top prisms accurate flat top prisms fast OK Cancel Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 32 Library Help Top lt 4 Back gt Controls in this dialog box Control Description Terrain bottom For the tensor case a notional bottom RL is also required make this well below the terrain elevation Radius of rings The ratio of 16 32 64 256 1024 is the traditional scalar gravity ratios As gradiometry falls off by one oredr of magnitude greater than scalar gravity a different ratio series with a sharper roll off is recommended eg use a finer cellsize and 9 27 81 243 Calculate scalar tensor It is recommended you start with flat top prisms and terrain correction using just 2 rings to make sure all is looking as it should eg the DTM grid is appropriate and the order of the terrain correction seems in order Then repeat the process with a higher number of rings and use the sloping top option Create tensor from inline or crossline Parent topic Process menu Library Help Top If you have FTG data from the contractor that is close to what was actually measured you may also have inline and crossline fields often called 11 12 13 C1 C2 C3 You also need a carousel angle which captures the angular orein
2. Previous Previous Clo This graph shows the drift for each loop in the first GMLS of the dataset The horizontal axis represents the time days since the survey began The vertical axis is the dial reading Each line segment represents one loop The length of the line segment indicates the time taken to complete the loop The gradient if any shows the drift at the same node p Se Standard Drift for gravimeter G101 55 06 82 59 Time days Drift normalised Parent topic View menu Library Help Top This graph shows the normalised drift for each loop in the first GMLS of the dataset The normalised graph shows each segment shifted up or down to fit a curve This gives some sense of a drift continuum for the GMLS The horizontal axis represents the time days since the survey began The vertical axis is the normalised dial reading INTREPID fits a polynomial to the gradients drift of the line segments loops It then shifts all line segments up or down so that they start on this polynomial 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 47 Library Help Top lt 4 Back gt Previous Screen dump to postscript Parent topic Use the graphics engine within this tool to create a postscript file with the loops View menu stations layout Z Specify Output Pos oe SG Recent Places Name Downloads BA 4 Unspecified 24 0 Libraries
3. This option allows you to drill down to individual stations by name to lasso groups and to query in a temporal spatial sense the readings so you can spot trends File Process Tools Spatial_Query Settings View Help Find Gravity Station Trace a Polygon double click to end Load Existing Polygon Save Current Polygon As Erase Traced Polygon Pseudo Profile View a In this section e Find gravity station e Trace a polygon e Load existing polygon Save current polygon e Erase traced polygon e Pseudo profile view Find gravity station Parent topic Spatial query Library Help Top Choose this option to get every entry in a StationName field to report Click on an entry in this list and the background graphics window will show the requested station in a purple highlight This is a reverse search Much the same can also be done just simply typing the station name into the top right hand side text window followed by a carriage return A text convention is also used to indicate which stations are nodes and repeats when you have a processed field loop observation dataset loaded The number of connections above one to other stations is recored by the white lines and also the gt 0 0 text code following the important stations 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 38 Library Help Top lt 4 Back gt i
4. Specify input Tensor Field Cancel Controls in this dialog box Control Description Input tensor Choose any tensor field in your database field and recompute equivalent inline and crossline components Tools menu Parent topic This collection of functions tend to be to the side of mainstream gravity processing Gravity corrections z z T54 File Process Tools Spatial_Query Settings View Help Gravity meter calibration Earth Tides Convert to WGS84 Convert Potsdam to IGSN71 Sort Data Base In this section Gravity meter calibration e Earth tides Convert to WGS84 Convert Potsdam to IGSN71 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 34 Library Help Top lt 4 Back gt e Sort database Gravity meter calibration Parent topic Tools menu Earth tides Parent topic Tools menu Library Help Top The AGSO field data format is designed to accomodate gravity readings collected from calibration ranges Using the known calibrated gravity stations INTREPID can calculate new instrument scale factors and can optionally apply these to all the gravity readings during the data reduction and network adjustment process Calibration and scale factor results are written to Section 3 of the processing report See Gravimeter calibration R29 for details Contact INTREPID if you wish
5. Note the gradient drift polynomial uses X MidTime Y Obs Drift The Calc Drift is the Least Squares Estimated drift Base StartTime interval Calc MidTime Obs Drift Calc Drift hrs hrs hrs from start per hr per hr status 83910104 0 0000 10 1833 5 0917 0 00209 0 00072 83910104 10 1833 13 7167 17 0417 0 00082 0 00071 83910104 23 9000 8 7000 28 2500 0 00101 0 00070 83910104 32 6000 328 4167 196 8083 0 00042 0 00057 83910104 361 0167 9 9167 365 9750 0 00211 0 00044 83910104 370 9333 325 9833 533 9250 0 00066 0 00031 83910104 696 9167 9 4000 701 6167 0 00866 0 00018 ignored 83910104 706 3167 0 5667 706 6000 0 03182 0 00017 ignored 0 00014 83910104 707 1167 85 9167 750 0750 0 00013 Final Goodness of fit for drift curve polynomial order 2 0 002152 ChiSqr Probability that obs ChiSqr for a correct model be less than this 0 000000 Final polynominal coeff for time 0 017289 time 2 0 000447 A long term drift correction found by integrating final polynominal drift curve Integrated Correction Polynomial coeff for time 0 000000 time 2 0 017289 time 3 0 000223 Correction Polynomial base value est long term drift correction 0 003658 Precision statistics INTREPID estimates and reports the precision statistics for the data after the drift correction process It calculates this from the variations in readings for nodes and other stations with more than one observation 11 3 Estimating Loop Precision for s
6. Spatial query option Provide a polygon dataset name this is a standard polygon dataset and can also be saved in any GIS format Erase traced polygon Parent topic This option simply erases the transient polygon graphic and resets back to a neutral Spatial query state Pseudo profile view Parent topic The longest dimension of the psuedo section is used to define an X axis All the Spatial query gravity data points that lie within the polygon are projected onto the section plot with the gravity reading as the Y axis You can mouse click on any of the crosses wiuthin this plot to get a station report in the underlying RHS reporting pane When you have loop data you can isolate individual field data records to get to a seeming outlier etc etc l Profile View 368 points from Loop Processing x x x x x x x x lt a z x Xx xxx x x 22205 82 43883 06 65560 30 Distance Settings menu Parent topic To change a setting choose a corresponding item from the Settings menu Gravity corrections n s T54 File Process Tools Settings View Help Tare Detection Limit Loop Adjustment Limit Repeat Reject Difference SkipEarthTideCorrection Strict viewOFNodesBeforeNetworkAdjustment Density gt Gravity Meter Drift gt Report Detail gt Database Layout gt Output Datum Yvwv v In this section Tare detection limit Library Help Top 2012 Intrepid Geophysics
7. X X X X X X X X X X 21 Gravity corrections T54 53 lt 4 Back gt These are stations with multiple readings in one loop only These points are useful cross reference points for corrections 7 2 Gravity Meter Loop Set 2 Loop ol AUNE Station 97052117 97053008 No Repeats 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 54 Library Help Top lt 4 Back gt 10 11 Total Number of Repeats 2 8 Data structure check This section reports Start and finish station Ties nodes in the loop e Possible tares in the data 8 1 Data Structure Check for Gravity Meter Loop Set 1 Loop FirstAndLast TimeOrder Tares Position Nodes 1 ok ok possible tare s in data Stationl Station2 Difference 83910104 97050001 28 134 97050001 83910104 28 197 ok ok ok 2 ok ok Reduce loop data to final Parent topic Gravity processing reports Library Help Top Report header kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkxkkk Intrepid Gravity v3 5 cut 62 static Start processing 20 4 2000 13 0 24 kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkxk Gravity Processing Report Starting from Loop Data Base and doing All adjustments INTREPID repeats and reports sections 1 8 as shown in Gravity data import 9 Meter corrections INTREPID lists each GMLS that it corrects using the gravimeter calibration file MeterCorre
8. correction at 167km and further is the traditional Earth curvature correction Calculate scalar terrain correction Calculate full tensor correction This is the classic case with rods and sloping top triangle prism modelling This terrain modelling uses the Holstein facet modelling code for a FTG case Number of calculation rings Tis comes from the Hammer chart idea of 2 to 5 rings The primary contribution comes from the closest terrain and this falls in the inner ring Primary cell size This cell size is independent of the underlying DTM grid as a resampling is used This drives the actual radius for each ring as the cell size is multiplied by the number of cells in each ring Density This is the assumed terrain or regolith density value If you use 1 g cc you can scale the calculated field later in thye spreadsheet Gravity database Digital terrain model grid The observed gravity database must include a field for the observation points X Y Z It is not actually necessary to have the actual observed field as the aim here is to create a field with the terrain correction fields without actually applying the corrections at this point in the process This is a standard geophysical grid that has the local DTM with good extents far beyond the gravity observation stations SRTM can be OK but generally something with better resolution is required Output report A very compre
9. gt Library Help Top INTREPID User Manual Library Help Top Gravity corrections T54 26 lt 4 Back gt and file selector to locate the file you require See Specifying input and output files in Introduction to INTREPID R02 for information about specifying files Menu options Option Description Open Gravity Database Use this to specify the gravity dataset which you wish to manipulate You may perform utility gravity transforms and terrain corrections on an existing gravity dataset It is also possible to open an XYZ or an existing principal facts database and make use of some of the data reduction and network adjustment tool functions In this case only some of the processing sequence can be applied to the data In this case you cannot then answer questions about differing precision of one reading vs another because Gravity datum changes etc so easily Survey Import Wizard The Import Wizard is the starting point for reduction and network adjustment of field data in AGSO or Scintrex format Dump Check CG5 Convert binary format CG5 data to readable ASCII Useful for viewing the data before importing Merge new survey with master database The Gravity Tool allows you to merge your current dataset with a master dataset of principal facts Fields to be merged must have the same names Missing fields are set to Null values This option calls a separate tool called merg
10. lt 4 Back gt INTREPID User Manual Gravity corrections T54 9 Library Help Top lt 4 Back gt adjustment These provide a useful measure of how well the survey data was collected and reduced After successful completion of the last step the gravity tool creates the following point dataset by default Survey DIR This is what we refer to as the principal facts database The final reduced gravity values consist of a single Observed gravity value per station The Freeair and Bouguer anomaly values are also calculated for each data point Utility gravity transforms Parent topic Gravity corrections T54 When field data is fully reduced using Reduce Loop Data to Final quantities such as the Freeair anomaly and the Simple Bouguer anomaly are created automatically as part of the processing sequence However you can also calculate stand alone gravity transforms and corrections using an existing database of gravity data The examples that follow are available in the Gravity Transforms options under the Process menu The Gravity tool creates new fields to store these values In this section Instructions for gravity corrections e Theoretical gravity e Free air anomaly Reverse free air anomaly Simple Bouguer anomaly e Reverse simple Bouguer anomaly e E tv s gravity correction e Velocity from E tv s gravity correction Instructions for gravity corrections Parent topic Utility gravity trans
11. Case 2 Scintrex style Changes can be made using the importGPS GPS Field Data menu and also the same keyword in the batch job file WGS84 is the default for the CG5 case Q In general I guess the position must be always in Geographic coordinates and cannot be projected coordinates No you can give each position data set in any coherent independent projection datum Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 65 Library Help Top Library Help Top 4 Back gt you like and even mix and match if you want We have samples of datasets being imported with GEODETIC and UTM etc that we can supply You also have the option of changing from the input Datum to another Datum on output Q Control Gravity Reference Control gravity value Accuracy of gravity values What exactly are these A This is an estimate of the precision of the fundamental tie in station and is often of an accuracy that is 3 or 5 times better than the standard loop collected with a CG3 for example for an absolute FG5 meter you should get an accuracy lt 2 ums Q Gravimeter Loop set Nominal scale factor What is this A Before you have conducted your own calibrations of your meter and there is provision for you to do this for both L amp R and Scintrex meters you are obliged to believe the manufacturer or other authority as to what the current scale factor is We
12. DIR eotvos_new ReportFile eotvos rpt RunType CALC_EOTVOS OutputUnits MILLIGALS TerrainType OCEAN_SURFACE DatumType IGSN71 In this example the density contrast used for the Bouguer correction is 1 17 g cc equivalent to 2 2 g cc total after addition of water density Eg land amp saltwater 1 17 2 2 1 03 I ntrepidTask Gravity compute Bouguer GravityDatabase datasets Survey9705_1 DIR ObservedGravity datasets Survey9705_1 DIR GRAV SimpleBouguer datasets Survey9705_1 DIR Bouguer_new StationElevation datasets Survey9705_1 DIR Elevation ReportFile bouguer rpt RunType SIMPLE_BOUGUER OutputUnits MILLIGALS TerrainType OCEAN_SURFACE flag to control density contrast selection DatumType IGSN71 Properties Density Fresh Water 1 0 Density Salt_Water 1 027 Density Ice 0 917 Density Land 2 67 Density LandMinusFreshWater 1 67 this following is the one being used in this case Density _MarineSedimentMinusSaltWater 1 17 the one for marine Density Marine Sediment 2 2 Density _LandMinusIce 1 753 A second example shows a terrain correction for a land based context Example task file V5 0 protbuf syntax gravity Usage fmanager batch gravity _terrain_correction task Compute terrain correction complete Bouguer for land gravity data Then add the terrain correction to the Bouguer field to cre
13. Documents a Music E Pictures a AGSO_Weekl amp 2 DAT H Videos al AGSO_Week1 DAT CG5_sample sgd _ Goulburn_SRTM_stitch 100m Goulburn_SRTM_stitch_100m ers 1 Computer E Goulburn_SRTM_stitch 100m isi ly Local Disk C E import_cg5_samplejob a Homegroup Help Parent topic You can use the help menu to display help text on the topics shown in the menu Gravity illustration below corrections T54 Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 48 Library Help Top 4 Back gt Help Gravity PDF INTREPID Contents INTREPID Search Intrepid Geophysics Library General Gravity Field data AGSO Field data SCINTREX Reduce data loops Files and databases Tensor Support Settings Batch processing About Using task specification files Parent topic Gravity corrections T54 Library Help Top You can use the help menu to display help text on the topics shown in the menu illustration below You can store sets of file specifications and parameter settings for Gravity Corrections in task specification job files At V5 0 we also support the use of GOOGLE protobuf syntax to accomplish the same function This move to the GOOGLE technology is a longterm strategic one designed to leverage off this kindness and strength As we then also publish the formal language syntax you can
14. Geophysics lt 4 Back gt INTREPID User Manual Library Help Top Controls in this dialog box Gravity corrections T54 28 lt 4 Back gt Control Description Gravity loop database This is the intermediate database with standardised fields that capture intermediately processed field data still in LOOP order Control gravity observations database Output database Your tie in to a national datum or an absolute station is kept in a much smaller seperate database This is not strictly necessary but your survey data cannot be interpreted or merged with other surveys until this is done properly The final principal facts data reduction from your newly acquired survey get written using standard feild names to this output gravity database Output report A very comprehensive report that pulls all your data apart reporting on loop design repeats drifts error analysis is automatically written by the tool to this file Please examine it carefully Gravity transforms Parent topic These calculator functions require supporting fields to function correctly and you Process menu also need to know the gravity datum if you wish for example to revese back to an observed gravity value from a FreeAir Some of the prompted fields are optional extras A SKIP button will present in this case Before a final calculation is executed after you have been prompted for all the necessary fie
15. Gravity Stations P N g 97051043 0 97051044 0 97053004 09 97052011 00 97051046 0 97051047 0 gt 97051048 0 97051049 0 97051050 0 97051051 0 97050043 0 gt 97050044 09 97051052 0 97051053 0 97051054 0 OK Cancel Trace a polygon Parent topic The aim here is to use the Spatial Query gt Trace a polygon to select a subset of the Spatial query gravity readings in a spatial sense regardless of when the data was acquired to define a psuedo section for which a profile of gravity can be viewed Intrepid Gravi a eS ll File Process Tools SpatialQuery Settings View Help 30 70 829 979603 310 Survey Statistics Locations 1055 Readings 1046 LoopSets 3 Loops 98 Nodes 121 Repeats 1 Globals 27 Tie Ins 1 Zoom In Zoom Out Query C Zoom Rectangle 2 eek DE 14 Fa e 144 Part 150 BD BBE Load existing polygon Parent topic Instead of doing on screen digitizing of a polygon you can choose an a existing Spatial query polygon dataset This can come from anywhere provided it meets the INTREPID format requirements eg Arc shape file something saved from the subset tool etc Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 39 Library Help Top lt 4 Back gt Save current polygon Parent topic You can save the polygon you have traced to a polygon dataset by choosing this
16. Help Top lt 4 Back gt Controls in this dialog box Controls Description Maximum loop When doing a loop levelling adjustment an iterative adjustment improvement in the mis fits will continue until the maximum mis fit is less than this limit There is usually no cause to change this value Repeat rejection difference Parent topic Settings menu This option sets the rejection tolerance value for repeat station values The default value is 0 20 mGal Precision Repeats Estimate Reject Tol OK Cancel Controls in this dialog box Controls Description Precision repeats Enter a value to specify an acceptable difference between estimate readings at the same station Skip Earth tide correction Parent topic Settings menu Skip Earth Tide Correction Turn off the Earth Tide correction The default is to include it while doing the standard land based loop processing stream This option also applies to marine processing for L amp R instruments etc The workflow for this case is tied up in batch processing options for this tool and is described in the marine gravity processing cookbook Strict view of nodes Parent topic Settings menu Library Help Top Strict View of Nodes A node or tie is defined as a station that appears in more than one loop INTREPID has two views of what constitutes a node 1 Strict rigorous view Station numbers that are repeated
17. default our Nominal Scale factor to 1 0 After calibration you may have a slightly better adjustment available so a number like 9995 may emerge A The gravity tool has an option for you to conduct your own calibration surveys and it calculates this number for you for each meter reader combination We can supply a sample calibration survey upon request Q Q Why is the dynamic range of the reported terrain correction TC range not the same as the dynamic range of the compl_boug field A If you turn off the Earth curvature correction they will be the same Q Can I compute a Bouguer correction for my FTG data and does it make sense to do so A The concept of a simple Bouguer slab correction for FTG is suspect even though you need to do a terrain correction to the Free Air In this later context a complete Bouguer FTG tensor has been terrain corrected assuming a constant density 2012 Intrepid Geophysics lt 4 Back gt
18. found 83910104 0000 is reading 12 17 18 only node found 83910104 0000 is reading 14 23 28 24 28 kkk Internal loop tied to node 97051277 0000 from reading 0 to reading 15 25 28 31 38 only node found 83910104 0000 is reading 11 44 58 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Gravity corrections T54 57 lt 4 Back gt Internal loop tied to node 83910104 0000 from reading 13 to reading 14 Internal loop tied to node 83910104 0000 from reading 14 to reading 15 kkk kkk 12 2 LoopAdjust for set number 2 Loop adjustment search control parameters Stop if Max Loop Change less than 0 010 Stop after Max Loop Itererations 20 Iteration 1 forward Average misclosure change 0 117216 New average 3251 291855 Running Sum of all changes 0 000000 Absolute Sum of all changes 0 000000 Max improvement at 97053023 of 0 413483 Iteration 2 backwards Average misclosure change 0 133276 New average 3251 367516 Running Sum of all changes 0 000000 Absolute Sum of all changes 0 000000 Max improvement at 97052021 of 0 223050 Iteration 3 forward Average misclosure change 0 072963 New average 3251 361520 Running Sum of all changes 0 000000 Absolute Sum of all changes 0 000000 Max improvement at 97053023 of 0 177428 Iteration 20 backwards Average misclosure change 0 017381 New average 3251 472899 Running Sum of all changes 0 000000 Absolute Sum of all changes 0 000000 M
19. or down to the station elevation plane which corresponds to the top of the simple Bouguer slab This is schematically shown in the figure below for a few prisms 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 19 Library Help Top 4 Back gt Station elevation thickness of Bouguer slab Reference Level Prisms in the innermost ring A have a sloping top to better adapt to terrain variations within a cell The gravity effect of prisms in outer rings B C is calculated using a vertical rod approximation to speed up the computation Each prism is assigned a standard density and the terrain correction is calculated at each station as the sum of effects due to all prisms contained within the radii This provides maximum precision in the region nearest to the station while allowing more efficient calculation further away To prevent edge effects you should choose a DTM that is larger than your survey area For best results your DTM should be large enough so that for each gravity station the area used to calculate the terrain correction is completely contained within the DTM In areas of high relief terrain corrections can be quite high In Australia gravity terrain corrections can be as high as 25 mGal and the terrain effect can extend for 50 km The terrain correction is added to the simple Bouguer anomaly to produce the Complete Bouguer anomaly In the case of land gravit
20. recommeded as the minimum eg Minimum cell size 5 Define first ring 5 gt 80m Second ring 80 gt 160 Third ring 160 gt 320 Fourth ring 320 gt 640 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 62 Library Help Top lt 4 Back gt Fifth ring 640 gt 1280 For mountainous regions use the following scheme 5 5 160 160 640 640 1280 1280 5120 5120 20480 For flatter regions use the following scheme 5 5 80 80 320 320 640 640 1280 1280 20480 Height ve above sea level Consistent units for distance and heights should be used eg meters B Land vs Marine vs Airbourne All calculated terrain corrections for Land are positive The Earth Curvature correction is negative for Land and can introduce small negative corrections increasing with the Height of your station above the Geoid On the other hand The Earth Curvature correction is mostly positive for Marine Both Submarine and Airborne terrain corrections can be both positive and negative If you require a submarine correction sea level is assumed as the observation height If you require an airborne correction gps height altitude is required as the observation height This is a vital ingredient for this situation This run is for a terrestrial correction only IMPORTANT NOTE The tool only calculates a terrain correction at an observation point where the Observed Gravity field a
21. some There is a great diversity in how you design successful gravity loop surveys with Scintrex tending to push the grid view more with the way the default meter wants to organise records Temporal and spatial coherence of the gravity readings are vital if one is to create a reduced dataset that accurately measures gravity anomalies in an area All survey styles can be accommodated in this tool though sometimes it does seem to be a trial if your planning was not well documented INTREPID has the capacity to retrieve duplicate readings at the same station as well the station data in a message box Turned off at present Intrepid Gravity 3 4 cut 61 StationNumber 97051233 Index 0 18 16 Dial 3016 367 Gravity 979482 492 StationNumber 97051233 Index 1 2 18 Dial 3016 102 Gravity 979482 411 StationNumber 97051233 Index 2 20 21 Dial 2956 081 Gravity 979482 801 StationNumber 97051233 Index 3 2 11 Dial 2955 704 Gravity 979482363 StationNumber 97051233 Index 4 18 14 Dial 2999 212 Gravity 979482 632 OK 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 7 Library Help Top lt 4 Back gt Note There is actually only one observed gravity record for each station in the reduced dataset The observed gravity for this station is the average of the displayed values See INTREPID gravity point datasets R28 for details of the gravity point dataset T
22. the commands gravity exe batch surv_034 Job 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Task specification file examples Gravity corrections T54 49 4 Back gt As part of the standard software distribution we give you example files Look in the jobs gravity and for V5 0 tasks gravity directories Here is an example of an Gravity Corrections task specification file using the new V5 0 protobuf syntax that is distributed The exact same job is also distributed in the jobs gravity area Example task file V5 0 protbuf syntax gravity Usage fmanager batch gravity_utilities task Shows 3 utility operations for the marine environment 1 compute Freeair 2 compute Eotvos 3 compute Bouguer IntrepidTask Gravity free_air GravityDatabase datasets Survey9705_1 DIR ObservedGravity datasets Survey9705_1 DIR GRAV FreeAir datasets Survey9705_1 DIR freeair_new ReportFile freeair rpt RunType FREE_AIR OutputUnits MILLIGALS TerrainType OCEAN_SURFACE DatumType IGSN71 IntrepidTask Gravity compute Eotvos GravityDatabase datasets Survey9705_1 DIR ObservedGravity datasets Survey9705_1 DIR GRAV CraftVelocity datasets Survey9705_1 DIR velocity filtered LineBearing datasets Survey9705_1 DIR Azimuth Eotvos datasets Survey9705_1
23. 08000 2 Control gravity data List of loop network control stations Library Help Top 2012 Intrepid Geophysics 97050129 000 97051037 000 97050083 000 4 Back gt INTREPID User Manual Gravity corrections T54 52 Library Help Top lt 4 Back gt 2 Control Gravity Data 2 1 Station List Station Obs Gravity Precision Datum Theoretical Comments 83910104 979603 3100 0 1000 IGSN71 979757 8421 Old AGSO Building Main Door 2 2 Primary Control Gravity Station 83910104 3 Gravity Meter Calibration Loop Data No Gravity Meter Calibration Data 3 Gravimeter calibration loop data Calibration data is optional See Gravimeter calibration R29 for details about this section 4 Gravimeter loop datasets For each GMLS this section lists Gravimeter details Operator details A summary for each loop 4 2 Gravity Meter Loop Set 2 Gravimeter LCR_G LCR Meter G132 Adjustment to Manufacturers Scale Factor 1 000000 Gravimeter Reader HReith Number of Loops 11 Loop Number Readings BaseIn BaseOut Start End 1 16 181 13 83910104 83910104 14 1 1998 7 15 14 1 1998 17 26 2 17 182 15 83910104 83910104 15 1 1998 7 9 15 1 1998 15 50 3 23 281 20 97051277 97051277 21 1 1998 8 26 21 1 1998 19 7 4 24 282 16 97051277 97051277 22 1 1998 7 47 22 1 1998 20 11 5 25 283 13 97051277 97051277 23 1 1998 7 37 23 1 1998 16 22 6 31 381 12 83910104 83910104 29 1 1998 8 16 29 1 1998 18 11 7 44 581 16 83910104
24. 13667 979603 310 0 0000 44 565 000 0 00 19 8502 43 3733 97050001 34 98807 149 02449 979573 562 0 0371 23 613 030 0 00 30 8983 37 6997 97053000 34 92465 149 13736 979579 053 0 0446 7 551 991 0 00 22 9309 38 8369 97051001 34 91920 149 16973 979579 231 1 556 609 0 00 24 9970 37 2875 97051002 34 93906 149 19984 979582 288 1 559 904 0 00 27 3852 35 2680 97051003 34 97221 149 21932 979581 461 1 576 656 0 00 28 9161 35 6117 97051004 34 99164 149 26253 979584 411 1 579 199 0 00 31 0013 33 8111 97051005 34 99776 149 22310 979581 655 1 586 229 0 00 29 8959 35 7031 97051006 34 98000 149 18748 979567 439 1 638 322 0 00 33 2680 38 1602 97051007 34 99683 149 15984 979563 917 1 654 965 0 00 33 4545 39 8361 Average Observed Free Air Bouguer 979516 864 35 341 40 100 Gravity output DB created Survey9705 in GEODETIC proj AGD66 datum 1046 stations output in newly created intrepid dataset data stored in gt Survey9705 Terrain correction report Parent topic Here is a sample terrain correction report Gravity processing reports kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkxkxkxk Intrepid Gravity v4 2 for Windows by TECHBASE1 Free Version Start processing 17 10 2008 20 47 47 kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkxk Gravity Complete Bouguer Report Intrepid Gravity v4 2 for Windows by TECHBASE1 Free Version 17 10 2008 20 47 47 Terrain Corrections A Conventions This method calculates a terrain corr
25. 6120 542243 to 5390784 200477 Z Range 377 794309 to 1192 490601 TC Range 1 062443 to 13 692027 kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk kkkkxk End processing 13 11 2008 16 23 6 Log terrain rpt kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk kkkkxk Frequently asked questions Parent topic Q Can the station name be numbers AND letters or only numbers ie Gravity 90001000 90001000R corrections T54 Case 1 AGSO style There are two key words that can be used for specifying the Station numbers in a GPS section e POSITION 98931602 119 48 05 28 23 24 59 48 552 0 or e LINE_POSITION 100 98931602 119 48 05 28 23 24 59 48 552 0 There is no provision for alphanumeric characters in the station numbers for either of the above Case 2 Scintrex style CG3 stations can have N S E or W in the station name and often do The station naming convention for a CG3 is often grid or line based and is quite at odds with the original AGSO inspired YYYYNNNN style convention We have generalized the rules to cope with common styles of station numbering Q For horizontal datum I get only AGD66 How can I change that to say WGS84 Case 1 AGSO style The line with the keyword POSITION defines what you want for both horizontal and vertical datums Just change it to suit your conditions For example POSITION UNKNOWN CLARKE ED50 PULKOVA NUTM23 0 00001 Note that you must use names that are known to POSC
26. 80 S8atm 8 74 0 000 99 h 0 000 000 035 6 h Where 52atm is the atmospheric correction in ums h height above ellipsoid not sea level in metres Sample processing report Calculating Free Air Anomaly Observed gravity field D cookbook gravity datasets Survey9705 obsgrav Latitude field Survey9705 Latitude Station Elevation field Survey9705 Elevation Meter Elevation field NO METER ELEVATION DATA BEING USED Output free air field D cookbook gravity datasets Survey9705 zzzz Gravity datum IGSN71 Terrain type land Gravity units Milligals Reverse free air anomaly Parent topic Utility gravity transforms Use this correction when your data contains a free air anomaly field but no observed gravity field INTREPID adds the free air correction and the theoretical gravity to the free air anomaly field to recreate the observed gravity field obsgrav FreeAir free air correction theoretical gravity Input field FreeAir Latitude Elevation Output field obsgrav Sample processing report Reversing Free Air anomaly to observed gravity Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 14 Library Help Top lt 4 Back gt Free air gravity field D cookbook gravity datasets Survey9705 FreeAir Latitude field Survey9705 Latitude Station Elevation field Survey9705 Elevation Meter Elevation field NO METER
27. 83910104 12 2 1998 8 9 12 2 1998 18 22 8 48 581 13 83910104 83910104 16 2 1998 8 16 16 2 1998 16 41 9 57 781 9 83910104 83910104 25 2 1998 7 57 25 2 1998 18 23 10 58 782 9 83910104 83910104 26 2 1998 6 57 26 2 1998 17 48 11 59 783 9 83910104 83910104 27 2 1998 7 8 27 2 1998 19 56 5 Node list A tie node is a station with readings in more than one loop Ties are important cross reference points for corrections Nodes are also important cross reference points for corrections X node in loop D node in loop used for drift control F fixed node in loop 5 4 Gravity Meter Loop Set 4 Number of Nodes from CreateNodeListFromLoops 6 Initial nodes 6 Loop 1 2 3 4 5 6 7 8 Node 97050001 D D X 98012078 X X 97051277 D D D Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Library Help Top 98010010 83910104 97053023 6 Global ties nodes Ties nodes common to more than one gravimeter 6 1 Global node list G132 G132 G6101 G101 G 651 Gravimeter Nodes 83910104 97050001 97053000 97053001 97051036 97051068 97051069 97051083 97051126 97051134 97052137 97051233 97051277 97052037 97052021 97052038 97052011 97053023 97053017 97051135 97052198 6 2 Number of global nodes 7 Internal loop repeat stations K mM OM OO MOM OM OO OM mM mm mM mM mm mM OM mM x x x X X X X X X X X X
28. ELEVATION DATA BEING USED Output gravity field D cookbook gravity datasets Survey9705 obsgrav Gravity datum IGSN71 Terrain type land Gravity units Milligals Simple Bouguer anomaly Parent topic Utility gravity transforms Library Help Top The simple Bouguer correction replaces the air in the Free Air anomaly with matter of a given density INTREPID uses the observed gravity field and the specified density and datum settings to calculate the simple Bouguer correction The simple Bouguer anomaly is calculated as follows Bouguer obsgrav theoretical gravity free air correction simple Bouguer correction obsgrav Subtract theoretical gravity Subtract free air Subtract Bouguer correction correction f ON Units datum terrain type density datum Latitude Bouguer Elevation Input field obsgrav Latitude Elevation Output field Bouguer You can experiment with different density settings to create a series of simple Bouguer anomaly fields for example Bouguer267 Bouguer250 Bouguer200 Simple Bouguer correction formula spherical cap For GA07 GRS80 the simple Bouguer correction is calculated using the following closed form equation for the gravity effect of a spherical cap of radius 166 7 km with a mean radius of 6 371 0087714 km and height relative to the ellipsoid Bouguer Correction BC 27Gp 1 p h AR Where Tis pi G is
29. EPID User Manual Gravity corrections T54 11 Library Help Top 4 Back gt latitude correction INTREPID automatically computes and subtracts the theoretical gravity when it calculates the free air anomaly and simple Bouguer anomaly Sample processing report Calculating theoretical gravity for all data base points Latitude field D cookbook gravity datasets Survey9705 Latitude Calculated gravity field D cookbook gravity datasets Survey9705 theograv Gravity datum Gravity units Library Help Top IGSN71 Milligals To convert data reduced to a different ellipsoid You may want to merge two datasets that were reduced to different ellipsoids If the datasets do not contain an observed gravity field you can use this option to revert to observed gravity for one of the datasets You can then reduce the observed gravity to the required ellipsoid as usual 1 From the Settings menu select the datum that was used for the original reduction Choose Theoretical Gravity to calculate the theoretical gravity that was subtracted from the observed gravity using this ellipsoid 2 Usethe spreadsheet editor to reapply add the theoretical gravity to the corrected gravity field to recreate the observed gravity field obsgrav See Step 2 of the complete Bouguer worked example in Gravity field reduction and correction C08 for an example of using the Spreadsheet tool 3 Select your preferred datum from the Settings menu for e
30. INTREPID User Manual Gravity corrections T54 1 Library Help Top lt 4 Back gt Gravity corrections T54 Top The INTREPID Gravity tool can apply gravity corrections and calculate gravity anomalies for land gravity data and also marine and airborne gravity data In this chapter Overview of the gravity corrections tool Key concepts for Land Gravity Acquisition e Data reduction and network adjustment e Utility gravity transforms e Terrain correction Gravity mode settings Specifying input and output files e Process menu Tools menu Spatial query Settings menu e View menu e Help e Using task specification files Gravity processing reports e Frequently asked questions For worked examples showing the use of the Gravity tool refer to the Cookbook Gravity field reduction and correction C08 Overview of the gravity corrections tool Parent topic You can use the help menu to display help text on the topics shown in the menu Gravity illustration below corrections T54 The INTREPID Gravity tool has four main functions Data reduction and network adjustment Import land gravity field data in either AGSO or Scintrex format and reduce the loop data to final Observed Gravity FreeAir and Bouguer anomalies This is a complete bundled processing sequence which involves several stages including gravimeter calibrations data integrity and loop structure checks Earth tide and gravimeter drift corre
31. al Gravity corrections T54 51 Library Help Top e 7 Internal loop repeat stations e 8 Data structure check Reduce loop data to final e Report header e 9 Meter corrections e 10 Earth tide 4 Back gt 11 Gravity drift corrections model statistics and estimate of precision e Corrections e Precision statistics e Node values 12 Node connections analysis amp levelling 18 Global adjustments 14 Applying meter scale factor to all loop data 15 Calculating adjustments to global nodes e 16 Final values e Terrain correction report Gravity data import Parent topic Report header Summary of the dataset characteristics Gravity processing reports kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkxkkk Gravity Field Data Checking Report Starting from AGSO field and checking loops GPS etc Intrepid Gravity v3 4 cut 61 20 3 2000 22 14 16 Survey 9705 Goulburn Regional Infill New South Wales 1 Position data Summary of the dataset characteristics 1 1 Position Set 1 Coordinate Reference Frame UNKNOWN Ellipsoid ANS Horizontal Datum AGD66 Vertical Datum AHD Coordinate Projection GEODETIC Position Accuracy 0 000001 Elevation Accuracy 0 020 Data Bounds Number of Stations 1054 Longitude Max Min 150 000188 97050366 000 148 499622 Latitude Max Min 33 997255 97052132 000 34 998606 Elevation Max Min 1266 371 98010003 000 282 5
32. alculate E tv s correction Eotvos line bearing craft velocity Units Input field Latitude line bearing and craft velocity Output field Eotvos WARNING The craft velocity is in units of knots Sample processing report Calculating Eotvos gravity for all data base points Latitude field D cookbook gravity datasets Survey9705 Latitude Line bearing field D cookbook gravity datasets Survey9705 bearing Craft velocity field D cookbook gravity datasets Survey9705 velocity Calculated Eotvos field D cookbook gravity datasets Survey9705 Eotvos Gravity units Milligals Applying the correction The E tv s correction is positive when the craft is moving to the east because when it moves with the earth centrifugal acceleration is increased and the downward pull is decreased and negative when its motion is westward Use the spreadsheet editor to add the E tv s correction to your observed gravity field to create a new E tv s corrected gravity field See Complete Bouguer anomaly worked example in Gravity field reduction and correction C08 for an example of using the Spreadsheet tool Velocity from E tv s gravity correction Parent topic Given the E tv s correction line bearing and latitude using this option INTREPID pe gravity computes the craft velocity that was required to produce just that E tv s effect ransiorms Library Help Top 2012 Intrepid Geophysics 4 Back
33. and arranged in time order are used as nodes The first and last stations in a loop are not used as nodes unless they are repeated Fixed stations are not used as nodes unless they are repeated 2 Relaxed view Station numbers that are repeated are used as nodes All first and last stations in a loop are used as nodes All fixed stations are used as nodes The default setting is the Strict View of Nodes 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 42 Library Help Top lt 4 Back gt Density Parent topic You can set the assumed density of material terrain as appropriate These values are Settings menu used for simple Bouguer and terrain corrections Validate your choice in the pop up that is provided just before a calculation is done The default density for land is 2 67 g cm SE Density 1 004 OK Cancel Controls in this dialog box Controls Description Density Enter a density value for the Bouguer slab correction You can specify land sea lake marine sediment and ice values Gravity meter drift Parent topic INTREPID has a choice of two drift models Settings menu The conventional short term linear drift uses a piecewise linear method to remove the drift for each loop The IgnoreRepeatsForShort option also allows you to ignore repeat stations for the purpose of drift calculations Long term polynomial drift is ca
34. ary fixed nodes for GMLS 1 by 976295 9897 Global Node Value 83910104 979603 3100 97050001 979573 4940 97053000 979579 0104 97053001 979571 2579 97051036 979576 9392 97051068 979536 9650 97051069 979547 4596 97051083 979538 7131 97051126 979540 2889 97051134 979432 1948 97052137 979458 3413 97051233 979482 4916 97051277 979549 1655 97052037 979523 5652 Secondary Fixed Node adjustment to GMLS 5 of 976439 6164 using an average adjustment via secondary nodes count 31 Adjusted secondary fixed nodes for GMLS 5 by 976439 6164 Global Node Value 97050001 979573 5846 97053000 979579 0865 97052011 979566 9397 97053001 979571 3244 97052021 979547 3547 97051036 979576 9323 97052037 979523 8117 97052038 979540 0260 97051069 979547 2874 97051083 979538 5529 Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 60 Library Help Top lt 4 Back gt 97051126 979540 1987 97051134 979432 1641 97053017 979455 7550 97051135 979457 2111 97052137 979458 3478 97052198 979452 5674 97051233 979482 6323 97051277 979549 4942 16 Final values A reduced set of data that is the best estimate of the gravity for each station This data is stored in the field bsgrav 16 Final Values Simple Bouguer Anomaly land Density 2 670 Datum IGSN71 Station Latitude Longitude Observed StdDev No Height Vert_Offset Free Air Bouguer 83910104 35 29333 149
35. aset and perform stand alone gravity transforms for example forward and reverse transformations of FreeAir and Bouguer anomalies or convert from one gravity Datum to another gravity Datum Key concepts for Land Gravity Acquisition Parent topic Gravity corrections T54 Library Help Top You can use the help menu to display help text on the topics shown in the menu illustration below Survey loop For land gravity surveys the basic data acquisition procedure is the loop It is required to remove the gravimeter s drift during the data reduction process The INTREPID Gravity tool requires that loops must start and stop on the same station unless one is a control base station in which case they are allowed to be different Survey network A land gravity survey network is a series of interlocking closed loops of gravity observations Gravimeter loop set GMLS The GMLS is defined as one gravimeter operator combination The INTREPID gravity tool allows for processing of large gravity datasets that could involve multiple gravimeters and operators over many years Nodes Nodes are gravity stations where more than one reading was observed Global nodes Global nodes are gravity stations common to more than one gravimeter Gravity base stations These are locations where the gravity value is well defined One or more main gravity base stations are used as a reference or control for local surveys The Global Adjustment proc
36. ate the terrain corrected Complete Bouguer field The process does not actually use the Observed Gravity field Earth curvature correction is irrelevant if radius is lt 167 km FE SHE SHE HE SHE SHE SHE HE HE Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 50 Library Help Top lt 4 Back gt The gravity datum choice does not affect the terrain correction output The terrain correction will be ve for the land case and ve for the airborne and submarine cases The terrain density should match the Bouguer density being added to Use the spreadsheet editor to add the terrain correction to the Bouguer field to create the terrain corrected Complete Bouguer field SHE SHE HE HE HE SHE SHE HE HE IntrepidTask Gravity terrain correction GravityDatabase datasets Survey9705_1 DIR ObservedGravity datasets Survey9705_1 DIR GRAV used as a flag field only DigitalTerrain datasets Goulburn_SRTM_stitch_100m ers DTM grid for this survey TerrainCorrection datasets Survey9705_1 DIR terrain_correction output for the correction StationElevation datasets Survey9705_1 DIR Elevation ReportFile terrain rpt RunType TERRAIN OutputUnits MILLIGALS TerrainType LAND_SURFACE DatumType POTSDAM Terrain Cell_Size 100 0 Max Circles 5 Earth_Curvature_Correction true UseDTM_ Elevatio
37. ax improvement at 97051277 of 0 028694 Total Iterations 20 Original average 3251 267805 Final Iter Average change 0 017381 Loop Adjusted values for nodes SUM OF DIFFERENCES OLD 0 000000 NEW 0 000000 Loop Adjusted values for Stations Station Old Value New Value Loop 16 18 83910104 3307 377 3307 367 98012062 3299 757 3299 747 98012065 3263 726 3263 716 98012066 3256 575 3256 565 Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 58 Library Help Top lt 4 Back gt Loop 59 78 83910104 3307 046 3307 367 97051069 3251 141 3251 442 98011120 3267 075 3267 379 97051126 3243 990 3244 298 98012151 3245 606 3245 926 98012150 3249 269 3249 590 98012152 3252 691 3253 013 98010207 3179 853 3180 183 83910104 3307 022 3307 367 12 4 Estimating Loop Precision for set number 2 after loop adjustment Number of repeat stations candidates for Precision Estimate 2 Number of repeat station differences actually used for current loop set 2 Precision estimate statistics for repeats for this loop Maximum 0 001131 Mean 0 017294 Mean Absolute Deviation 0 016163 Variance 0 000522 Standard Deviation 0 022858 Skew 0 353553 Kurtosis 2 750000 13 Global adjustments Adjustments between loops within a GMLS INTREPID compares the global tie values for all pairs of GMLS If the corrections have been performed properly there should be a constant difference between the gra
38. coefficients WGS84 9780326 7714 0 00193185138639 0 00669437999013 6371008 7714 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 18 Library Help Top lt 4 Back gt Datum ay ao a3 Ro GA07 formula coefficients GA07 9780326 7715 0 001931851353 0 00669438002290 6371008 7714 Terrain correction Parent topic Gravity corrections T54 The complete Bouguer anomaly reduction includes the simple Bouguer slab correction earth curvature correction and terrain correction The INTREPID complete Bouguer anomaly option calculates a terrain response for gravity data You must provide a digital terrain model DTM grid which is used to calculate the terrain correction required for each gravity station After the terrain correction has been calculated the correction can be applied to the Bouguer anomaly using the INTREPID spreadsheet editor Terrain correction can be calculated for either land marine or airborne data Full tensor gravity terrain corrections for new generation data acquisition systems are also supported Use this option also for Falcon then use the spreadsheet functions to re organise the FTG tensor to create a Falcon tensor Generally it is best to assume a 1 g cc density for the terrain correction phase then use the spreadsheet editor to scale the terrain correction with a va
39. ctions network adjustment and global tie in to gravity base stations A principal facts database is created from the reduced data Terrain correction Using a Digital Elevation Model DEM calculate terrain corrections for either land marine or airborne data The terrain correction can then be used to compute the Complete Bouguer anomaly Full tensor gravity terrain corrections are also supported Moving Platform Gravity and Gradiometry Support INTREPID has support for many instruments and systems for gathering gravity or Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 2 Library Help Top 4 Back gt gradiometry from a craft that is moving This covers the older L amp R sea meters including a direct algorithmic link to the original LaCoste decorrelation of wave action accelerations from the gravity This came via a collaboration with Herb Valiant of ZLS Also supported is the inline amp cross line geometry matrix transforms for the Lockhead Martin Full tensor gravity gradiometry system The FALCON instrument is also fully supported though some of the support is distributed through several tools especially the gfilt FFT tool as some of the transforms have to be done using Foyurier transforms using gridded data The GTXX Rio VKX and Sanders instrumental systems have also been processed using this tool Utility gravity transforms Open an existing gravity dat
40. ctions for set number 1 MeterCorrections for set number 2 MeterCorrections for set number 3 MeterCorrections for set number 4 MeterCorrections for set number 5 10 Earth tide INTREPID lists each GMLS that it corrects using an internally stored Earth tide model EarthTide correction for set number 1 EarthTide correction for set number 2 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 55 Library Help Top lt 4 Back gt EarthTide correction for set number 3 EarthTide correction for set number 4 EarthTide correction for set number 5 11 Gravity drift corrections model statistics and estimate of precision Corrections INTREPID finds the difference between the readings at the start or finish station at the beginning and end of the loop It then interpolates a correction for each observation in the loop to correct this discrepancy assumed to be instrument drift 11 1 Gravity Meter Drift Correction for set number 2 Least Squares Polynomial Fitting multi loop Rejecting Too small an Interval Time Segment for Base Stations Skipping time segment as too small less than 0 480 of hour Base 83910104 Obs Drift 0 121752 Time interval 0 233333 hrs Initial Goodness of fit for drift curve polynominal of order 2 is 0 021089 ChiSqr Probability that observed ChiSqr for a correct model be less than this is 0 000000 Rejecting Outlier Intervals Time Segments for Base
41. d loop in the first GMLS The data used in this manual is supplied as part of the sample_data cookbooks gravity_land It comes from a Geoscience Australia gravity regional survey near Goulbourn NSW and was acquired in 1997 So whilst this is a reference manual by doing an AGSO data format import of the file AGSO_Week1 amp 2 DAT you will be able to see and reproduce quite a few of the screen states described within Of course as this Gravity tool covers a very large set of circumstances this guideline only applies to the land gravity acquisition and data reduction functionality 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 6 Library Help Top Library Help Top lt 4 Back gt File Process Tools Spatial Query Settings View Help Long E ast Lat North Survey Statistics Locations 1055 Readings 1 LoopSets 3 values Zouma Zoom Out C Query C Zoom Rectangle one 55 Jiran E 149 ED BD E 144 3D BD E 159 BB BDE The convention above is that a single black dot represents a gravity station with just one reading Left mouse click a station to view the station data including loop number loop set and sequence number in the loop The dial value is the actual number from the meter before calibration corrections The white lines show nodes that have many readings connecting key stations in the loop network This regional layout may seem foreign to
42. de decimal Elevation metres Month mm Year tyw Interval minutes 5 Time diff Greenwich hrs 4 OK Cancel Controls in this dialog box Controls Description Title A title Latitude where on the earth Longitude where on the earth Elevation where on the earth Month what month are you interested in Year what year are you interested in Interval dump values for every interval in minutes Time difference offset in time from GMT Convert to WGS84 Parent topic Use this to convert an Observed gravity field from a non WGS84 gravity datum to the Tools menu WGS84 gravity datum Specify the new gravity field name in the Specify Output Observed Gravity Field dialog box Specify Input Observed Gravity Field Cancel Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 36 Library Help Top lt 4 Back gt Controls in this dialog box Controls Description Specify input observed You are prompted for an observed gravity field in gravity field your database together with its datum Convert Potsdam to IGSN71 Parent topic Use this to convert an Observed gravity field from the Potsdam gravity datum to the Tools menu IGSN71 gravity datum Sort database Parent topic Sort the database on any indexed field Sorting the database on Station number is a Tools menu useful way of checkin
43. e exe that does location and precision checks on the new data compared to the master data and attempts to arbitrate or make a judgement about which records are better Exceptions are written to a log file for reprocessing editing Do not use this option without some planning and thought Check the tutorial first Edit Gravity Database Aliases This supports normal assigning and re assigning of the standard INTREPID alias names Load Options Select a Grdop task specification file to preload the interactive session with all the required file and parameter settings See Using task specification files for information about task specification files Save Options Save the current Grid Operations file specifications and parameter settings as a task specification file See Section Using task specification files for more information 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 27 Library Help Top lt 4 Back gt Process menu Parent topic Intro text Gravity corrections T54 In this section Reduce loop data Gravity transforms Complete Bouger anomaly Complete Bouger anomaly advanced options Create tensor from inline or crossline e Create inline or crossline from tensor Reduce loop data Parent topic Intro text Process menu Databases Required for Gravity Field Processing Library Help Top 2012 Intrepid
44. e resolution of the DTM grid It also controls the radius of each ring See the Advanced options below Specify the DTM grid cell size to start with Increasing the size increases the ring radii The result is less accurate but it runs faster Density Land The density in g cm assigned to prisms on land Density Seawater The density in g cm assigned to prisms in the sea Advanced options Setting Description Terrain Bottom RL Full tensor gradient terrain corrections for land air sea are supported The Holstein polyhedra modelling method is used to calculate the tensor response of the terrain The method requires a bottom RL to determine the height of the prisms 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Library Help Top Gravity corrections T54 23 4 Back gt Setting Description Radius in cells of Ring 1 5 The radius of the rings of terrain influence in primary cell sites can be individually modified Calculate scalar tensor terrain corrections using The method of sloping prisms is the more accurate but slower option Note that this only affects the prisms in the innermost ring Outer rings always use the rod approximation pscalar cape or flat top prisms tensor case Press the first Browse button to select your gravity dataset Press the second Browse button to select
45. ead into the program 21 Number of Null records 0 Determining gravity data limits X Range 473069 674332 to 477129 768455 Y Range 5386120 542243 to 5390784 200477 Z Range 316 380000 to 1227 000000 Using Inverse distance gridding for DTM elevation interpolation Adding observed elevations to dtm list Moving station elevations onto DTM grid Calculating terrain response Calculating for each row and column of DTM grid Reporting observed gravity data density for each grid cell The density is found by finding the distance of all observations to the closest edge of the cell This distance is compared to the radii and an appropriate density is found If a observation is found within a cell the density is set to the maximum The algorithm stops when a minimum density is found or all points have been searched 0 most dense 5 means no observations Problem domain is rows 32 cols 32 Scanning box row 1 central easting 450425 0 central northing 5364325 0 Scanning box row 2 central easting 450425 0 central northing 5365925 0 Scanning box row 31 central easting 450425 0 central northing 5412325 0 Scanning box row 32 central easting 450425 0 central northing 5413925 0 Terrain Complete 16896 prisms amp 34999 rods calculated Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 64 Library Help Top lt 4 Back gt X Range 473069 674332 to 477129 768455 Y Range 538
46. ection TC either for the vertical component of gravity or the full gravity gradient tensor It does not modify the observed gravity field After the TC is calculated you must add the corrections to your Bouguer corrected values Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 61 Library Help Top lt 4 Back gt For traditional scalar vertical component of Gravity The gravity effect is calculated using the vertical edge prism model for density ring 0 or optionally and more exactly a sloping top triangle model and the thin rod model for density rings 1 4 The prism model is assumed to lie directly below the gravity observation to a depth equal to the absolute value of the difference in height between the gravity station and the averge height of the terrain at the prism The trianglular prism has the advantage of a sloping top As the near field terrain effects errors are greatest this proves to be a major improvement reference Woodward O J 1975 The Gravitational Attraction of Vertical Triangular Prism geophysical prospecting 23 pp 526 532 Obviously each prism triangle rod model is offset in X Y from the gravity station For Full Vector and Full Tensor Gradiometry of Gravity Gravity component and Gradient Tensor calculations are performed using a facet technique for forming sloping top triangular prisms for the inner ring The outer rings are fou
47. essing stage ties all the GMLS survey stations back to these base stations The nature of the Global adjustment depends upon the number of Control stations Where there is a single Control station INTREPID holds that station fixed and adjusts all other stations to it However where there is more than one Control station INTREPID calculates a global adjustment by averaging the changes to each Control station made as a result of the network processing In this case no single Control station remains fixed It is presently not possible in INTREPID to influence the relative weightings of the Control stations 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 3 Library Help Top 4 Back gt Data reduction and network adjustment Parent topic Gravity corrections T54 You can use the help menu to display help text on the topics shown in the menu illustration below Field data reduction and network adjustment can only be applied to land gravity data which has the survey loop structure clearly defined The process consists of two stages Data Import and Reduce Loop data to Final The intent here is to provide high redundancy through good survey loop design with one or more base stations master nodes for each loop and repeat stations that may not be nodes The design of the software also makes the distinction for each Meter Operator pair as the care taken by an individual with a gravity me
48. et number 2 after drift Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 56 Library Help Top lt 4 Back gt Number of repeat stations candidates for Precision Estimate 2 Number of repeat station differences actually used for current loop set 2 Precision estimate statistics for repeats for this loop Maximum 0 009414 Mean 0 009781 Mean Absolute Deviation 0 000368 Variance 0 000000 Standard Deviation 0 000520 Skew 0 353553 Kurtosis 2 750000 Node values INTREPID reports the drift results for each tie node showing original and corrected values 11 4 Print node values for set number 2 after drift corrections Node 83910104 Loop Original Reading Drifted Val 16 181 3130 391 3307 377 16 181 3130 262 3307 349 17 182 3130 374 3307 350 17 182 3130 199 3307 353 31 381 3130 513 3307 304 31 381 3130 448 3307 278 44 581 3130 279 3306 963 44 581 3130 147 3306 880 12 Node connections analysis amp levelling Loop adjustment and misclosure statistics Adjustments between loops within a GMLS INTREPID makes an interpolated correction to all readings based on discrepancies between readings at stations with more than one observation within each loop 12 0a CreateNodeListFromLoops for set number 2 Number of Nodes from CreateNodeListFromLoops 10 12 1 NodeConnectionLevelling for set number 2 Loop connection search commenced not by time 16 18 only node
49. forms Library Help Top gt gt To perform gravity corrections 1 Choose Gravity Transforms from the Process menu 2 Inthe Mode dialog boxes specify the required settings see Gravity mode settings for details 3 In the Gravity Transforms dialog box i Gravity Tansfoms TT A Select Gravity Operation Note projection conversion for Latitude is automatic C Theoretical Gravity C Earth Tide Correction Free Air Anomaly Reverse Free Air Anomaly C Simple Bouguer Anomaly C Reverse Simple Bouguer Anomaly C Eotvos Correction Velocity from Eotvos Correction Browse Output Data Base to Hold Processed Gravity Values C Intrepid 4 5 sample_data cookbooks gravity land des2 Survey9705 lt lt Prev Finish Cancel 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 10 Library Help Top lt 4 Back gt Specify the gravity dataset for correction Select the correction that you require Choose Finish 4 INTREPID asks you for the required input and output fields see below for details INTREPID does not ask for a field name if there is a corresponding valid alias 5 INTREPID displays the current settings if any to use in the calculation Please Choose Calculate Reversed Free Air Anomaly Datum IGSN71 Yes No If you wish to change the settings choose No to cancel gravity correction and then modify the gravity settings as requi
50. g for repeat stations The INTREPID database format is very flexible The primary focus is its ability to handle groups of fields associated with a profile With the classic random point nature of a regional gravity database the default key fields such as StationNumber may conatin many duplicate readings as this field does not have to be a primary key In the standard field loop reduction process the final principal facts process does reduce the readings back to just one entry for each station This function gives you the ability to reorder the data rows to force all the readings for each station to be in order when viewed in a spreadsheet or dumped via export to an ASCII file Sort Groups Indexed Fields Sort Keys Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 37 Library Help Top 4 Back gt Controls in this dialog box Controls Description Sort groups Function name Indexed fields Choose the field s that you want to sort the random records in the database by eg StationNumber Sort keys This is the chosen field s prior to the sort being actually undertaken Spatial query Parent topic Gravity corrections T54 Gravity data is collected often regionally in temporal loops and spatial radom points You may suspect that data in one region has some sort of a drift or error and you wish to find the outlier
51. gravity free air correction obsgrav Subtract Subtract theoretical free air FreeAir gravity correction Elevation Input field obsgrav Latitude Elevation Output field FreeAir Free air correction formula Here is the formula for free air correction using the full formula expressed as a vertical gradient For POTSDAM and IGSN71_AGSO 5g 3 086 h For IGSN71 GRS67 5g 3 08768 0 00440 sino h 0 000001442 h For ISOGAL80 5g 3 086 h 7 3 10 8 h For WGS84 5g 3 083293357 0 004397732 cos h 7 2125 10 7 h For GA07 GRS80 5g 3 087691 0 004398 sino h 7 2125 107 h Where dg is the free air correction to be subtracted in ums per metre h is the height of the gravity meter above the ellipsoid p represents degrees of latitude 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 13 Library Help Top 4 Back gt Correction for the mass of the atmosphere Mass of atmosphere is not included in theoretical gravity for datums older than WGS84 thus there is no need to correct for it when calculating a free air anomaly This correction is automatically subtracted from the normal gravity For POTSDAM IGSN71_AGSO IGSN71 ISOGAL80 O8atm For WGS84 stations above sea level p 1047 0 116 O8 atm 8 7e For WGS84 stations below sea level d8atm 8 7 For GA07 GRS
52. gt INTREPID User Manual Library Help Top Gravity corrections T54 17 4 Back gt Input field Latitude line bearing and E tv s correction Output field craft velocity WARNING Latitude field Line bearing field Calculated velocity Eotvos field Gravity units The E tv s correction is in units of milligals INTREPID computes the craft velocity in units of knots Sample processing report Calculating velocity from Eotvos gravity for all data base points uuu DU Milligals Gravity constants for various datums Parent topic Utility gravity transforms cookbook gravity datasets Survey9705 Latitude cookbook gravity datasets Survey9705 bearing cookbook gravity datasets Survey9705 velocity cookbook gravity datasets Survey9705 Eotvos The following table shows the constants used in theoretical gravity formulas Datum ay ag ag Ro 1930 amp POTSDAM amp ISOGAL65 formula coefficients POTSDAM 9780490 0 0 0052884 0 0000059 6371229 3154 1967 amp ISOGAL84 formula coefficients IGSN 71_AGSO 9780318 46 0 0053024 0 0000058 6371031 5014 IGSN 71 9780318 456 0 005278895 0 000023462 6371031 5014 World Geodetic System 1972 amp WGS80 formula coefficients ISOGAL80 9780332 7 0 005278994 0 000023461 6371008 7714 World Geodetic System 1984 amp WGS84 formula
53. h a radius of 167 km from the station Select this option if your survey covers a wide area This only applies to scalar terrain corrections Calculate Scalar Terrain correction This is the default setting INTREPID calculates the terrain correction for the vertical component of gravity Calculate Full Tensor correction Number of Calculation Rings Id you select this option INTREPID calculates a full tensor terrain correction together with all components of the gravity vector Note that the gravity gradient tensor is in the ENU system Note that tensor terrain corrections compute a forward model of the gravity tensor based on the DTM at each observation point This is different to scalar correction which computes the effect of the deviation from the infinite slab or spherical cap approximation You must be licensed for Gravity 2 to use this option These are the rings of terrain influence surrounding the observation point Specify a range between 1 and 5 Choosing fewer rings provides less coverage but faster processing Choosing 5 rings gives maximum coverage and maximum accuracy but slower processing The radius of the area processed approximately doubles for each outer ring if you use default settings Remember that most of the terrain influence occurs in the inner rings close to the station Primary Cell Size Controls the prism cell size which is used to model the terrain surface This parameter depends on th
54. he station data records shown are from the imported loop data where Heading Description Station Number Station number Index GMLS number Loop number Reading number within loop Dial Raw field gravity measurement as read from the gravimeter The data is now calibrated and scaled Gravity Corrected observed gravity field For stations with multiple reading contains the average only Stage 2 Data Reduction of Loop Data Intrepid Gi File Tools Spatial_Query Settings View Hel Reduce Loop data to final Gravity transforms Terrain correction Create Tensor from Inline Crossline Create Inline Crossline from tensor The next step is to apply another 8 steps including loop levelling to produce a principal facts dataset from the field data There is also a tie in to one or more absolute base stations using a least squares drift algorithm to estiamte the observed value Free Air and a Bouguer together with an error estimate where more than one occupation of a gravity station was undertaken The overall accuracy of the survey is also estimated Follow the wizard prompts You come to the point where the initial Loop Database is requested below Choose Finish Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 8 Library Help Top lt 4 Back gt T Databases Required for Gravity Field Processi Gravity Loop Da
55. hensive report is created every time this option is run A full explanation of all the options is recorded in this report Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 31 Library Help Top lt 4 Back gt Control Description Treatment of elevation Interestingly the accuracy of the survey observation data height of the gravity observation station often is out of sorts with the DTM grid so the option exists to locally adapt the DTM to include the local survey heights However this may not work and you may have to settle for the DTM view of the elevation at the station to avoid pimples Local elevation If you want to use the local observation of interpolation method elevation and mix this with the DTM this requires a local interpolation two methods are available inverse distance squared and a MINQ Complete Bouger anomaly advanced options Parent topic Here is finally where the ring dimensions are finalised The radius of the inner ring is Process menu 16 cellsize This inner ring is always carefully modelled with high resolution prisms and the option for sloping top prisms does make quite a difference r _ ie Advanced Options Terrain Bottom RL 0 000 Radius in cells of Ring 1 18 Radius in cells of Ring 2 32 Radius in cells of Ring 3 64 Radius in cells of Ring 4 256
56. ibrary Help Top lt 4 Back gt the slab The gradient tensor response of a Bouguer slab is thus identical to zero everywhere and the concept of a simple Bouguer correction is not applicable in the tensor case Instead a forward model of the terrain has to be calculated to account for effects of topography on the gravity tensor As with the scalar case the terrain surrounding the gravity station is divided into prisms The prisms extend from a reference level usually the geoid or ellipsoid to the terrain elevation cf the figure below In the innermost ring sloping top prisms are used for high accuracy whereas flat top prisms are used in the outer rings to speed up the computation A density is assigned to each prism and the tensor terrain correction at a gravity station is given as the sum of the gradient tensor response from all prisms inside the concentric rings Reference Level With the evaluation of the tensor terrain correction a forward model of the full gravity vector is also calculated Note that the vertical component of the gravity vector is different to the value from the scalar terrain correction The former is the response of a complete forward model whereas the latter accounts for the mass missing from or in excess of an infinite Bouguer slab Finally the tensor terrain correction has to be subtracted from the tensor data to remove the effect of topography This can be done using the spreadsheet editor N
57. ields created by the Terrain Correction calculations The default setting is Standard See INTREPID gravity point datasets R28 for details of fields Tare Detection Limit Loop Adjustment Limit Repeat Reject Difference v Skip Earth Tide Correction v Strict View Of Nodes Density gt Gravity Meter Drift gt Report Detail gt v Standard Output Datum Complete vwY Options in this menu Options Description Standard Geoscience Australia has defined a standard set of fields for the principal facts gravity stations available using the GADDS web based data delivery system This is powered by INTREPID JETSTREAM Complete Optional extra gravity fields can also be generated by the processing within this tool when doing field data reduction Output datum Parent topic The spatial XY datum can be changed for output It does not have to be the same as Settings menu the input spatial datum Select the datum you require from the Select Datum dialog box For instance all the land based survey loops maybe recorded using a GPS and Latitude Longitude pairs At the very end of the processing you may wish to present the principal facts in a projected map format with an Easting and a Northing Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top View menu Parent topic Gravity corrections T54 Drift rate Parent topic Vie
58. inspect the language for extra hints as to what new functions or undocumented functions are available within this and every other tool Example of duplicated processes in the old and new syntax are distributed at v5 0 and we also routinely put these same processes through the automatic batch testing proceedures GOOGLE parsers are pretty good at reporting syntax errors down to line number and column gt gt To create a task specification file with the Gravity Corrections tool 1 Specify all files and parameters 2 If possible execute the task choose Apply to ensure that it works 3 Choose Save Options from the File menu Specify a task specification file INTREPID adds the extension job INTREPID creates the file with the settings current at the time of the Save Options operation For full instructions on creating and editing task specification files see INTREPID task specification job files R06 files gt gt To use a task specification file in an interactive Gravity Corrections session Load the task specification job file File menu Load Options modify any settings as required then choose Apply gt gt To use a task specification file for a batch mode Gravity Corrections task 1 Type the command gravity exe with the switch batch followed by the name and path if necessary of the task specification file For example if you had a task specification file called surv_034 job in the current directory you would use
59. itioners generate anomaly numbers that are difficult to reproduce as a simple mistake has been made in choosing the right parameters Please review amp choose appropriate settings Gravity Datum Type C POTSDAM IGSN71 IGSN71_AGSO C ISOGAL80 WGS84 C GA07 Output gravity units C milliGals umj s 2 microGals Gravity Acquisition Environment Land Marine Airborne Lake Ice Next gt gt Cancel Select Type of data to import AGSO Gravity Field Data C SCINTREX CG3 Gravity Field Data y Select detailed acquisition environment C SCINTREX CG5 Gravity Field Data Land surface l Allow Close Repeats at same station Land subsuifacs Next gt gt Cancel Next gt gt EE 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Library Help Top Settings and formats Gravity corrections T54 25 4 Back gt Setting or format Description AGSO format You are prompted for data file names output report file name and output database names Scintrex formats You are prompted for data file names output report file name and output database names Survey Number Used to extract just that survey number from the data file Survey Suffix Only relevant for the formal AGSO import you may ignore it Override meter settings regarding coordinate type Usually the values coming out of the meter are showing LatLong though in other cases they ma
60. land Surface gravity acquisition so choose Land Surface The field data can also be presented in various pre defined formats One is the AGSO gravity field format which is future proof by reqyuiring data to be in a flat ASCII file and also requiring all the necessary data to be in just one file Choose AGSO Gravity Field Data 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 4 Library Help Top 4 Back gt Process to loop data Parent topic Data reduction and network adjustment Library Help Top The following sequence of 8 processing steps are applied to the data Position Data Check Control Data Check Calibration Calculation Loop Data Check Locate Nodes Loop Ties Locate Global Nodes Repeat Nodes Check Data Structure Integrity Check ON OORA WO ND After the import process is finished INTREPID displays a report file to the screen We recommend you check the report carefully In particular scroll to the bottom of the report file and ensure that all 8 processing steps were applied to completion Bad data records time reversals excessive tares duplicate loop numbers can all cause the processing sequence to stop prematurely If this is the case you must go back to the input data and resolve the problem before proceding further After successfully completing the data import the gravity tool creates the following point datasets Survey_ControlDB DIR This da
61. lculated using a weighted least squares fit to the nodes with an outlier rejection criteria A 2nd order drift rate curve is derived The area under this curve is found by integration and this is the model of the drift adopted Long term polynomial drift is the default setting Tare Detection Limit Loop Adjustment Limit Repeat Reject Difference v Skip Earth Tide Correction v Strict View Of Nodes Density gt Gravity Meter Drift gt J Long term polynomial Report Detail gt gt y Short term linear Database Layout gt gt Ignore Repeats for Short Output Datum Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 43 Library Help Top Report detail Parent topic Settings menu Library Help Top 4 Back gt Options in this menu Options Description Long term Use a long term view of the metre drift as modelled in a polynomial piecewise polynomial drift curve to help level the survey Short term linear A short term linear drift curve is considered adequate for most surveys As you lean towards doing geodetic quality work switch to long term drift modelling Ignore repeats Variability at a station can distort gradients in the drift curve so trun off You can select brief or verbose processing reports The default setting is Brief See Gravity processing reports for details of the brief imp
62. lds to conduct your required calulation you will get a summary pop up describing what you are attempting to do Please check and verify that what this reports is what you intended to do C Theoretical Gravity Free Air Anomaly C Simple Bouguer Anomaly C Eotvos Correction r x Select Gravity Operation Note projection conversion for Latitude is automatic Browse Output Data Base to Hold Processed Gravity Yalues C intrepid V4 5 sample_data cookbooks gravity land des2 Survey9705 C Earth Tide Correction C Reverse Free Air Anomaly C Reverse Simple Bouguer Anomaly Velocity from Eotvos Correction lt lt Prev Finish Cancel Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Controls in this dialog box Gravity corrections T54 29 lt 4 Back gt Control Description Select gravity operation Choose one of the 8 options above Output database Any database can be used to manage manipulate gravity observations The importance of these calculator functions is that data from any source and age can have reverse forumulae applied say reverse out of Potsdam then go forward to ISOGAL This also applies to the moving platform Eotvos correction Complete Bouger anomaly Parent topic Intro text Process menu Gravity 1 Lic Estimates Gz F Calculate Full Tensor Correction Airbor
63. lt 4 Back gt INTREPID User Manual Gravity corrections T54 40 Library Help Top lt 4 Back gt e Loop adjustment limit Repeat rejection difference Skip Earth tide correction Strict view of nodes e Density Gravity meter drift Report detail e Database layout Output datum Tare detection limit Parent topic A tare is an unacceptable difference between data acquired at successive stations It Settings menu may be caused by a meter being knocked or dropped and causes the subsequent readings to be higher or lower than before The Tare Detection limit is the maximum acceptable tare If a tare exceeds this value INTREPID insert a warning in the processing report file ES Max Tare OK Cancel Controls in this dialog box Controls Description Maximun tare The default is 20 mGal and this comes from experience in the field You would like to know if your meter appears to have been bumped from one session to the next Loop adjustment limit Parent topic The Loop Adjustment Limit is the limit of error for network adjustment corrections Settings menu The loop adjustment stops when the maximum change for an iteration is less than the specified limit The default value is 0 01 mGal Maximum Loop Adjustment Residual 9 0000 OK Cancel Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 41 Library
64. ne tensor subsample l Earth Curvature Correction Infinite slab replaced by spherical cap of radius 166 735 km Calculate Scalar Terrain Correction amp reninConecin os Fa Terrain correction using DTM amp Hammer method Gravity 2 Lic Estimates gravity tensor amp Gx Gy Gz Applicable for land airborne amp marine cases nb Airborne tensor supports subset amp interpolation Number of Calculation Rings 1 2 SE Ca C5 Primary Cell Size 100 0 Density Land 2 670 Density Marine Sediments 2 290 Density Sea water 1 027 Browse Advanced Options Gravity Database C Intrepid v4 5 sample_data cookbooks gravity land des2 Survey9705 DIF Browse Digital Terrain Model Grid Browse Output Report File Name terrain rpt Include Obs Pt in DTM Treatment of Elevation Observation Data C Use elevations from Gravity Observations Re estimate Elevation from DTM at Obs Pt Local elevation interpolation method Inverse distance Minimum curvature lt lt Prev Finish Cancel M Report terrain calculations to ASCII report Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Library Help Top Controls in this dialog box Gravity corrections T54 30 q Back gt Control Description Earth curvature correction For a scalar terrain correction the
65. ns_At_ Observation true Add_Obs_Elevations_To_ DTM true LocalInverseDistanceInterpolator true UseSlopingTopPrisms true Number_CPUs 2 this tests multi threading Properties Density Land 2 67 density to use in terrain calcs Gravity processing reports Parent topic This section contains annotated processing report samples for import loop reduction Gravity and terrain correction The Gravity tool also generates reports for individual corrections corrections See the description of the individual corrections earlier in this manual T54 ae a for individual correction sample report listings The INTREPID Gravity tool generally appends reports to the current processing report file In some cases it enables you to specify the file name for the processing report and continues to append reports to this file throughout the session If you do not specify a report file name it uses processing rpt except for terrain correction its default report name is terrain rpt Report files are always in the INTREPID current directory current directory when you launched the Gravity tool In this section e Gravity data import Report header Summary of the dataset characteristics e 1 Position data e 2 Control gravity data 3 Gravimeter calibration loop data 4 Gravimeter loop datasets e 5 Node list 6 Global ties nodes Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manu
66. ort and loop reduction report The verbose processing report includes Section la Check Print of positions The station number latitude longitude and height for each station Section 10 Earth tide corrections The Loop no Station no latitude longitude elevation time GMT Earth tide and adjusted gravity for each station Section 11 2 Final Drift Control Adjustments Drift control data for each loop sequence Section 12 2 Loop Adjustments Further detail about the loop adjustments Tare Detection Limit Loop Adjustment Limit Repeat Reject Difference v Skip Earth Tide Correction v Strict View Of Nodes Density gt gt Gravity Meter Drift gt b Report Detail gt a v Brief Database Layout gt gt Verbose Output Datum 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 44 Library Help Top lt 4 Back gt Options in this menu Options Description Brief The default report type is brief and is usually adequate for all needs Verbose If a survey is giving you trouble and will not level very weel try turing on the extra reporting Database layout Parent topic INTREPID writes a standard set of fields to the gravity datasets The field names Settings menu follow the ASEG standard naming convention All supporting fields are populated directly by the program The Complete form of the layout contains additional f
67. ote The full gravity vector and the gravity gradient tensor are calculated in the ENU coordinate system i e the x axis points east the y axis points north and the z axis points up You have to convert the tensor terrain correction first before you can subtract it from your gravity gradient tensor data if the latter is expressed in a different coordinate system such as NED north east down or END east north down Computing a terrain correction Parent topic Terrain correction Library Help Top From the Process menu select Terrain Correction anomaly File Tools Spatial_Query Settings View Help Reduce Loop data to final Gravity transforms Terrain correction Create Tensor from Inline Crossline Create Inline Crossline from tensor 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity corrections T54 21 Library Help Top lt 4 Back gt The Mode box requires you to choose appropriate settings for the gravity Datum units and survey environment After you select the correct modes the main dialog box appears Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Library Help Top Library Help Top Parameters Gravity corrections T54 22 q Back gt Parameter Description Earth Curvature Correction Converts the geometry for the correction from an infinite slab to a spherical cap wit
68. r sided flat top prisms The Gravity and magnetic potential components and tensor gradients for the mass above missing to the side of your observation point is computed We only report the gravity components and tensor Tensor units are always Eotvos regardless of what you want for the vertical component The algorithm is by Holstein and is written up in Geophysics The size of the model is dependent on the distance from the gravity observation There are five possible observation density rings specified by the user as radii The number of models increases by a factor of two in each ring so that at maximum observation denisty 0 there will be 256 models for every model at the lowest density eg 4 The sign convention for elevation is heights above sea level are positive and bathymetry depths should always be negative This is your responsibility The elevation used to calculate the correction for any cell is the average elevation of the cell This is calculated by gridding the centre of all the unit cells that comprise the cell This involves alot of gridding but ensures a very accurate result The firstrad and lastrad variables define which of radii will be calculated The minimum radii is 0 and the maximum is 4 The radii pairs describe the observation density eg 10 50 50 250 250 1000 1000 5000 5000 20000 As gravity effect decreases as the square of distance a scheme where the cell sizes reduce every doubling of distance is
69. red see Settings menu for details To continue choose Yes To cancel choose No INTREPID creates the new field in the gravity dataset and appends a processing report to the current processing report file If you have not specified a report file name during the current INTREPID session it is named processing rpt by default You can e View the processing report using a text editor Use the Spreadsheet Editor to view the new data e Use the Visualisation tool to view the data graphically See Steps 2 and 3 of the complete Bouguer worked example in Gravity field reduction and correction C08 for details Theoretical gravity Parent topic Utility gravity transforms Library Help Top The theoretical gravity also called normal gravity is based on a mathematical model of the earth s gravity field It takes into account that the earth is an ellipsoid rather than a sphere and therefore the force of gravity changes with latitude Each ellipsoid model has a corresponding gravity datum INTREPID uses the latitude and datum to create a theoretical gravity field Calculate Theoretical Latitude theoretical gravity gravity field Units Datum Input field Latitude Output field Theoretical gravity theograv The effect of latitude is removed by subtracting the theoretical value of gravity from the observed values This process of subtraction is also known as a 2012 Intrepid Geophysics 4 Back gt INTR
70. riety of density values to minimize the correlation of the observed gravity signal with the terrain response This principle applies even more so for gradiometry as from experience 80 of the measured signal is usually due to the terrain response Gravity tool licensing Parent topic Terrain correction If you are licensed for Gravity 1 you can calculate normal vertical gravity terrain corrections for land airborne and marine environments If you are licensed for Gravity 2 you can calculate normal vertical and horizontal component gravity terrain corrections as well as full tensor terrain corrections for land airborne and marine environments Scalar terrain corrections Parent topic Terrain correction Library Help Top When simple Bouguer gravity anomalies are calculated for land gravity data the gravity station is assumed to be located on a horizontal plane This assumption is wrong if there is local varying topography In this case a terrain correction must be applied to the data The terrain correction algorithm divides the region surrounding a gravity station into concentric rings of increasing radii Each ring labelled A B and C in the figure below is subdivided into cells These cells are smallest in the innermost ring and increase in size with each ring similar to the well known Hammer method for terrain corrections A mean elevation is assigned to each cell and prisms are formed by projecting the cells up
71. s 16 Final Values Simple Bouguer Anomaly Terrain type land Density 2 670 Gravity datum IGSN71_AGSO Station Latitude Longitude Observed StdDev No Height Vert_Offset Free Air Bouguer 83910104 35 29180 149 13793 979603 310 0 0000 44 565 000 565 00 9 7995 53 4221 97050001 34 98653 149 02575 979573 630 0 3017 23 613 030 613 03 20 9889 47 6071 97053000 34 92311 149 13862 979579 171 0 2667 7 551 991 551 99 13 0920 48 6739 97051001 34 91766 149 17099 979579 775 1 556 609 556 61 15 5854 46 6973 97051002 34 93752 149 20110 979582 801 1 559 904 559 90 17 9376 44 7138 97051003 34 97068 149 22058 979581 946 1 576 656 576 66 19 4305 45 0953 97051004 34 99010 149 26379 979584 837 1 579 199 579 20 21 4519 43 3585 97051005 34 99623 149 22436 979582 031 1 586 229 586 23 20 2932 45 3038 97051006 34 97846 149 18874 979567 771 1 638 322 638 32 23 6218 47 8043 97051007 34 99529 149 16110 979564 220 1 654 965 654 97 23 7743 49 5140 97051008 34 94436 149 15306 979572 780 1 589 025 589 02 16 3217 49 5882 97051009 34 88974 149 13475 979560 296 1 645 721 645 72 25 9815 46 2725 97051010 34 85413 149 13500 979566 550 1 604 947 604 95 22 6806 45 0110 After the Reduce Loop data process is finished INTREPID displays the appended report file on the screen Again we recommend that you check the report thoroughly Sections 11 and 12 contains precision statistics computed after drift and after loop Library Help Top 2012 Intrepid Geophysics
72. t that point is non Null You can use this as a way of limiting where you want calculation to be done for your survey C Data Input reporting Geoid GA07 Cell 1000 00 is the minimum sub cell size Cell 16000 00 is the maximum sub cell size Your input Density is 2 670 g cc Output units are Milligals Terrain correction calculated using sloping top triangles Calculating standard VERTICAL GRAVITY terrain correction Calculating LAND based Terrain Correction Gravity DB opened D Intrepid cookbook gravity datasets Longford Co ordinates are in TMAMG55 proj AGD66 datum XX Y Y Hts D Intrepid cookbook gravity datasets Longford Elevation Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 63 Library Help Top lt 4 Back gt Digital Terrain Model grid opened D Intrepid cookbook gravity datasets Longford_Terrain in TMAMG55 proj AGD66 datum DTM grid nulls 127536 Radii for gravity terrain correction estimates around each observation Ring 1 start 0 0 end 1600 0 Ring 2 start 1600 0 end 3200 0 Ring 3 start 3200 0 end 6400 0 Ring 4 start 6400 0 end 25600 0 Ring 5 start 25600 0 end 102400 0 Report on Local Improvement Estimation scheme for Digital Elevation data Allocating swap space for gravity observation requirements 6421 points or 4 MBytes Reading observed data file The number of observed records inc nulls 21 Number r
73. tabase that has been imported previously Each loop must start and end with a node unless there is just one loop Browse Gravity Loop Data Base C Intrepid v4 5 sample_data cookbooks gravity land des2 Survey9705_LoopDB DIR Control Gravity Reading Database Node s that have a control or tie in reading available Browse Control Gravity Observations Database C Intrepid V4 5 sample_data cookbooks gravity land des2 Survey9705_ControlIDB DIFI Output Gravity Principal Facts Database Loop are adjusted drift corrected tides amp precision calc Browse Output Directory name To Hold Processed Gravity databases C Intrepid V4 5 sample_data cookbooks gravity land des2 Survey9705 DIR Browse Output Report File Name processing rpt lt lt Prev Finish Cancel Reduce loop data to final data Parent topic After the Data Import phase you can reduce the loop data to final data From the Data reduction Process menu select Reduce Loop data to final INTREPID asks you for another and network ee eee Mode review and for output dataset names The following sequence of processing steps are then applied to the data e Meter Correction uses the gravimeter calibration file Earth Tide Correction e Meter Drift Correction Node Levelling network adjustment Global Adjustment of Loopsets Apply Meter Scale Factor Global Adjustment tie in to Control Report Final Value
74. taset contains the Control gravity station details Survey_LoopDB DIR This dataset contains the gravity survey data The structure of this dataset reflects the order of the aquisition loops The gravity tool displays the field loop data that has just been imported INTREPID uses the following symbols to display the gravity dataset rs Gravity station location of a gravity measurement 4 Ties nodes base station or station common to more than one loop x Click a station to view the data for that station INTREPID displays the station data in a message box Repeated links between stations Usually shown as white lines 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 5 Library Help Top Library Help Top 4 Back gt StationNumber 97051233 Index 0 18 16 Dial 3016 367 StationNumber 97051233 Index 1 2 18 Dial 3016 102 StationNumber 97051233 Index 2 20 21 Dial 2956 081 StationNumber 97051233 Index 3 2 11 Dial 2955 704 StationNumber 97051233 Index 4 18 14 Dial 2999 212 where Heading Description Station Station number Number Index GMLS number Loop number Reading number within loop Dial Raw field gravity measurement as read from the gravimeter The data is uncalibrated and unscaled Note this numbering system begins at zero not one A station with an index of 0 1 2 is third station of the secon
75. tation of the rotating GGI s within the Lockhead Martin instrument use the advanced alias assigment in the ProjectManager tool to set these fields in your database to the corresponding alias This must be done before you can successfully recreate the tensor field from its parts Choose this option specify the output tensor field name and the option to form the tensor takes very liuttle time to compute Note that FTG data from this instrument is universally declared and formed in a left handed coordinate reference frame with East North Down The tensor training coyurse contains a great trouve of practical information about the details of all the gradient instruments in use today Specify Output Tensor Field Enter New Field Name Existing Fields Ok Cancel 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 33 Library Help Top lt 4 Back gt Controls in this dialog box Controls Description Enter new field name required name for the formed tensor field Existing fields Use the alias facility as described above to tie the observed inline and crossline fields whatever they are named to their function Create inline or crossline from tensor Parent topic This is the reveres process to the option above Given a FTG field decompose it back Process menu to its inline and crossline parts with the carousel angle held constant to an azimuth of 0 degrees
76. ter is also very characteristic 3 levels of error analysis are undertaken in the following 16 steps of data reduction Preliminary set up Parent topic Data reduction and network adjustment For non Scintrex gravimeters each meter has a table of manufacturer supplied gravimeter calibration values also called instrument factors These must be included in a special INTREPID configuration file The file is INTREPID installation folder config gravimeter cfg Scintrex meters use a scale factor of 1 0 as a special case and the gravimeter configuration file is not used Data import formats Parent topic Data reduction and network adjustment Data import Parent topic Data reduction and network adjustment Library Help Top The field data must be in one of the following three formats AGSO format Scintrex format CG3 Scintrex format CG5 For details of the file formats see Gravity import file formats R27 From the File menu select Survey Import Wizard Select the data format to import Process Tools Spatial_Query Settings View Help Open Gravity Database Survey Import Wizard Dump Check CG5 Merge new survey with master database Edit Gravity Database Aliases Load Options Save Options Quit The Mode box requires you to choose appropriate settings for the gravity Datum units and survey environment See Gravity mode settings The next section mostly applies to
77. the gravitational constant 6 67428 x 1071 m kg s Mohr and Taylor 2001 p is density in tm typically 2 67 tm h is the ellipsoid height in metres of the station R R h the radius of the earth at the station R is the mean radius of the earth 6 371 008 771 4 km GRS 80 value from Moritz u amp are dimensionless coefficients with following definitions 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 15 Library Help Top lt 4 Back gt u 1 3 n n where n h R A 1 3 d fS 82 f 8 k p m In n f 5 f 8 k 2 where d 3cos2a 2 f cosa k sin2a p 6cos asin a 2 4sin a 2 Ro R m sin2acos a 3kf n 2 sin a 2 sin a 2 a S R with S Bullard B Surface radius 166 735 km Sample processing report Calculating Simple Bouguer Anomaly Observed gravity field D gravity import_data Survey9533_0710 Bouguer Latitude field D gravity import_data Survey9533_0710 Latitude Station Elevation field D gravity import_data Survey9533_0710 Elevation Meter Elevation field NO METER ELEVATION DATA BEING USED Bouguer anomaly field D gravity import_data Survey9533_0710 Bouguer2 Gravity datum IGSN71 Terrain type land Density 2 670 Gravity units Milligals Reverse simple Bouguer anomaly Parent topic INTREPID calculates the observed gravity from the simple Bouguer gravit
78. to have access to examples of land gravity meter calibrations AGSO Gravity Field Processing AGSO gravity field data in ASCII file with blocks of data separated by KEY WORDS e g LOOP GRAVIMETER POSITION Data integrity check report file Browse AGSO Gravity Field Data Browse Output Report File Name lt lt Prev Finish Cancel Controls in this dialog box Controls Description AGSO gravity field data An ASCII file that contains field observations from a calibration exercise so there are many repeats and possibly 2 or more meters occuping several well known and observed gravity stations Output report standard report file for capturing results Most gravity field data format is designed to accomodate earth tides The value of gravity at any point on the Earth varies during the course of the day because of the tidal attraction of the sun and the moon INTREPID automatically applies Earth tide corrections during the data reduction and network adjustment process INTREPID uses the Longman formula Earth Tide corrections may also be calculated manually and the results written to a report file Select Earth Tides from the Tools menu Specify the location and time interval Specify the name of the report file 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 35 Library Help Top lt 4 Back gt Latitude decima Longitu
79. u do not have one You can also specify an optional gravity units field Press Skip if you do not have one Now specify the output terrain correction field name The default name is compl_boug Choose OK INTREPID starts calculating the terrain correction After INTREPID has computed the terrain correction you may use the INTREPID spreadsheet editor to add it to the simple Bouguer anomaly to create the complete Bouguer anomaly 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 24 Library Help Top lt 4 Back gt Note however if you are dealing with tensor data the tensor terrain correction has to be subtracted from the full tensor data See Complete Bouguer anomaly worked example in Gravity field reduction and correction C08 for more information on the technical capabilities Gravity mode settings Parent topic Gravity corrections T54 Library Help Top You can use the help menu to display help text on the topics shown in the menu illustration below You can change a number of INTREPID settings during a Gravity processing session Every time a dataset containing Gravity data is referenced you must explicitly confirm the following essential information This ensures that the units geoid ellipsoid and equations that you are expecting to use are in fact the ones chosen While elements of gravity data reduction appear simple it is a known fact that many pract
80. vimeters or perhaps a difference with an observable linear trend when the ties are arranged chronologically Each GMLS has so far been treated independently Examine the global nodes and work out best fit adjustment for the whole Populate secondary fixed nodes for GMLS 1 Global Node Value 83910104 3307 3203 97050001 3277 5042 97053000 3283 0206 97053001 3275 2682 97051036 3280 9495 97051068 3240 9753 97051069 3251 4699 97051083 3242 7234 97051126 3244 2991 97051134 3136 2051 97052137 3162 3516 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 59 Library Help Top lt 4 Back gt 97051233 3186 5019 97051277 3253 1758 97052037 3227 5754 14 Applying meter scale factor to all loop data You can specify a scale factor for each gravimeter usually 1 INTREPID applies this scale factor to each set of loop data See Gravimeter calibration R29 for details about scale factors and calibration 15 Calculating adjustments to global nodes Adjustments to tie each GMLS to the network control station INTREPID compares the global tie node values to the network control station The global tie has a known gravity INTREPID adjusts all ties accordingly 15 Calculating adjustments to global nodes Doing tie in to control value at fixed stations Primary Fixed Node adjustment to GMLS 1 Fixed Node Adjustment 83910104 976295 990 Mean 976295 989737 Adjusted second
81. w menu Library Help Top Gravity corrections T54 45 4 Back gt Cancel Options in this dialog box Options Description Select datum Select the Ellipsoid datum you wish to have the data calculated in Options for graphically displaying the drift of the gravimeter View Help Drift Rate Drift Standard Drift Normalised Screen dump to Postscript In this section e Drift rate e Drift standard e Drift normalised Screen dump to postscript This graph shows the drift rate for each tie in the first GMLS of the dataset in this case G132 amp G651 This includes ALL ties nodes the ties at the beginning and end of each loop loop ties and other ties within the GMLS Use the Next and Previous buttons to view other GMLS in the dataset The horizontal axis represents the time days since the survey began shown over 128 days The vertical axis is the drift divided by the time difference dial reading hr 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 46 Library Help Top Drift standard Parent topic View menu 4 Back gt l i TO O O T E O i Drift Rate for gravimeter G132 Drift Rate for gravimeter G651 5 59 DriftRate DriftRate 5 48 4 046 g ax ER x XAA R g x CK OH XXa x soaa OGRE E W a xxx ppt 4 42 96 63 Time days 0 00 7 96 15 91 23 87 Time days
82. xample WGS84 Calculate the theoretical gravity using this preferred datum 4 Use the spreadsheet editor to subtract the revised theoretical gravity from the observed gravity Theoretical gravity formula Older gravity datums approximate normal gravity using truncated polynomial expansions Recent gravity datums use Somiglianas closed form solution For POTSDAM and IGSN71_AGSO Gn a 1 ag sind ag sin 2 For IGSN71 and ISOGAL80 G a 1 ag sino ag sin For WGS84 and GA07 GRS80 1 a sino a G oo 1 a sing Where G is theoretical gravity in ums p represents degrees of latitude 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 12 Library Help Top 4 Back gt Aj a9 ag are constants listed in the table of constants See Gravity constants for various datums Ro is the mean radius of the earth Free air anomaly Parent topic Utility gravity transforms Library Help Top The free air correction compensates the observed gravity for the fact that it was measured at a given height above or below the datum It assumes however that there is nothing but air between the geoid or ellipsoid and the observation point INTREPID calculates the free air correction from the elevation and observed gravity fields and the terrain type The free air anomaly is calculated as follows FreeAir obsgrav theoretical
83. y anomaly Utility gravity field transforms obsgrav Bouguer simple Bouguer correction free air correction theoretical gravity Input field Bouguer Latitude Elevation Output field obsgrav This is useful if you have data that is missing an observed gravity field and want to process it using different settings or corrections Sample processing report Calculating Simple Bouguer Anomaly Reversing Simple Bouguer anomaly to observed gravity Bouguer anomaly field D cookbook gravity datasets Survey9705 Bouguer Latitude field Survey9705 Latitude Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 16 Library Help Top lt 4 Back gt Station Elevation field Survey9705 Elevation Meter Elevation field NO METER ELEVATION DATA BEING USED Output gravity field D cookbook gravity datasets Survey9705 obsgrav Gravity datum IGSN71 Terrain type land Density 2 670 Gravity units Milligals E tv s gravity correction Parent topic The E tv s correction is required for gravity measurements taken from a moving ca gravity platform The meter s velocity over the surface adds vectorially to the velocity due to ransftorms the earth s rotation varying the centrifugal acceleration and hence the apparent gravitational attraction Use this correction for marine and airborne survey data before applying Latitude and FreeAir corrections Latitude c
84. y be a local grid or UTM coordinates Gravity Datum Type This is one of Potsdam IGSN71 IGSN71_AGSO IGSN71_NZ ISOGAL80 WGS84 GAO7 INTREPID uses the standard International Formulae and there are references to regional tie ins You can easily define new Datums as required Please contact technical support with details of any other required tie ins Output Gravity Units Gravity Acquisition Environment INTREPID uses either mGal ums or wGal Specify the units used in the data you intend to import or process before you start the process The default unit is mGal One milligal mGal 10 pms INTREPID uses different processing parameters for land marine and airborne gravity data You can select Land Marine Airborne Lake or Ice The default environment is Land Specifying input and output files Parent topic Gravity corrections T54 Library Help Top You can use the help menu to display help text on the topics shown in the menu illustration below Introduction to input and output files Intrepid Gravity File Process Tools Spatial_Query Settings View Help Open Gravity Database Survey Import Wizard Dump Check CG5 Merge new survey with master database Edit Gravity Database Aliases Load Options Save Options Quit In each case INTREPID displays an Open or Save As dialog box Use the directory 2012 Intrepid Geophysics 4 Back
85. y the terrain correction is positive everywhere This is not necessarily true for airborne and marine terrain corrections Please note that INTREPID calculates the scalar terrain correction using the common convention that the vertical component of gravity is positive the z axis is pointing down A full description of the terrain correction method used in the INTREPID software can be found in the following reference Application of terrain corrections in Australia by N Direen T Luyendyk Geoscience Australia see Application of terrain corrections in Australia C13 Tensor terrain corrections Parent topic Terrain correction Library Help Top The algorithm to calculate the terrain correction for full tensor gravity gradiometry data is essentially the same as in the scalar case However there is one distinct difference It is well known that for land based gravity measurements the simple Bouguer correction overestimates the gravity effect of the material between the gravity station and the reference level geoid or ellipsoid in the presence of significant relief The terrain correction accounts for this by calculating the effect of missing or excess mass due to variations in topography On the other hand the gravity effect of a infinite Bouguer slab is independent of the location and height of a gravity station on or above 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity corrections T54 20 L
86. your DTM grid Press the third Browse button to optionally select a name for your output report file You hve the option of writing the calculated terrain values to the report file Treatment of Elevation Observation Data For ground gravity data if the elevations calculated from the DTM differ significantly from those measured with the gravity readings the option exists to replace all station elevations by those interpolated from the DTM grid for calculating the terrain correction This is the default setting Note Do not replace observation elevation if you are processing airborne or marine data Include Observation Point in DTM The elevation at each gravity station location must be estimated by interpolating from the DTM grid You have the option of including the gravity reading elevations along with the DTM data for the interpolation process The default setting is not to do this Local elevation interpolation method The interpolation of the elevation can be done using the method of either inverse distance default or minimum curvature Press Finish You are now prompted for a flag field This can be any field in the dataset which contains valid data If the field contains any Null values INTREPID skips the terrain calculation for those records INTREPID asks you for the ground elevation field relative to the geoid You can also specify an optional meter elevation relative to the geoid Press Skip if yo

Gravity corrections (T54)

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