Gravity field reduction and correction (C08)
Contents
1. Here is a sample job file for the INTREPID Gravity tool Process Begin Name gravity InputFieldData D Intrepid sample_data cookbooks gravity datasets SCINTREXAMPLE2 DAT ReportFile processing_sci_1l rpt LoopDataBase D Intrepid sample_data cookbooks gravity datasets Survey9807_LoopDB_23 AbsoluteDataBase D Intrepid sample_data cookbooks gravity datasets Survey9807_ControlDB_23 OutputCorrectedField D Intrepid sample_data cookbooks gravity datasets Survey9807_23 Parameters Begin OutputUnits Milligals Checking Begin Tolerance 0 01 Repeat_Tolerance 0 2 Maximum_Tare 20 0 Checking End Terrain Begin Cell Size 100 0 Density 2 67 Max Circles 2 Earth Curvature Correction No Terrain End Levelling Begin Max Loop_Iterations 20 Maximum_Loop Change 6 277438562e 66 DriftType Long_Term_Polynominal IgnoreInternalRepeats No NodesInTime No Levelling End Method Begin RuntType SCINTREXField Scintrex Begin SurveyNumber 9807 Projection GEODETIC Datum WGS84 Scintrex End Method End Datum Begin Output AGD66 Datum End TerrainType LAND SURFACE DatumType IGSN71 Verbose_Report No Complete DataBase Fields No Parameters End Process End Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 6 Library Help Top 4 Back gt Worked example overview This worked example demonstrates the following operation2. By appending each new week s data to the import file you can build up long term drift and multiple base ties This increases the accuracy of the reduced data 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 3 Library Help Top 4 Back gt Comprehensive Processing The gravity processing options include e Gravimeter calibration Earth tides for a specified time E tv s corrections e Free air anomaly e Simple Bouguer anomaly e Terrain corrections e Theoretical gravity options e Merge with master dataset Output The Gravity tool produces detailed processing reports that include warnings about possible tares in your data It stores the corrected gravity values in an INTREPID dataset so that you can process this data using other INTREPID tools Note Gravimeters are prone to have steps in their drift curves Some mechanical hitch or stick catches or releases and causes the subsequent readings to be higher or lower than before These jumps are called tares and may be several gravity units in magnitude It is often difficult to determine where in the loop a tare has occurred unless it 1s large enough to be obvious Note Terrestrial gravity data that 1s available to the public in Australia is generally accurate to 0 03 mGal The accuracy depends on the method of acquisition How the INTREPID Gravity tool works Parent topic Gravity field reduction and co
3. Corrections INTREPID finds the difference between the readings at the start 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 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 loop ties and other stations with more than one observation Node values INTREPID reports the drift results for each tie node showing original and corrected values Node Connections Analysis Adjustments within each loop and Levelling 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 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 14 Library Help Top 4 Back gt a E Global Adjustment 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 gravimeters or perhaps a difference with an observable linear trend when the ties are arranged chronologically Applying Meter Scale factor Youcan specify a scale
4. Intrepid Gravity 3 7 release cut 251 static Drift Rate for gravimeter 310219 451 Time days Close 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 25 Library Help Top lt 4 Back gt Standard Drift window intrepid Gravity v3 7 release cut 2751 static Normalised Drift window i Intrepid Gravity v3 7 release cut 751 static Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 26 Library Help Top 4 Back gt Gravity field reduction and correction using Scintrex CG 5 format data worked example Parent topic In this worked example we perform the following operations Gravity field reduction and correction C08 Steps to follow Start Gravity 1 Import Scintrex CG 5 format binary data 2 Check the processing report Field data import 1 Inthe INTREPID Project Manager locate the Cookbooks gravity dataset directory install_path sample_data cookbooks gravity KS 2 Start the Gravity tool From the Gravity menu choose Field Reduction E 3 Check the following menu items in the Settings menu and change 1f necessary pA e Specify Density gt Land 2 670 4 From the File menu choose Import CG5 The Scintrex CG5 gravity survey data import dialog box will appear Specify the name of the CG 5 format data file CG5_sa
5. INTREPID will assume GPS height as the observation height Tip Earth Curvature Correction Select this check box if your survey covers a wide area and your DTM grid is highly precise INTREPID applies an additional term to the correction at distances greater than 167km from the observation point Tip Minimum Cell Size This controls the prism cell size which is used to model the terrain surface This parameter will depend on the resolution of the DTM grid It also controls the radius of each ring Specify the DTM grid cell size to start with Increasing the size increases the ring radii The result will be less accurate but it will run faster Tip Density Specify the density underlying the survey The default value for land is 2 67 For marine surveys specify 1 64 This is 2 67 1 03 Tip Number of Calculation Circles These are the rings of terrain influence around the observation point Specify a range between 2and 5 Specify 2 for faster processing but less coverage Specify 5 for maximum coverage but slower processing The radius of the area processed approximately doubles for each new ring Keep in mind that most of the terrain influence occurs in the inner rings Choose Finish 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 29 Library Help Top Check for errors in the terrain correction Library Help Top lt 4 Back gt Specify the input and output
6. has collected data from large regional surveys performed by many operators using many gravimeters Large Surveys Sets The Gravity tool allows you to process large gravity data sets that could involve multiple gravimeters and operators Gravimeter Loop Sets GMLS A key INTREPID concept is the gravimeter loop set GMLS that is defined as one operator gravimeter combination A person may collect gravity data with a particular gravimeter over a number of days or weeks The operator gathers the data in gravity loops where the start and finish of the loop are at a station loop tie that has a known observed gravity value The Gravity tool uses one or more main gravity base station as a control base tie The gravity stations of a GMLS are tied back to this main station Loop drift control All gravity meters drift Meter drift is caused by mechanical stresses and strains in the mechanism as the meter 1s moved subjected to vibration knocked unclamped reset subjected to heat stresses etc with INTREPID s innovative approach to drift control you can correct The short term linear drift on each loop This is the traditional approach The long term drift polynomial or linear for each GMLS INTREPID corrects this drift using a polynomial drift function calculated from a least squares fit through drift rates obtained from tie points You can view and query the drift windows to identify stations with abnormal drift data
7. map It shows the reduced and corrected gravity dataset Intrepid Gravity 3_5 cut 62 static File Process Tools Settings View Click a station to view the gravity values Click a station to view the station data and calculated gravity values INTREPID displays the station data in a message box Intrepid Gravity Staton umber 97051233 Index 0 18 16 Dial 3016 367 Gravity 979482492 StationNumber 97051233 Index 1 2 18 Dial 3016 102 Gravity 979482411 StationNumber 97051233 Index 2 20 21 Dial 2956 081 Gravity 979482 801 StationNumber 97051233 Index 3 2 11 Dial 2955 704 Gravity 979482 363 Staton umber 97051233 Index 4 18 14 Dia 2999 212 Gravity 979482 632 OK Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 16 Library Help Top View drift rates for each gravimeter loop set Library Help Top lt 4 Back gt 16 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 The 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 gravit
8. Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 28 Library Help Top Library Help Top lt 4 Back gt For treatment of elevation observation data select Use Elevations from Gravity Observations Complete Bouguer Gravity Processing Complete Bouguer gravity Correction Terrain comecion using OTM amp Hammer method Gravity 1 Lic Estimates Gr Gravity 2 Lic Estimates Gx Gy Gz Full Gii tensor Applicable for land airborne amp marine cases Earth Curvature Correction infinite slab replaced by spherical cap of radius 166 735 km Calculate Scalar Terrain Comection Calculate Full Tensor Correction Number of Calculation Rings 2 C3 CA FG Primary Cell Size Density 2 6 0 Advanced Options Browse Gravity Database Browse Digital Terain Model Grid Browse Output Repor File Name Tresiment of Elewstion Observation Data Use elevations trom Gravity Observations Ae estimate Elevation from OTM al Obas Pi M Include Obs_Pt in OTM Local elevation interpolation method Inverse distance C Minimum cunrature cPrev Finish Cancel Tip Submarine Terrain Correction Select this check box for marine gravity surveys INTREPID will assume sea level as the observation height It will use the Nettleton marine correction model Tip Airborne Terrain Correction Select this check box for airborne gravity surveys
9. Specify completeBouguer as the new Field Name Bouguer terraincorr as the Initial value for the field Field Name COmpleteBouquer Data Type Signed 16BitInteger Siqned32Bitlnteger C IEEE4ByteReal EEE amp ByteReal Logical C Character C Byte W idth Group By l Create as Formula Initial Bouguersterraincorr OK Cancel i Choose OK INTREPID displays the new completeBouguer column If the field name does not display properly change the column width as follows e Choose Format Column from the Format window e Select the completeBouguer column Specify a width of 15 characters Column Format Select Column Stationno Longitude X latitude Bouguer Specify Format Width characters is D ecimal Places Apply Cancel 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 32 Library Help Top 4 Back gt View the 7 To use the Visualisation tool to view the complete Bouguer anomaly let oar Open the Visualisation tool and load the Survey 9705 dataset anomaly Assign the completeBouguer field to point size and colour a Intrepid NT Visualisation v3 4 cut 61 File Line Display Point Display Grid Display Window Help DATASET T ype x ee Line FID1 FID2 Dist m 0 0 Value km oom In zoom Out House Mode C Query C Zoom References Parent topic N G Direen with contr
10. factor for each gravimeter to all loop data usually 1 INTREPID applies this scale factor to each set of loop data If the scale factor value in the import file is equal to 1 and you supply calibration data in the import file INTREPID performs a calibration process and adjust the scale factor value Calculating adjustments to Adjustments to tie each GMLS to the network control global nodes 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 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 grav see INTREPIDINTREPIDGravity processing reports in Gravity corrections T54 for annotated excerpts from a processing report Check that INTREPID was able to close each loop in Section 12 and that the misclosure converged each time e INTREPID identified a primary fixed tie to use for each GMLS in Section 15 If not INTREPID creates a reference point by averaging all ties in the GMLS Choose Close when you have finished examining the processing report INTREPID saves this processing report in the current directory Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 15 Library Help Top 4 Back gt View the 15 View the Gravity window gravity dataset
11. fields Prompt Field Specify the Observed Gravity Field Bouguer Specify optional observed gravity units field Skip Select the Ground Elevation Field if not Elevation aliased Specify the Output Terrain Correction Field terraincorr Tip Intrepid does not use the Observed Gravity field in the calculation It is used as a data flag Tip If the Elevation field is defined as an Alias in the isi file you will not be prompted for it Tip For marine surveys use the Bathymetry field for Ground Elevation INTREPID will start the processing and create the terrain correction field terraincorr When the process is finished close the Gravity window Now examine the report file in a text editor Tip If the report shows errors adjust the parameters and repeat the calculation The report will give you some background information about terrain corrections and will summarise the parameters and files in use It will include a table of the circle radii for the gravity terrain correction estimates around each observation This is important to know since the extent of your DTM grid will determine the extent of the terrain correction coverage you can achieve Scroll down to the end of the report Asummary for the Z Y Z and TC parameters for the gravity dataset are given TC is the computed terrain correction terraincorr Terrain corrections for land base surveys should all be positive 2012 Intrepid Geophysics 4 Back gt IN
12. land marine or airborne gravity data Please review amp choose appropriate settings Gravity Datum Type C POTSDAM IGSN71 IGSN 71_AGSO C ISOGAL amp 0 WGS84 Output gravity units milliGals umfs 2 microGals Gravity Acquisition Environment Land Marine Airborne Next gt gt Cancel Choose Next Scintrex Gravity Field Processing SCINTREX gravity field data import Need the scintrex field data file and the gps field data file Survey Number 3807 Survey Suffix Heter Yertical Offset m Browse SCINTREX Gravity Field Data F DFA_softwarefcookbook gravity SCll Browse GPS Field Data F DFA_software cookbook gravity SC Browse GPS Field Data Projection Name Data willbe converted to Geodetic on input MGA50 Browse GPS Field Data Datum Name GDA494 Browse Output Report file name 960 7_processing rpt lt lt Prev Next gt gt Cancel Specify 9807 as the Survey Number This is the YY MM of the survey acquisition date Tip For this worked example we chose arbitrary values for the survey number and suffix based on the survey date YYMMDD GA allocates all Australian survey numbers Contact GA if you require more specific information 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 22 Library Help Top Library Help Top lt 4 Back gt The survey number become
13. Ge tone i ft gee ANT FIDi 0 Sts i S Pir fear ie a Pea apne FID Oe Al treacle eee int Ne viens TF Dist m 42496 542 ad Pur Oat erie re ee roty 3 gd Ae sree i Z Value 0 229233 ae bis ar Hye wpb Sete EGS Dy Paty Km p anh i iss x j se es af Ne Fi ALT MAA Oat Rere ET s es rr N F pee eT e T As F eaa F x ae ie Ste tek Hyon eA ee oom In rd re H t io i R a Ty A wi i Hn TEN TE ri oiy 74 G 2i me as ig s itt SAE if A ae has z b a 1 0 2 hate f gt a i 0 a a Ce ie ue i r ai Ar E m i SN Plea ani amp he nee Zoom Previous X EMES Stee 4 ha House Mode Query Zoom or Trace 5 If you have time rerun the terrain correction this time using more or less circles of influence or try changing the prism cell size Note the effect these new parameters have on the computed terrain correction Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 31 Library Help Top Calculate the complete Bouguer anomaly Library Help Top lt 4 Back gt 6 Tocalculate the complete Bouguer anomaly use the Spreadsheet Editor to add the terrain correction to the simple Bouguer anomaly as follows completeBouguer Bouguer terraincorr In the Project Manager select the Survey 9705 dataset Choose Spreadsheet from the Editors menu Choose Create New Field from the Edit menu
14. INTREPID User Manual Gravity field reduction and correction C08 1 Library Help Top 4 Back gt Gravity field reduction and correction C08 iep The goal of gravity field reduction and correction is to remove from the observed gravity data any components that would be present if we were dealing with a simple and virtually homogenous earth Whatever remains will be anomalous Ideally anything that is left over is the result of density inhomogenities due to local geology and perhaps of local exploration interest The diagram below is a conceptual flow chart for the gravity correction process adapted from Chapin 1996 Observed Gravity corrected for drift earth tide etc Homogenous Earth Static Measurement at surface of Datum Moving platform Observed Subtract Static tV ravit E tv s measurement g y correction Subtract theoretical gravity Measurements above or below datum Pens Free air anomaly correction Rock above datum Subtract Simple Bouguer Bouguer correction anomaly Variable terrain Labels on arrows Add terrain Complete indicate the correction Bouguer assumptions made anomaly The Gravity Corrections tool imports data from a gravimeter and applies gravity field reduction and correction techniques In this tutorial e Gravity field reduction and correction C08 How the INTREPID Gravity tool works e Notes about the worked examples e Gravity field reduction and correction using AG
15. SO format data worked example e Gravity field reduction and correction using Scintrex CG 3 format data worked example e Gravity field reduction and correction using Scintrex CG 5 format data worked example e Complete Bouguer anomaly worked example References Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 2 Library Help Top lt 4 Back gt Location of sample data for Cookbooks Where install_pathis the path of your INTREPID installation the project directory for the Cookbooks sample data is install_path sample_data cookbooks For example if INTREPID is installed in C Program Files Intrepid Intrepid4 5 then you can find the sample data at C Program Files Intrepid Intrepid4 5 sample_ data cookbooks For information about installing or reinstalling the sample data see Sample data for the INTREPID Cookbooks in Using INTREPID Cookbooks R19 For a description of INTREPID datasets see Introduction to the INTREPID database G20 For more detail see INTREPID database file and data structures R05 Advantages of the INTREPID Gravity tool Parent topic Gravity field reduction and correction C08 Advantages Library Help Top The INTREPID Gravity tool has evolved from collaborative work with Geoscience Australia formerly AGSO the Australian Geological Survey Organisation Geoscience Australia GA
16. TREPID User Manual Gravity field reduction and correction C08 30 Library Help Top lt 4 Back gt 3 Use either the INTREPID Project Manager or the Dataset Statistics tool to view a histogram of the terrain correction field Note that while the mean value is low there are some high outlier values This may indicate that there are some data errors in the gravity point dataset a Intrepid Histogram v4 0 Release Candidat ioj xi File Options Help J xl Basic Statistics For terraincorr Value Minimum 24 462043 Maximum 513 702905 0 30306 Mean 25 761956 Frequenc Ma absbey 31 646202 variance 3002 085794 UCU Std Dey 54 791293 Cumulative RMS 828 992121 33 90970 Skew 4 261184 kurtosis 23 565442 Samples 1041 Mulls 0 CurveType unknown 4 Grid the terraincorr field using the INTREPID Gridding tool and saving the grid as terrcorrgrid View the grid in the INTREPID Visualisation tool It should be coherent You can also use the Visualisation tool to view the gravity point dataset and assign the value of the terrain correction to the symbol colour Intrepid isualisation 4 0 Release Candidate Cut 15 File Line Display Point Display Grid Display Polygon Display Window Help loj x GRID LINE PN T SurveyS 705_ 1 0 POLY La he Ae a Suse oe z SER Signal teraincorr ee H cia it he a z ANS ae m r451l20 2 A Bare E Ar e ae ieie ET A Y 6171763 5 St ei a ce pee ar Se Line
17. ain correction 1s added to the simple Bouguer anomaly to produce the Complete Bouguer anomaly HH For a complete reference to the technique s used to apply the terrain corrections in the gravity module the interested reader is referred to Application of terrain corrections in Australia written by N G Direen with contributions from T Luyendyk The Intrepid cookbook C13 contains a full reprint of this reference paper Steps to follow Calculate the In this worked example we shall use AGSO Survey 9705 the Goulburn regional infill terrain survey which we imported and corrected in the first part of this tutorial However it is correction important to note that INTREPID cannot use geodetic data for this process Therefore we have prepared a copy of the dataset with projected coordinates The dataset is called Survey9705_1 and is in the cookbooks gravity directory The DTM we will use is called goulburn_dtm ers It is a requirement that the DTM and the gravity point dataset have the same Datum and Projection Both datasets have been converted using the INTREPID Projection Conversion tool to WGS84 TMAMG55 1 From the Gravity tool Process menu choose Complete Bouguer anomaly Terrain Corr Specify the following Minimum Cell Size 150 Number of Calculation Circles 4 Gravity Database Survey9705_1 Digital terrain model grid goulburn_dtm ers Output report filename 9705 terrain rpt Library Help Top 2012 Intrepid
18. binary dataset See Data for gravity cookbook examples C12 for a full description of data Location of sample data for Cookbooks Where install_pathis the path of your INTREPID installation the project directory for the Cookbooks sample data is install_path sample_data cookbooks For example if INTREPID is installed in C Program Files Intrepid Intrepid4 5 then you can find the sample data at C Program Files Intrepid Intrepid4 5 sample_ data cookbooks For information about installing or reinstalling the sample data see Sample data for the INTREPID Cookbooks in Using INTREPID Cookbooks R19 For a description of INTREPID datasets see Introduction to the INTREPID database G20 For more detail see INTREPID database file and data structures R05 Use of INTREPID visualisation tools The Gravity tool displays certain aspects of the data Most illustrations in this worked example show the data as displayed by the Gravity tool You can also use the Visualisation tool to display the data Tip Assign the observed gravity to point colour and the elevation or an anomaly field to point size Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 5 Library Help Top 4 Back gt Batch mode operation As well as using the gravity tool interactively you can also perform all operations in these worked examples using job files in batch mode
19. brary Help Top 4 Back gt Repeat the 22 Exit from the Gravity tool and repeat steps 1 16 with the week 1 amp 2 data Use the steps for Week 1 following file names amp 2 data AGSO gravity field data AGSO Week1 amp 2 DAT Output Report file 9705 _Week1 amp 2_ processing rpt 4650 Gravity Field Processing 4650 gravity field data in ASCII file with blocks of data separated by KET WORDS e g LOOP GRAYIMETER POSITION Data integrity check report file Browse AGSO Gravity Field Data D Intrepid cookbook qravity datasets AGSO_Week142 _DAT Browse Output Report file name 9705 Wreekl amp 2_ processing_rpt lt lt Prev Finish Cancel After import the Gravity window displays the Week 1 amp 2 data as follows a Intrepid Gravity 3 5 cut 62 static Oy x File Process Tools Settings View 23 Exit from the Gravity tool Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Gravity field reduction and correction C08 20 4 Back gt Gravity field reduction and correction using Scintrex CG 3 format data worked example Parent topic In this worked example we perform the following operations Gravity field reduction and 1 correction D C08 3 Steps to follow Start Gravity 1 Field Reduction Import 4 Scintrex data file Library Help Top Import and reduce a set of loop data Check the processi
20. d during an earlier session Tip Step 1 Choose Reduce Loop to Final Tip Step 2 Specify the two import datasets the dataset to hold the corrected values and the report file name Do not use File Open Gravity Dataset Choose Finish 13 INTREPID does the following e Redisplays the Gravity window e Appends the Gravity Processing Report to the existing processing report Displays the report using a text editor Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 13 Library Help Top 4 Back gt 14 Scroll down to view the Gravity Processing Report The Gravity Processing Report contains the following sections ee 5 Node List A tie node is a station with readings in more than one loop Ties nodes are important cross reference points for corrections Global Nodes Ties nodes common to more than one Gravimeter Internal Loop repeat These are stations with multiple readings in one loop stations only These points are useful cross reference points for corrections Data Structure Check This section reports Start and finish station Loop ties nodes Possible tares in the data 9 Meter Corrections A list of each GMLS that INTREPID corrects using the gravimeter calibration file Earth Tide A list of each GMLS that INTREPID corrects using the internally stored Earth tide model Gravity Drift Corrections Adjustments within each loop
21. enu pd bii INTREPID displays the Standard Drift window The graph displays the drift for loop set each loop in the first GMLS Tip 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 Intrepid Gravity 3 5 cut 62 static 3307 11 Standard Drift for gravimeter G 132 Click an end point aoe tie to view station ii ee details st 3229 54 i K ed x x 3190 75 TE va SS 3151 96 k a 3113 17 0 00 TETH 15 59 23 38 31 17 Time Previous A line segment with a steep slope indicates a large drift rate between the start and end of the loop Click on the x at the end of a line segment to view the loop tie node station number and dial reading Intrepid Gravity 3 5 cut 62 static Station 37051069 Drit 3251 2671 OK Choose OK to close the message box Choose Next to view the standard drift for the next GMLS Close the Standard Drift window Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 18 Library Help Top View normalised drift for each gravimeter loop set Library Help Top 19 20 21 lt 4 Back gt Display the Normalised Drift window From the View menu choose Drift Normalised INTREPID display
22. ibutions from T Luyendyk Application of terrain corrections Gravity field in Australia Geoscience Australia Projects National Gravity Databases terrain reduction and corrections correction C08 Alice S Murray Ray M Tracey Best Practice in Gravity Surveying Geoscience Australia 2001 Library Help Top 2012 Intrepid Geophysics 4 Back gt
23. id Gravity v3 7 release cut 251 static g 2 joj x File Process Tools Settings wiew Help Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 24 Library Help Top Reduce dataset and calculate corrections View drift windows Library Help Top 4 Back gt From the Process menu choose Reduce Loop Data to Final You do not need to change any of the files or paths Databases Required for Gravity Field Processing Gravity Loop Data Base that has been imported previously Each loop must start and end with a node Browse Gravity Loop Data Base D Intrepid cookbook qravity datasets Survey9807_LoopDB_23 Control Gravity Reading Data Base Node s that have an control reading available Browse Control Gravity Observations Data Base D Intrepid cookbook gravity datasets Survey980 _ControlIDB_2i Output Gravity Principal Facts Data Base Loop are adjusted drift corrected tides amp precision calc Browse Output Data Base to Hold Processed Gravity Values D Intrepid cookbook qravity datasets Survey9807_23 Browse Output Report file name 960 7_processing rpt lt lt Prev Finish Cancel When you select the drift windows from the View menu INTREPID displays the following graphs Since there is only one GMLS in this survey each drift window displays the drift for the entire dataset Drift Rate window
24. ing the INTREPID Import tool Reduce Loop Data to Final also validates the data repeats sections 5 8 in the processing report file 7 INTREPID does the following e Displays a message indicating that the Import has completed e Displays the Gravity window Generates a report of the import and data checking process e Displays this report the Gravity Field Data Checking Report using a text editor 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 9 Library Help Top 4 Back gt 8 View the report The Gravity Field Data Checking Report contains the following sections 1 Report header Position Summary of the dataset characteristics Data Control Gravity Data List of loop network control stations Gravity Meter Calibration Calibration data is optional Loop Data Gravity Meter Loop For each GMLS this section lists Datasets e Gravimeter details e Operator details e A summary for each loop 5 Node List A tie node is a station with readings in more than one loop Ties are important cross reference points for corrections Global Nodes Ties nodes common to more than one gravimeter Internal Loop repeat These are stations with multiple readings in one loop stations only These points are useful cross reference points for corrections 8 Data Structure Check This section reports Start and finish station Ties nodes in the loop e Possible ta
25. mple sgd INTREPID will generate a report and display this report SCINTRES CG5 gravity survey data import Choose which scintrex survey data block Hote no GPS field data is entered with this option Survey Number 0 Survey Suffix Meter Vertical Offset m Browse Create Intrepid Gravity DataBase a Browse Output Report file name F DFA_softwarefcookbook gravity CG5_sample rpi OK Cancel 5 Enter the Survey Number and Survey Suffix 6 Click Browse to create an Intrepid gravity database Enter the name of the database and click OK 7 Click OK on the data import dialog box Intrepid will import the data and inform the user that the import is complete Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 27 Library Help Top 4 Back gt Complete Bouguer anomaly worked example Parent topic The INTREPID complete Bouguer anomaly option calculates a terrain correction for Gravity field gravity It uses a digital terrain model DTM grid to calculate the terrain correction reduction and for each gravity station correction C08 This correction attempts to account for local topography rather than assuming that the gravity station is sitting on a horizontal plane In areas of high relief these corrections can be quite high In Australia gravity terrain corrections can be as high as 25 meals The terr
26. ng report Correct the raw data as required and import again if necessary Reduce the data and apply gravity corrections In the INTREPID Project Manager locate the Cookbooks gravity dataset directory install_path sample_data cookbooks gravity Start the Gravity tool From the Gravity menu choose Field Reduction Check the following menu items in the Settings menu and change if necessary Specify Density gt Land 2 670 Tip The settings menu enables you to change a variety of settings The default settings are appropriate for this worked example If you need to change the settings do this before importing or opening a dataset See Gravity mode settings in Gravity corrections T54 for details From the File menu choose Survey Import Wizard Specify Scintrex Gravity Field Data as the file type Select Type of data to import C AGSO Gravity Field Data SCINTREX Gravity Field Data Next gt gt Cancel Choose Next 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 21 Library Help Top Library Help Top lt 4 Back gt The gravity Datum Units and Environment settings box will appear Check the following menu items and change if necessary Select Gravity Datum Type gt IGSN71 Select Output Gravity Units gt milliGals Select Gravity Acquisition Environment gt Land Tip INTREPID automatically adjusts its corrections for
27. propriate settings Gravity Datum Type C POTSDAM IGSN 1 IGSN 71_AGS5S0 C ISOGAL amp 0 WGS 84 Output gravity units milliGals umfs 2 microGals Gravity Acquisition Environment Land Marine Airborne Next gt gt Cancel 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 8 Library Help Top Library Help Top 4 Back gt Choose Next 4650 Gravity Field Processing 4650 gravity field data in ASCII file with blocks of data separated by KEY WORDS e g LOOP GRAYIMETER POSITION Data integrity check report file Browse AGSO Gravity Field Data D Intrepid cookbook gravity datasets AGSO_Week1 DAT Browse Output Report file name 9705 Wreekl_processing rpt lt lt Prev Finish Cancel Choose Browse to locate the input file AGSO_Week1 DAT Modify the Output Report file name to include the survey number as follows 9705 Weekl1_ processing rpt Tip If a report of that name already exists INTREPID will append the new processing report to the existing file INTREPID saves the processing report in the directory in which the Gravity tool was launched Choose Finish INTREPID imports the data Tip INTREPID does basic validation data checking and field checking see parts 1 8 in the processing report file You can bypass this step if you start with data that you imported in some other way for example us
28. ravity 1 Field Reduction 2 3 Import week 1 4 data file Library Help Top q Back gt In the INTREPID Project Manager locate the Cookbooks gravity dataset directory install_path sample_data cookbooks gravity Start the Gravity tool From the Gravity menu choose Field Reduction Check the following menu items in the Settings menu and change if necessary e Specify Density gt Land 2 670 Tip The settings menu enables you to change a variety of settings The default settings are appropriate for this worked example If you need to change the settings do this before importing or opening a dataset See Gravity mode settings in Gravity corrections T54 for details Tip At this point you would check that the import file has a valid format See Gravity import file formats R27 for specification From the File menu choose Survey Import Wizard Specify AGSO Gravity Field Data as the file type Select Type of data to import AGSO Gravity Field Data C SCINTREX Gravity Field Data Next gt gt Cancel Choose Next The gravity Datum Units and Environment settings box will appear Check the following menu items and change if necessary Select Gravity Datum Type gt IGSN71 Select Output Gravity Units gt milliGals Select Gravity Acquisition Environment gt Land Tip INTREPID automatically adjusts its corrections for land marine or airborne gravity data Please review amp choose ap
29. res in the data See INTREPIDINTREPIDGravity processing reports in Gravity corrections T54 for annotated excerpts from a processing report Examine Section 8 Data Structure Check to verify that the figures and statistics are reasonable The report flags errors with the symbol Check that e All loops are closed Unclosed loops indicate an integrity problem e Tares are not excessive or unexplained Tip At this point you can edit the import data to restore integrity and remove tares See Gravity import file formats R27 for details of import file format and tips for editing them Tip After you edit the file repeat from Step 3 Choose Import Wizard Choose Close when you have finished examining the processing report INTREPID saves the processing report in the directory in which the Gravity tool was launched Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 10 Library Help Top 4 Back gt View the 9 View the Gravity window It shows the raw loop data that you have just imported gravity dataset INTREPID uses the following symbols to display the gravity dataset map i Gravity station location of a gravity measurement Ties nodes base station or station common to more than one loop Repeated links between stations Intrepid Gravity 3_5 cut 62 static Oo x File Process Tools Settings View Click a
30. rrection C08 Library Help Top The Gravity tool imports a raw gravity data file performs corrections and creates an adjusted and corrected gravity point dataset Three native file formats are supported AGSO format Scintrex CG 3 ASCII format and Scintrex CG 5 binary format In the process of creating the dataset it performs the following adjustments and corrections Instrument drift corrections Earth tide corrections e Loop adjustments by instrument or operator or both e Adjustment by least squares to absolute readings e Calculation of e Free air anomaly Simple Bouguer anomaly You can also use the tool to calculate standalone gravity transformations eg reverse Free air Theoretical gravity Eotvos as well as gravity terrain corrections also known as Complete Bouguer anomaly This tool assumes that all stations have accurate GPS locations and heights We can assist you to process older barometric data Contact our technical support service for further information 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 4 Library Help Top 4 Back gt Notes about the worked examples Parent topic Gravity field reduction and correction C08 The worked example datasets The following worked example datasets are provided AGSO format survey of Goulburn New South Wales Australia Scintrex CG 3 ASCII format dataset Scintrex CG 5 format
31. s Import some data e AGSO format data from a LaCoste amp Romberg gravimeter e Scintrex CG 3 format data from a Scintrex gravimeter Reduce the loop data to one record for each station This will include observed gravity Free air and simple Bouguer anomalies Calculate a gravity terrain correction Use the Spreadsheet Editor to add the terrain correction to the simple Bouguer anomaly to produce the complete Bouguer anomaly The following flowchart gives an overall view of the typical gravity processing steps Raw data Import Point dataset Reduced corrected point dataset with free air and simple Bouguer anomalies Reduce loop data to final Calculate and apply terrain correction Complete Bouguer anomaly Digital Terrain Model grid Gravity field reduction and correction using AGSO format data worked example Parent topic Gravity field reduction and correction C08 Library Help Top In this worked example we perform the following operations oO oo fF W N Import and reduce a set of loop data week 1 Check the processing report Correct the raw data as required Discard the imported data week 1 Append more raw data to the import data file week 1 amp 2 Import reduce and correct the complete set of data 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 7 Library Help Top Steps to follow Start G
32. s a prefix for e The existing station number For example station 282 will now have a station number of 9807282 e The Dataset name Specify 23 as the Survey Suffix This is the DD of the survey acquisition date INTREPID will append _23 to the dataset name If the gravity meter has a vertical offset you can specify it here Choose Browse to locate the Scintrex gravity field data file SCINTREXAMPLE2 DAT Choose Browse to locate the GPS field data file SCINTREXAMPLE2 GPS Choose Browse to change the GPS field data Projection name to MGA5O In the Select Projection box double click on Projection Type TransverseMercator then scroll down and choose Projection MGA5O Choose Browse to change the GPS field data Datum name to GDA94 Tip INTREPID will convert the data it to geodetic coordinates upon import Modify the Output Report file name to include the survey number as follows 9807_processing rpt Tip If a report of that name already exists INTREPID appends the new processing report to the existing file INTREPID saves the processing report in the directory in which the Gravity tool was launched Choose Next Choose Load from the Scintrex Gravity Field Processing dialog box Locate SCINTREXAMPLE2 ABS in the Specify Ascii File With Control Stations dialog box INTREPID displays the list of loop network control stations the file contents Scintrex Gravity Field Processing SCINTREX gravity field da
33. s the Normalised Drift window The graph shows the normalised drift for each loop in the first GMLS 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 Tip 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 Intrepid Gravity 3 5 cut 62 static 5 amp 1 Normalised Drift for gravimeter G1 32 Previous Next Close A line segment that deviates significantly from the polynomial indicates an abnormal drift rate Click on the x at an end of a line segment view the station number at the start or end of a loop Intrepid Gravity 3 5 cut 62 static Station 970521371 OK Choose OK to close the message box Choose Next to view the Normalised Drift for the next GMLS Close the Normalised Drift window Now that you have examined the Week 1 data thoroughly you can make adjustments to the import data using a text editor if required before appending the next week s data Note In this demonstration you won t actually append the data We supply a second import file containing both weeks data 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 19 Li
34. station to view the station data Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 11 Library Help Top Library Help Top lt 4 Back gt Click on a station to view the data for that station INTREPID displays the station data in a message box Te eo cut 6 amp 1 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 Tip Note this numbering system begins at zero A station with an index of 0 1 2 is third station of the second loop in the first GMLS Choose OK to close the message box Tip To output the gravity station diagram to a PostScript file choose from the View menu Screen dump to Postscript INTREPID saves this file in the current directory 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 12 Library Help Top 4 Back gt Reduce dataset 10 From the Process menu choose Reduce Loop Data
35. ta import Add Control Gravity Stations Note this setting for gravity datum overrides all others Gravity datums supported are WGS84 POTSDAM ISOGAL80 IGSN71 IGSN71_AGSO Use the load button to add Edit 3209 979389 26 WGS84l Load lt lt Prev Finish Cancel Choose Finish 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 23 Library Help Top 4 Back gt 7 INTREPID now imports the data Tip INTREPID performs basic validation data checking and field checking see parts 1 8 in the processing report file You can bypass this step if you start with data that you imported in some other way for example using the INTREPID Import tool Reduce Loop Data to Final also validates the data repeats sections 5 8 in the processing report file 8 INTREPID does the following e Displays a message indicating that the Import has completed e Displays the Gravity window Generates a report of the import and data checking process e Displays this report the Gravity Field Data Checking Report using a text editor Carry out steps 9 Carry out steps 7 16 from the AGSO exercise 7 16 from N l AGSO exercise otes e Continue to use report file 9807_processing rpt INTREPID will append the processing report data to this file e Scintrex data is not usually collected in GMLS After import the Gravity window displays the Scintrex data as follows Intrep
36. to Final d calculat l nant i 11 The Datum Units Environment box will appear These settings will be the same as before Click Next 12 The database dialog box will appear You do not need to change any of the files or paths for this example Databases Required for Gravity Field Processing Gravity Loop Data Base that has been imported previously Each loop must start and end with a node Browse Gravity Loop Data Base survey9 05 LoopDB Control Gravity Reading Data Base Node s that have an control reading available Browse Control Gravity Observations Data Base survey9 705 ControlDB Output Gravity Principal Facts Data Base Loop are adjusted drift corrected tides amp precision calc Browse Output Data Base to Hold Processed Gravity Values survey9 705 Browse Output Report file name 9705 Wreekl_processing_rpt lt lt Prev Finish Cancel Tip In this operation INTREPID creates the reduced and corrected gravity point dataset from the ControlDB and LoopDB datasets It performs meter corrections Earth tide corrections gravity drift corrections loop tie drift analysis and levelling It then produces a single record for each station with the following gravity data fields grav Corrected observed gravity for stations with multiple readings this is the average of those readings FreeAir Free air anomaly Bouguer Simple Bouguer anomaly Tip To do a loop reduction on a file you importe
37. y measurement as read from the gravimeter The data is now calibrated and scaled Gravity Corrected observed gravity field For stations with multiple readings INTREPID lists each reading although the dataset will contain the average only 17 Display the Drift Rate window Choose Drift Rate from the View menu INTREPID displays the Drift Rate window The graph shows the drift rate for each tie in the first GMLS of the dataset This includes ALL ties nodes the ties at the beginning and end of each loop loop ties and other ties within the GMLS Tip The horizontal axis represents the time days since the survey began The vertical axis is the drift divided by the time difference dial reading hr Intrepid Gravity 3 5 cut 62 static Drift Rate for gravimeter G1 32 Each point indicates the drift rate for a tie A high rate of drift may indicate a data error 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Gravity field reduction and correction C08 17 Library Help Top 4 Back gt Click a point to view the station number and the dial value Intrepid Gravity v3 5 cut 62 static Station 97051263 Yalue 3233 4451 OK Choose OK to close the message box Choose Next to view the Drift Rate window for the next GMLS When you have finished viewing drift windows close the Drift Rate window View standard 18 Display the Standard Drift window Choose Drift Standard from the View m
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