Gravity corrections available (T54a)
Contents
1. Use the spreadsheet editor to subtract the E tv s correction from the observed gravity field obsgrav to create a new observed gravity field See Complete Bouguer anomaly worked example in Gravity field reduction and correction C08 for an example of using the Spreadsheet tool Other gravity operations Gravity Wizard Use this to start processing gravity data part way through the usual processing sequence Select Gravity Wizard from the Process menu Observed gravity adjustments and corrections File Process Settings View Gravity meter calibration Earth Tides Convert to Wa See Gravity meter calibration Datasets can contain calibration data You can use this data to recalculate the scale factors for a gravimeter Information about scale factors is included in Section 3 of the processing report See Gravimeter calibration R29 for details Select Gravity Meter Calibration from the Tools menu Specify the dataset that contains the calibration data Library Contents Top 4q Back gt INTREPID User Manual Gravity corrections available T54a 9 Library Contents Top 4 Back gt Earth Tides The Sun moon and planets affect gravity at any place on the earth Use this to calculate the tidal corrections for a specified region of the Earth INTREPID uses the Longman formula Select Earth Tides from the Tools menu Specify the location and time interval INTREPID automatically applies Earth tide2. INTREPID User Manual Gravity corrections available T54a 1 Library Contents Top 4 Back gt Gravity corrections available T54a Top The INTREPID Gravity tool can calculate individual gravity corrections or gravity anomalies for a gravity point dataset It creates new fields to store these values Use the reverse anomaly corrections to recreate an obsgrav field for a dataset that has only FreeAir or Bouguer fields Note You automatically create the free air anomaly and the simple Bouguer anomaly when you reduce loop data to final To perform these corrections 1 Check that you have specified the appropriate datum and density settings see Gravity tool settings T54b for details 2 Choose Gravity Transforms from the Process menu Gravity Transforms Select Gravity Operation Note projection conversion for Latitude is automatic Free Air Anomaly C Reverse Free Air Anomaly C Simple Bouguer Anomaly Reverse Simple Bouguer Anomaly C Theoretical Gravity C Eotvos Gravity Correction Browse Output Data Base to Hold Processed Gravity Values D Intrepid cookbook gravityfimport_datafSurvey9533_0710 lt lt Prev Finish Cancel Specify the gravity point dataset for correction Select the correction that you require Choose Finish 3 INTREPID prompts you for the required input and output fields see below for details INTREPID will not prompt for a field name if there is a corresponding valid alias Lib
3. Survey9705 Elevation Meter Elevation field NO METER ELEVATION DATA BEING USED Output gravity field D Intrepid cookbook gravity datasets Survey9705 obsgrav Gravity datum IGSN71 Terrain type land Density 2 670 Gravity units Milligals Eotvos gravity correction Use this correction for marine and airborne survey data before reducing loop data to final The correction compensates for measurements taken from a moving platform It calculates the difference between the angular velocity of the observation platform and that of the rotating earth Latitude calculate line bearing E tv s Eotvos craft velocit correction Units Input field Latitude line bearing and craft velocity fields Output field Eotvos Tip INTREPID currently uses the simple velocity correction We are developing a more complex method using cross correlation Library Contents Top 4q Back gt INTREPID User Manual Gravity corrections available T54a 8 Library Contents Top 4 Back gt Sample processing report Calculating Eotvos gravity for all data base points Latitude field D Intrepid cookbook gravity datasets Survey9705 Latitude Line bearing field D Intrepid cookbook gravity datasets Survey9705 bearing Craft velocity field D Intrepid cookbook gravity datasets Survey9705 velocity Calculated Eotvos field D Intrepid cookbook gravity datasets Survey9705 Eotvos Gravity units Milligals Applying the correction
4. an a sphere Each ellipsoid model has a corresponding gravity datum INTREPID uses the latitude and datum to create a new theoretical gravity field Calculate the or etic al Latitude theoretical gt gravity gravity field Urits datum Input field Latitude Output field Theoretical gravity theograv INTREPID also automatically subtracts the theoretical gravity as one of the steps when it calculates the free air anomaly and simple Bouguer anomaly Library Contents Top 4q Back gt INTREPID User Manual Gravity corrections available T54a 3 Library Contents Top 4 Back gt Sample processing report Calculating theoretical gravity for all data base points Latitude field D Intrepid cookbook gravity datasets Survey9705 Latitude Calculated gravity field D Intrepid cookbook gravity datasets Survey9705 theograv Gravity datum IGSN71 Gravity units 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
5. corrections during the import process Information about the Earth tide correction is included in Section 2 of the processing report Convert to WGS84 Use this to convert a gravity dataset to the WGS84 datum Specify the new gravity dataset name in the Specify Output Dataset dialog box Library Contents Top 4q Back gt
6. ellipsoid 2 Use the 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 CO8 for an example of using the Spreadsheet tool 3 Select your preferred datum from the Settings menu for example WGS84 Calculate the theoretical gravity using this preferred datum 4 Use the spreadsheet editor to subtract the revised theoretical gravity from the observed gravity Library Contents Top 4q Back gt INTREPID User Manual Gravity corrections available T54a 4 Library Contents Top 4 Back gt Free air anomaly 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 sea level 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 gravity free air correction subtract free air correction subtract j gt Freedir datum Units terrain type 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 gradi
7. ent For IGSN71 GRS67 the formula is as follows Robbins 1981 and Fils Butt Hawke 1998 See Free air references in Gravity references R32 Sgh 5h 2g a 1 f m C3f 5m 2 sin 6g a h where equatorial gravity on the ellipsoid 9780318 456 ums f flattening coefficient 1 298 25 semi major axis radius of the ellipsoid 6378160 m centrifugal force at equator g 0 0034498014 latitude height above ellipsoid This can be expressed as Free Air correction 3 08768 0 00440 sin h 0 000001442 h ums per metre Library Contents Top 4q Back gt INTREPID User Manual Library Contents Top Sample processing report Gravity corrections available T54a 5 4 Back gt Calculating Free Air Anomaly Observed gravity field obsgrav Latitude field Station Elevation field Meter Elevation field Output free air field ZZZZ Gravity datum Terrain type Gravity units Reverse free air anomaly D Intrepid cookbook gravity datasets Survey9705 Survey9705 Latitude Survey9705 Elevation NO METER ELEVATION DATA BEING USED D Intrepid cookbook gravity datasets Survey9705 IGSN71 land Milligals 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 obsgra
8. rary Contents Top 4 Back p INTREPID User Manual Gravity corrections available T54a 2 Library Contents Top lt 4 Back gt 4 INTREPID displays the current settings if any to use in the calculation Please Choose Calculate Reversed Free Aw Anomaly Datum IGSH1 Yes No If you wish to change the settings choose No to cancel gravity correction and then modify the gravity settings as required see Gravity tool settings T 54b for details To continue choose Yes To cancel choose No INTREPID creates the new field in the gravity point 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 The processing report does not display automatically but you can locate the file in the current directory You can e View the processing report using a text editor e Use the Spreadsheet Editor to view the new field data e Use the Visualisation tool to view the data graphically Tip Assign the elevation field to point colour and the observed gravity or an anomaly field to point size See Steps 2 and 3 of the complete Bouguer worked example in Gravity field reduction and correction C08 for details Theoretical gravity The theoretical gravity is a mathematical model of the earth s gravity field It takes into account that the earth is an ellipsoid rather th
9. s for example Bouguer267 Bouguer250 Bouguer200 Sample processing report Calculating Simple Bouguer Anomaly Observed gravity field D Intrepid cookbook gravity import_data Survey 9533_0710 Bouguer Latitude field D Intrepid cookbook gravity import_data Survey9533_0710 Latitude Station Elevation field D Intrepid cookbook gravity import_data Survey 9533_0710 Elevation Meter Elevation field NO METER ELEVATION DATA BEING USED Bouguer anomaly field D Intrepid cookbook gravity import_data Survey 9533_0710 Bouguer2 Gravity datum IGSN71 Terrain type land Density 2 670 Gravity units Milligals Library Contents Top 4q Back gt INTREPID User Manual Gravity corrections available T54a 7 Library Contents Top 4 Back gt Reverse simple Bouguer anomaly INTREPID calculates the observed gravity from the simple Bouguer gravity anomaly field 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 Intrepid cookbook gravity datasets Survey9705 Bouguer Latitude field Survey9705 Latitude Station Elevation field
10. v FreeAir free air correction theoretical gravity Input field Output field Sample processing report FreeAir Latitude Elevation obsgrav Reversing Free Air anomaly to observed gravity Free air gravity field FreeAir Latitude field Station Elevation field Meter Elevation field Output gravity field obsgrav Gravity datum Terrain type Gravity units Library Contents Top D Intrepid cookbook gravity datasets Survey9705 Survey9705 Latitude Survey9705 Elevation NO METER ELEVATION DATA BEING USED D Intrepid cookbook gravity datasets Survey9705 IGSN71 land Milligals 4q Back gt INTREPID User Manual Gravity corrections available T54a 6 Library Contents Top 4 Back gt Simple Bouguer anomaly The simple Bouguer correction replaces the air in the Free Air anomaly with rock 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 Add free air correction subtract Boweuer correction Subtract theoretic al gravity Bouguer Units datum terrain type density datum Input field obsgrav Latitude Elevation Output field FreeAir You can experiment with different density settings to create a series of simple Bouguer anomaly field
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