Processing Parameters

Use the Gravity > Processing Parameters menu option (GRPARM GX) to view and modify the gravity project parameters.

If the current database is a "single survey database" (i.e., data is imported from a single gravity instrument dump file or from a single "day" survey file), you will be presented with the Survey Parameters dialog. Otherwise, the Global Survey Parameters dialog will display. The Survey Parameters dialog includes survey specific parameters together with "global" parameters that apply to the entire project.

Survey Parameters dialog options

Database

If the current database is a survey database, this entry simply displays the name of the database for your information.

Script Parameter: GRAVRED.SURVEY_DATABASE

Source file

The name of the original source data file.

Script Parameter: GRAVRED.FILE

Survey date

The survey date.

Script Parameter: GRAVRED.DATE

Operator

The instrument operator.

Script Parameter: GRAVRED.OPERATOR

Instrument

The instrument ID, which is the instrument name or number which designates a particular instrument.

Script Parameter: GRAVRED.INSTRUMENT

Instrument scale factor

The instrument scale factor if you are not using a calibration file. If you are using a calibration file, this entry must be blank.

Script Parameter: GRAVRED.INSTRUMENT_SCALE

OR scale factor file

The name of an instrument scale factor calibration file. If you are using a single scale factor, this field must be left blank. See the GRCALIB GX for information on how to create a calibration scale factor file.

Script Parameter: GRAVRED.INSTRUMENT_SCALE

The script parameter can either be a calibration file name or a scale factor. If the setting is a number, it is assumed to be a scale factor.

Hours to GMT (+ in W)

This is the time difference between the time recorded in the survey database and GMT. This is a positive number in the Western hemisphere. If this field is left blank, no tide correction is applied.

Script Parameter: GRAVRED.GMT_DIFFERENCE

Global Survey Parameters dialog options

Base station database

The name of the database that contains base station information. See the GRBASEDB GX topic for information on the base station database.

Script Parameter: GRAVRED.BASE_DATABASE

Location database

The name of the database that contains location information. See the GRXYEDB GX for information on the base station database.

Leave this entry blank if there is no location database, and the location data is already entered into the gravity survey database. The location data, if specified, will be merged into the survey database as part of the drift correction. The longitude & latitude of each station are used to calculate the gravity earth tide correction.

Script Parameter: GRAVRED.LOCATION_DATABASE

Latitude correction

Select a gravity latitude formula from the list.

The first time the latitude correction is executed, the five formulas defined using Oasis montaj expressions are saved in the file "Gravity_Latitude.lst" located in the folder %USERPROFILE%\Documents\Geosoft\Desktop Applications \etc.

If you would like to provide alternate equation(s), append them in the "Gravity_Latitude.lst" file.

For syntax details, see the Application Notes section under the Latitude Correction topic.

Script Parameter: GRAVRED.FORMULA

Free-air correction

Select an elevation (free-air) correction formula from the list.

The elevation correction formulas are defined using Oasis montaj expressions (the full ellipsoid expression as a function of height and latitude from Heiskanen and Moritz (1967), and the constant gradients 0.308596 mGal/m and 0.3086 mGal/m) and are stored in a file named "Gravity_Free_Air.lst" located in the folder %USERPROFILE%\Documents\Geosoft\Desktop Applications \etc.

You can add other formulas by editing this file. For syntax details, see the Application Notes section under the Free Air Anomaly topic.

Script Parameter: GRAVRED.FREE_AIR

Earth density

The density of the earth in g/cm³ for calculating the Bouguer correction.

Script Parameter: GRAVRED.DENSITY_EARTH

Water density

The density of water to be used for the Bouguer correction for gravity readings over water (on ice or marine surveys). The density of fresh water is 1.0 g/cm³ ; the density of salt water ranges from 1.02 to 1.07 g/cm³.

The "Water" channel is assumed to contain the depth of water below the gravity station for calculating the water correction, which is only applied if a "Water" channel exists.

Script Parameter: GRAVRED.DENSITY_WATER

Ice density

The density of ice to be used for the Bouguer correction for gravity readings on ice. The density of freshwater ice is 0.97 g/cm³ and varies for sea water.

The "Ice" channel is assumed to contain the thickness of ice below the gravity station for calculating the ice correction, which is only applied if an "Ice" channel exists. The depth of "Water" must include the "Ice" thickness.

Script Parameter: GRAVRED.DENSITY_ICE

Survey type

The gravity survey types provided are:

  • Ground survey (including on lake surface)
  • Shipborne survey (marine survey)
  • Airborne survey

Script Parameter: GRPARM.SURVEYTYPE

Application Notes

The GRPRAM GX enables you to view and change the parameters of a gravity project or of an individual gravity survey database. The changes will propagate to all applicable Gravity and Terrain Correction GXs: the associated parameters in the corresponding dialogs will default to the values set through GRPRAM GX.

This GX can be used from a script to change or set any global parameters that may be required by other GXs in a script. All global parameters begin with "GRAVRED".

To change the survey parameters of a specific file from a script, set the "GRAVRED.FILE" parameter to the file name.