Simulate Absolute Gravity Stations

Use the Simulate Absolute Gravity Stations option (geogxnet.dll(Geosoft.GX.Gravity.SimulateAbsoluteGravityStations;Run)*) to calculate Earth’s gravity at a given latitude and elevation.

The option is available with the Gravity and Terrain Correction extension under the following menus:

• Gravity > Gravity Corrections
• Moving Platform Gravity > Corrections by Steps

Simulate Absolute Gravity Stations dialog options

Base station database	The input base station sdatabase name. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.BASE_STATION_DATABASE
Line selection	You have the option to calculate the absolute gravity for the displayed line, selected lines or all lines in the database. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY. LINE_SELECTION [D – Displayed line, S- Selected lines (default), A- All lines]
Latitude channel	The input channel containing the stations’ latitudes. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.LATITUDE_CHANNEL
Elevation channel	The input channel containing the stations’ elevations from sea level. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.ELEVATION_CHANNEL
Absolute gravity channel	The output channel in which to save the absolute gravities in milligals. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.ABSOLUTE_GRAVITY_CHANNEL
Add to existing gravity channel	If checked, the calculated absolute gravities are added to the values of the recorded gravity. Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.ADD_TO_GRAVITY_CHANNEL [0- No, 1-Yes]
Gravity channel	This is a contextual entry. If the option Add to existing gravity channel is selected, you are prompted to enter the input gravity channel to be added to the simulated absolute gravity. The output summed values are saved in the database under the channel named "Absolute gravity channel _Gravity channel". Script Parameter: SIMULATE_ABSOLUTE_GRAVITY.GRAVITY_CHANNEL

Application Notes

A base station database contains the absolute gravity value at specific longitude/latitude location(s) of established gravity base stations. The absolute gravity does not change with time and will be the reference base to level the survey data to. The absolute gravity should be in the vicinity of the survey area. Generally, the absolute base station gravity value will be close to the gravity value at that latitude and elevation above sea level. Using the Moritz gravity equation, given the base station height H above sea level at latitude Lat, the gravity value at the station should be approximately:

If there aren’t any stations in the vicinity of the survey to loop through during the survey, you can establish one. Gravity readings are relative to the established base station values. As long as you set properly a base station, you should be able to conduct the survey. Find a flat area away from the known dense geology to establish as your base station. This station will be then your reference station to level the data to.

To establish a base station reading, you can survey the gravity response at the established base station, then correct it for tide and drift, and then run the absolute gravity simulation at that location’s latitude. This is a single value per base station.

The latitude correction is in the order of 980,000 mGals. Prior to applying the latitude correction, gravity data is drift corrected and leveled to the base station absolute reading. If the base station is not in the order of 980,000 mGals, and rather at a much smaller magnitude, applying the latitude correction will yield gravity values in the order of -980,000 mGals, which is a source of confusion.
By allowing to set the base station to a range around 980,000, the final corrected gravity will be within an expected range.