Drift Correction

Use the Gravity Drift Correction dialog (GRDRIFT GX) to carry out the following components of a gravity reduction:

• Instrument scale factor correction

• Earth tide correction

• Instrument height correction

• Drift correction

• Absolute gravity calculation

Gravity Drift Correction dialog options

The dialog box options are displayed for you to confirm all the information required to perform the drift correction. In normal processing, all entries should already be set appropriately.

Database	The name of the current gravity database will be displayed for confirmation. Script: GRDRIFT.DATABASE
Source file	The name of the source file will be displayed for confirmation.
Survey date	The survey date. Script: GRAVRED.DATE set by GRPARM
Operator	The instrument operator. Script: GRAVRED.OPERATOR set by GRPARM
Instrument	The instrument ID (name, number). Script: GRAVRED.INSTRUMENT set by GRPARM
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: GRAVRED.INSTRUMENT_SCALE set by GRPARM
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: GRAVRED.INSTRUMENT_SCALE set by GRPARM
Base station database	The name of the database that contains base station information. See the GRBASEDB GX for information on the base station database. Script Parameter: GRAVRED.BASE_DATABASE
Location database	The name of the location database that contains location. 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 is used to calculate the gravity earth tide correction. Script Parameter: GRAVRED.LOCATION_DATABASE set by GRPARM
Hours to GMT (+ In West)	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 set by GRPARM

Application Notes

This GX assumes that a gravity survey database is loaded. The GRLOAD GX should be used to create the database.

The absolute gravity at base station(s) is determined by looking up the station (or Line and Station if a Line channel exists) in the base station database. If a Station exists in the base station database, it is assumed to be a base station. If a station does not exist in the base station database, it is assumed to be a normal survey reading. The "Type" channel will be set to 0 for base station readings and "1" for survey readings based on the presence of the stations in the base station database.

Base station readings may be repeated any number of times to improve the statistical accuracy of the reading. If base station readings are repeated the readings after applying the terrain correction and reading times are averaged to determining the closure error and drift correction. The resulting drift corrected database will show the closure error next to the first database reading. Note that the averaging process is carried out as part of the reduction and the readings shown in the survey database will not be changed. If you want to remove a base reading from the averaging process, delete the reading by clicking on the reading line in the left column of the spreadsheet and press the delete key.

Drift Correction Formulas

The following are the formulas applied in the drift correction process:

1. Instrument Scale Factor

The instrument scale factor corrects a reading to a relative milligal value based on an instrument calibration. The correction can either be constant throughout the instrument range, or it can be derived from a user supplied calibration table.

Rc = R * S(R)

where,

Rc	corrected reading in milligals
R	instrument reading in dial units
S(R)	scale factor (dial units/milligal), which will be a function of the reading if a calibration table is provided.

2. Tide Correction

If a relative time difference to Greenwich Mean Time is provided on the constants line (via the gm= parameter), all readings are corrected for earth tides due to the position of the sun and the moon at the time and location of the observation. The full formula is too complex to list here, but can be obtained from the Dominion Observatory of Canada. The scale and tide corrected result (Rt) will be stored in the channel "_Gscale" in the survey database.

Rt = Rc + Gt

where

Rt	tide corrected reading
Rc	scale corrected reading from 1.
Gt	tide correction

3. Instrument Height

Each reading is corrected for the height of the instrument above the station or base at which the elevation is measured:

Rh = Rt + 0.308596 * Hi

where

Rh	Instrument height corrected reading
Rt	tide corrected reading from 2.
Hi	Instrument height in metres

 

Note that all readings in the following formulas are assumed to be corrected for the instrument scale factor, optional tide correction and instrument height.

4. Drift Correction

A drift is calculated based on the closure error between the first and last base reading in each loop:

D = [(Rb2 - Rb1) - (Gb2 - Gb1)] / (Tb2 - Tb1)

where

D	drift in milligals/hour
Rb1	Base 1 reading, tide and instrument height corrected.
Tb1	Base 1 time
Gb1	Base 1 absolute G in milligals
Rb2	Base 2 reading, tide and instrument height corrected.
Tb2	Base 2 time
Gb2	Base 2 absolute G in milligals

Note that Rb1, Rb2, Tb1 and Tb2 will be averaged if there are repeat base readings.

5. Absolute Gravity

The absolute gravity is the earth's gravitational attraction at the observed station.

Ga = Gb1 + (Rh - Rb1) - (T - Tb1) * D

where

Ga	Absolute gravity in milligals
Gb1	Base 1 absolute G in milligals
Rh	Instrument height corrected station reading from 3.
Rb1	Base 1 reading
T	Reading time
Tb1	Base 1 reading time
D	drift from 4. in milligals/hour