Gravity Corrections

Use the Gravity Corrections option from the Gravity > Gravity Corrections menu (geogxnet.dll(Geosoft.GX.Gravity.GravityCorrections;Run)*) to carry out some or all of the following components of a gravity reduction:

• Instrument scale factor correction

• Earth tide correction

• Instrument height correction

• Drift correction

• Absolute gravity calculation

Gravity Corrections dialog options

Input gravity channel	The name of the input channel containing the gravity readings. Script Parameter: GRAVITY_CORRECTIONS. INPUT_CHANNEL
Output gravity channel	The name of the corrected gravity channel. Only the corrections indicated by a checked box will be applied. Script Parameter: GRAVITY_CORRECTIONS. OUTPUT_CHANNEL
Line selection	You have the option to calculate the gravity corrections for the displayed line, selected lines or all lines in the database. Script Parameter: GRAVITY_CORRECTIONS.LINE_SELECTION [D – Displayed line, S- Selected lines(default), A- All lines]
Scaling	If checked, scaling is applied, and you will have to provide the scaling parameters. Most current instruments are calibrated to read the gravity in milligals and do not need scaling. This option is provided in order to support older systems. Refer to your instrument user’s manual to find out if you need to apply scaling. Script Parameter: GRAVITY_CORRECTIONS. APPLY_SCALE_FACTOR [0- No, 1-Yes] Scaling method If the Scaling option is checked, either provide a single scale factor to multiply all the gravity readings by, or provide a scaling table that defines the scaling factors as a function of amplitude. Script Parameter: GRAVITY_CORRECTIONS. SCALING_METHOD [0- value, 1-File] Scale factor Or Scale factor file Depending on the selection above, you will be prompted to provide an input for one of the options below: Scale factor Script Parameter: GRAVITY_CORRECTIONS. SCALE_FACTOR Scale factor file Script Parameter: GRAVITY_CORRECTIONS. SCALE_FACTOR_FILE See the Application Notes for more information on Instrument Scale Factor and Instrument Calibration File.
Tide correction	If checked, tide correction will be applied/added to the gravity values, and you will be prompted for the tide parameters. Some instruments apply the Earth tide correction to the gravity readings. Refer to your instrument user's manual to determine if you need to apply tide correction. Do not check this box if the tide correction has already been applied at the instrument level. Check this box if you would like to replace the tide correction applied at the instrument level. Script Parameter: GRAVITY_CORRECTIONS.APPLY_TIDE_CORRECTION [0- No, 1-Yes] Tide correction method Select Onboard to indicate that the Earth tide correction is provided as a channel in the database but not yet applied to the gravity readings. Refer to the instrument user’s manual to find out if the tide correction is systematically applied at the instrument level. Select Calculate if it is to be calculated and applied. Note that if the entry selected in the Output tide channel exists (see below), an overwrite confirmation dialog will be prompted. The tide correction is added to the gravity values. Script Parameter: GRAVITY_CORRECTION. TIDE_CORRECTION_METHOD [0- Onboard, 1-Calculate] Instrument tide channel Select the instrument tide channel. This field is enabled if the Onboard mode is selected or when the Replace instrument ETC data option is checked with the Calculate mode. Many gravity meters output the tide correction as a field, which is imported as a channel, generally called "ETC". If the current database has an ETC channel, it will be automatically selected. Script Parameter: GRAVITY_CORRECTIONS. INSTRUMENT_TIDE_CHANNEL Hours to GMT (West +) This entry appears if you selected to calculate the tide correction. Provide the time difference in units of decimal hours between the time recorded in the survey database and GMT. This is a positive value in the Western hemisphere. This value is added to the time channel prior to calculating the tide correction. To circumvent the eventuality of the dataset crossing midnight, the tide correction calculation takes into account both the Time and Date channels (see below). If you have opted to apply the tide correction, this is a mandatory entry. The default is 0 for a gravity database created from a CG-6 file import. If the current gravity database has been created from a CG-5 file import, the entry defaults to the GMT offset value stored in the header of the imported file. See the Application Notes under Import Gravity Survey for more details on CG-5 and CG-6 dump files. Script Parameter: GRAVITY_CORRECTIONS. HOURS_TO_GMT Output tide channel Enter the name for the channel to contain the calculated tide correction values. This field is enabled if you opted to calculate the tide correction. If you select a channel from the drop-down list or you enter a channel name that already exists, an overwrite confirmation dialog will be prompted. Select "No" to preserve the existing tide correction channel and to write out the calculated tide correction to a different channel. Script Parameter: GRAVITY_CORRECTIONS. OUTPUT_TIDE_CHANNEL Replace instrument ETC data This option is contextual and appears if you have selected above to calculate the Earth tide. If you do know that the Earth tide has been calculated at the instrumentation level and added to the gravity readings, but you prefer to recalculate it at a higher precision and reapply it, this is the option that allows you to remove the Earth tide effect calculated at the instrument level and replace it with the Earth tide calculated in Oasis montaj. If you have opted to calculate the tide correction and there is already an imported tide channel, you may want to subtract the latter first and then add the calculated tide correction; in this situation you will preserve the existing tide correction channel, and you will need to provide a different channel name for the output tide correction channel. Script Parameter: GRAVITY_CORRECTIONS.SUBTRACT_ETC [ 0- No, 1-Yes]
Instrument height correction	If checked, height correction is applied, and you will have to provide the height above ground channel name and the height correction equation. The height correction is added to the gravity values. Script Parameter: GRAVITY_CORRECTIONS. APPLY_INSTRUMENT_HEIGHT_CORRECTION [ 0- No, 1-Yes] Instrument height channel Select the instrument height channel name. If the current database has a "Height" channel, it will be automatically selected. Script Parameter: GRAVITY_CORRECTIONS. INSTRUMENT_HEIGHT_CHANNEL Height correction equation Select one of the three equations for height correction. See the Instrument Height section under Application Notes for further details. Script Parameter: GRAVITY_CORRECTIONS. HEIGHT_CORRECTION_EQUATION [ 0- 0.308596 , 1-0.3086, 2 – Ellipsoid (Heiskanen & Moritz)] Latitude channel This is a contextual parameter. If the "Ellipsoid" option has been selected above, you will be prompted to provide the latitude channel name required for the height correction. Script Parameter: GRAVITY_CORRECTIONS.LATITUDE_CHANNEL
Drift calculation	If checked, instrument drift correction is applied, and you will have to provide the drift correction channel name. To calculate the drift properly, you should start and end your survey at the same base station. Script Parameter: GRAVITY_CORRECTIONS.APPLY_DRIFT_CORRECTION Drift correction channel Provide the name of the channel in which to place the drift correction values. See the Drift Correction section under Application Notes for further details. Script: GRAVITY_CORRECTIONS. GRAVITY_CORRECTIONS. DRIFT_CHANNEL
Absolute gravity calculation	If checked, base station correction to absolute gravity is applied, and you will have to provide the base station information. See the Absolute Gravity section under Application Notes for further details. Script Parameter: GRAVITY_CORRECTIONS.APPLY_ABSOLUTE_GRAVITY_CORRECTION Base station database Provide the name of the database that contains base station information. Script: GRAVITY_CORRECTIONS. BASE_STATION_DATABASE Gravity channel Provide the name of the gravity channel in the base station database. Script Parameter: GRAVITY_CORRECTIONS. BASE_GRAVITY_CHANNEL
[More] Click to expand the section and select the date and time channels: these are mandatory fields for Tide correction and Drift calculation.
Date channel	Select the date channel name. If the current database has a "Date" channel, it will be automatically selected. Script: GRAVITY_CORRECTIONS.DATE_CHANNEL
Time channel	Select the time channel name. If the current database has a "Time" channel, it will be automatically selected. Script: GRAVITY_CORRECTIONS.TIME_CHANNEL

Application Notes

This dialog assumes that a gravity survey database is loaded.

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. Generally, base stations have a station number of at least an order of magnitude larger than the survey stations.

To improve the statistical accuracy of the data, readings may be repeated any number of times during the survey before moving to the next survey location. This includes repeating the base station readings. If base station readings are repeated in the survey database, after applying the preceding corrections and before calculating the drift and/or base station corrections, the readings and reading times at the base stations are averaged.

Gravity Correction Formulas

The following are the formulas applied in this 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 S(r), where the scale factor depends on the amplitude of the readings.

Where:

Gs	Corrected gravity reading in milligals
R	Instrument reading in dial units
S(r)	Constant scale factor (dial units/milligal) or Calculated from a supplied calibration table as a function of the gravity reading. In this case a calibration table must be provided.

1.1 Instrument Calibration File

Lacoste and Rhomberg meters are delivered together with a table that indicates the factory calibration. You can create your own calibration file directly from this table by formatting the information into a CSV file in the format noted below. The Scintrex CG-3/CG-5/CG-6 instruments are internally calibrated and should produce values already scaled to milligals.

With most instruments, you may wish to perform your own calibration if highly accurate gravity readings are required. This may also be necessary if you will be using more than one meter on a survey and you want to ensure that all meters are calibrated against the same scale.

A calibration file is a standard Geosoft ASCII table file, which contains the columns "Instrument", “Scale”, "Milligal". Comments must start with an ”/” and can be added to the beginning of the file.

An example is provided below:

Although the calibration table includes a scale column, it is not used. The scaling simply finds the rows surrounding the instrument reading and then linearly interpolates the value from the "Milligal" column. The scale column is reported for you to verify that the calibration survey is acceptable, since the scale should change very little between readings. Each scale value in the table is scale for the meter range from the reading on the scale line to the next reading. The last scale in the column is the average scale for the meter across the calibration range. If the scale is reasonably linear, you may choose to use the instrument scale alone.

To calibrate a gravity meter, you will need a set of "base stations" at which you know the absolute gravity, and which span the gravity range of interest for a particular survey. Government organizations often maintain sets of base stations specifically intended to be used for calibrating gravity instruments. If you do not have access to a set of calibration stations, you can establish your own by selecting two or more locations that you know will span the range of interest. You can use a combination of elevation and longitude to create a gradient. You will need to establish the calibration base stations using a well-calibrated instrument, and then use these bases for a calibration survey for your other instruments.

To conduct a calibration survey, simply take readings with each instrument at all calibration base stations. The more readings the better (by looping back and forth), and it is usually best to start and end on the same station, so that each base station is read twice (except for the station in the centre of the loop). By reading stations twice, and assuming a roughly equal time interval between readings, the averaging of readings will remove the effect of any instrument drift.

2. Tide Correction

If a relative time difference to Greenwich Mean Time is provided, the tidal correction is calculated using the Longman (1959) equations and saved in the tide correction channel.

Readings are then 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; please see the References section below. The tide correction (G_t) is added to the reading.

Where:

Gst	Output tide corrected channel
Gs	Gravity reading optionally scale corrected (1. Instrument Scale Factor / Calibration File)
Gt	Tide correction

Tidal Correction Precision

The tidal effect is a function of location (longitude, latitude) and time (date and time). In order to obtain the desired precision in the calculated tidal effect, these channels (Longitude, Latitude, Date, Time) should be defined as double precision channels. Specifying any one of them in single precision may result in errors in the order of 10^-2 milliGals. If you use the Gravity Import tool, these will be created as double precision channels; however, if you use the Oasis montaj Import tool, ensure that you create the channels as double precision float type.

The formats below are only a display representation:

date -> year/month/day

time -> hour:minute:second

geographic -> degree:minute:second

These values are stored internally as double precision floating point values.

3. Instrument Height

Each reading is corrected for the height of the instrument above the station or base at which the elevation is measured. There are three alternative equations:

Where:

Gsth	Instrument height corrected reading
Gst	Gravity values containing the optional corrections 1. & 2. above
H	Instrument height in metres
L	Latitude channel

4. Drift Correction

Gravity surveys are conducted in loops. The survey begins with a reading at a designated base station and ends with a reading at a designated base station.

The simplest survey pattern is to conduct a single loop survey, starting and ending at the same base station.

For surveys of a longer duration you may add more loops. In addition to visiting the designated base station at the start and end of the survey, you may take additional readings at the designated base station during the survey in a multiple loop pattern.

In both these instances the drift correction could simply be a relative correction. The first base station reading is the base to which subsequent readings at the base station are adjusted. The correction is calculated as linear drift in time between each two visits to the base station.

Where:

D	Drift in milligals/hour
Gsthb1	Base 1 reading, potentially tide and instrument height corrected
Tb1	Base 1 time
Gsthb2	Base 2 reading, potentially tide and instrument height corrected
Tb2	Base 2 time

If the survey spans over a vast area and revisiting the same base station is impractical, your survey can contain more than one base station and furthermore each base station can be visited more than once. In this case, if the readings at the different base stations are not absolute gravity readings, they must be calibrated to each other. The drift is then calculated as follows:

Where:

Ab1	Height adjusted Base 1 absolute Gravity in milligals
Ab2	Height adjusted Base 2 absolute Gravity in milligals

G_sthb1, G_sthb2, T_b1, and T_b2 will be averaged if there are repeat base readings.

If the loop is closed on the same base station, (A_b2- A_b1) is zero.

5. Absolute Gravity

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

Where:

Ga	Absolute gravity in milligals
Ab1	Base 1 absolute G in milligals
Gsth	Base 1 reading
Gsthb1	Instrument height corrected station reading (3. Instrument Height)
T	Reading date+time
Tb1	Base 1 reading date+time
D	Drift in milligals/hour (4. Drift Correction)

References