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:
-
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] |
||||||||||
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]
|
||||||||||
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]
|
||||||||||
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
|
||||||||||
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
|
||||||||||
[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.
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.
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:
/ Gravity calibration file
/ Survey date: 2018/11/14
/ Instrument: G 123
/
Instrument, Scale, Milligal
5090.00, 1.02514, 981327.9626
5100.00, 1.02506, 981337.8061
5110.00, 1.02499, 981347.6989
5120.00, 1.02492, 981357.5904
5130.00, 1.02487, 981367.5831
5140.00, 1.02483, 981377.6262
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 (Gt) 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 |
Gsthb1, Gsthb2, Tb1, and Tb2 will be averaged if there are repeat base readings.
If the loop is closed on the same base station, (Ab2- Ab1) 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) |
*The GX tool will search in the "...\Geosoft\Desktop Applications \gx" folder. The GX.Net tools, however, are embedded in the geogxnet.dll located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" folder, then supply the GX.Net tool in the specified format.
References
- I.M. Longman, "Formulas for computing the tidal accelerations due to the moon and the sun", Journal of Geophysical Research, vol. 64, no. 12 (December 1959), pp. 2351-2355. DOI: https://doi.org/10.1029/JZ064i012p02351.
- W. A. Heiskanen, and H. Moritz, Physical Geodesy (San Francisco: W. H. Freeman and Company, 1967).
Got a question? Visit the Seequent forums or Seequent support
© 2023 Seequent, The Bentley Subsurface Company
Privacy | Terms of Use