Create Calibration from Survey

Use the Gravity > Calibration > Create Calibration from Survey menu option (GRCALIB GX) to create a calibration table from a calibration survey database.

Create an Instrument Calibration Table dialog options

Source file	This is the name of the source survey data file that was used to create the calibration survey database. This information if provided for you to verify that this is the correct survey.
Output calibration file (*.csv)	Name of the output instrument calibration file. The calibration data is written to a comma delimited ASCII file (CSV file) that contains columns of instrument reading, calibrated milligal equivalent and the interval scale factor.
Base station database	Specify the name of the base station database that contains the absolute gravity of the calibration base stations. Each base station on the calibration loop must have an entry in the base station database. There should be a column for the base station number, absolute gravity and longitude, latitude at the station. The longitude, latitude is used to calculate the earth tide correction.
Hours to GMT (+ in West)	The time difference (in hours) between the recorded survey time and Greenwich Mean Time (GMT). The time difference is used to calculate earth tides at the time of each base station observation.
Instrument	Specify the name of the instrument that is calibrated. This name is written as information in the output calibration file.

Application Notes

The GX will create a gravity instrument calibration file.

Common relative gravity meters should be calibrated to determine the true relationship between the observed meter readings and gravity in milligals. This information is compiled and stored in a calibration file, which in turn is used to convert instrument meter readings to equivalent milligal values.

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 CG3 is internally calibrated and should produce values already scaled to milligals. For a CG3 instrument, you can set the instrument scale factor to 1.0. However, with most instruments, including Scintrex CG3 and Lacoste and Rhomberg meters, you may wish to perform your own calibration if very 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 insure that all meters are calibrated against the same scale.

A calibration file is a standard Geosoft ASCII table file, which contains a "Instrument", "Milligal" and "Scale" column as in the following example:

Although the calibration table includes a scale column, it is not used by GravRed, which simply looks up a linearly interpolated 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 organisations 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.

To reduce the calibration data the absolute gravity at each observation is calculated. Note that calculations are applied to the known absolute gravity rather than the instrument reading, so this is almost the inverse of reducing readings to absolute gravity. The reduction sequence is as follows:

1. The earth tide is calculated and added to the known absolute gravity.

2. The effect of the instrument height (normally negative) is added to the absolute gravity.

3. The meter readings and the absolute gravity at repeated stations are averaged, which reduces "noise" and removes the effect of drift.

4. The data is sorted by meter reading and the table is created.

The calibration table is presented as a text file in your standard editor for you to review and modify is required.