Import Gravity Survey

Use the Gravity > Import > Import Gravity Survey menu option (geogxnet.dll(Geosoft.GX.Gravity.ImportGravitySurvey;Run)*) to import a gravity survey data file into a new Geosoft Database. The data file may be either in Geosoft RAW data format, or it may be in a supported digital instrument format. A new database will be created from the imported data.

Import Gravity Survey dialog options

File type	Select one of the Scintrex instrument dump formats (CG-3, CG-5 or CG-6), or select "Geosoft RAW file" for a Geosoft format text file. Script Parameter: GRLOAD.TYPE
Survey data file	Select the survey data file, which is either a Scintrex instrument dump file or a Geosoft RAW format file. Script Parameter: GRLOAD.DATA
Output database	Specify the name of the database in which to import the data. By default, this database has the same base name as the input file. Script Parameter: GRLOAD.OUTPUT_DATABASE
Longitude channel Latitude channel	These entries appear only if the selected type is CG-6 and there is more than one set of Latitude /Longitude fields in the input file. In this eventuality, you are prompted to determine which set to proceed with as the designated pair of position channels. Script Parameter: GRLOAD.LONGITUDE Script Parameter: GRLOAD.LATITUDE
Date format	This entry appears if the selected type is CG-6. From the available drop-down list, select the date format you wish to use for the import. Script Parameter: GRLOAD.DATE_FORMAT [1 - YYYY/MM/DD; 2 - DD/MM/YYYY; 3 - MM/DD/YYYY]
Location database	This entry appears if the selected type is CG-3 or CG-5. If provided, the position information is extracted from this file. Script Parameter: GRLOAD.LOCATION_DATABASE
Base station database	Select the base station database. This is an optional entry. If provided, the station numbers of the survey database are compared with the station numbers in the base station database to identify the base station readings of the survey database. Subsequently, the Type of all base station rows in the survey database is then set to 0. All other rows are assumed to be survey stations and their Type remains as 1. Script Parameter: GRLOAD.BASESTATION_DATABASE
Import into a single line	Select this option to import the data into a single line. Script Parameter: GRLOAD.IMPORT_SINGLE_LINE [0 - No; 1 - Yes]

Application Notes

Gravity survey files contain observed gravity data for normal loop-style gravity surveys. A gravity "loop" has the first and last gravity observations on an existing gravity base station with any number of other gravity observations in between. There may be other gravity base stations located within a loop to create sub-loops. A gravity survey file contains gravity survey information from a single gravity instrument, normally collected during a single day by the same operator. There will be a separate gravity database created for every gravity survey file in a gravity project.

All station numbers greater or equal to 90000 are assumed to be base stations. All station numbers less than 90000 are assumed to be survey stations.

Most gravity surveys are conducted based on a station numbering scheme in which all gravity stations will have a defined and unique number. The station number is used to determine if the station is a base station or a normal survey reading, and to look-up other information such as survey location information which may be determined separate from the gravity survey.

After import, a Geosoft gravity survey database will be created with the same name as the original data file but with extension ".gdb". The gravity survey database will contain the following channels regardless of the presence of this information in the gravity file.

Gravity Survey Database Channels

Channel	Data type	Description
Line	ASCII 24	Line number (name)
Station	ASCII 24	Station number or station name. Station numbers (or names) should be unique within a gravity project. The only exception is when a project is conducted along lines and the station number is used to indicated the location along survey lines. In this case, the Line/Station combination must be unique.
Type	short	0 - base; 1 - station; 2 - repeat station Repeat stations will in fact be determined by GravRed based on a common station number.
Date	double	Reading date
Time	double	Reading time
Reading	double	Instrument dial reading
Alt	double	Instrument height above Elevation in metres
Longitude	double	Longitude (may be calculated from X,Y)
Latitude	double	Latitude (may be calculated from X,Y)
Elevation	double	Station elevation
Tide	double	Tidal correction
Dec_TimeDate	double	Dec. reading time + date (DEC.TIME+DATE)
Dur	long	Duration of measurement
Ice	double	Ice thickness
Rej	long	Rejected noise
SD	double	Standard deviation
Slope	double	Local terrain slope in degrees. *See notes below.
Temp	double	The instrument (gravity sensor) temperature
Terrain	double	Terrain correction
TiltX	double	The X-axis tilt of the instrument
TiltY	double	The Y-axis tilt of the instrument
Water	double	Water depth (includes ice)
X	double	X location in local coordinate system . The position information is extracted from the location database if provided.
Y	double	Y location in local coordinate system. The position information is extracted from the location database if provided.

Distance units will be metres by default. RAW format files can specify different distance units using the "un=" parameter. The distance units can also be specified by editing the channel information after the data has been imported.

The slope channel was intended to be used as a better estimation of local slope than a DEM could provide at a certain cell resolution. However, this was before the advent of CG-5. The CG-5 and later instruments are able to compute near terrain corrections in Hammer zones. When using an old instrument, it is possible to incorporate local slope measurements\ observations made at stations in the calculation. As per Nagy (1962): "The slope angle is used to calculate the sum of four sloping triangle sections in the near section. In the near zone (0 to 1 cells from the station), the algorithm sums the effects of four sloping triangular sections, which describe a surface between the gravity station and the elevation at each diagonal corner".

If you do not provide any slope data in the channel, the local slope of the DEM grid will be used.

Geosoft RAW Data File

A Geosoft RAW data file is an ASCII file that contains raw gravity observations. The format has a three-line header followed by rows and columns of data. Data from all non-digital instruments should be entered into a RAW format file. A blank file named "gravity.raw" that can be used as a template, can be found in the "...\Geosoft\Desktop Applications \etc\" folder.

RAW Data File Format:

Line 1:	Title line - anything you like
Line 2:	The reduction parameters are used to specify data constants.
	dt=	Year/month/day of first reading. A Date data column may be used instead. Times that pass through 24:00 will increment the date. (example -dt=1988/12/14; no default)
	gm=	Time difference to GMT, positive in the western hemisphere. (example gm=2.5; default gm=0.0).
	sf=	Instrument scale factor or the name of a calibration file. (examples sf=1.001, sf=SN105.SFC; default sf=1.0).
	dn=	Densities of the earth, water and ice for the Bouguer correction (examples dn=2.6,1.04,0.95; default dn=2.67,0,0).
	un=	Distance, elevation and instrument height units respectively (M-metres, F-feet,C-cm.,I-in.). (Example un=MFC; default un=MMM)
	gf=	Gravity formula (1930, 1967 or 1980). (Example gf=1930; default gf=1967). You may also establish your own unique gravity formulas (see GRBOUG GX for more information).
	Dms	Present if longitude, latitude coordinates are written as deg.mmsss. If not present, coordinates are assumed to be in decimal degrees.
	pj=	For local coordinate systems, this is the longitude, latitude coordinate of the local coordinate system and the rotation angle of the coordinate axis relative to geographic North (degrees counter-clockwise). For example, "pj=-83.5,42.8,15" indicates that the local coordinate origin is located at longitude –83.5, latitude 42.8 and the local grid is oriented 15 degrees East of Geographic North. If the longitude, latitude is known at a specific location on the local coordinate grid, you can specify that location as the 4th and 5th parameters. For example "pj=-83.5,42.8,15,1000,500" means that the geographic point (-83.5,42.8) is located at local coordinate (1000,500).
	op=	An operator identification, such as "op=jbl".
	in=	The instrument number or instrument identification.
Line 3:	Data column labels. The data column labels must be exactly as specified. The Stn, Time, and Rdng columns are required. All other data is optional.
	Stn	Station number (negative for base stations)
	Time	local reading time as hhmm or hh:mm:ss. If times pass through 24:00, the date will automatically be incremented to the next day unless there is a data column.
	Rdng	The instrument reading.
	Line	Line number if non-unique stations are used.
	T	Reading type: 0 base station 1 normal gravity station If Type is not specified, negative stations or stations numbered 90000 or greater are assumed to be base stations. All other readings are assumed to be normal gravity stations.
	Date	The reading date if not specified on line 2 by the dt= parameter.
	In_Ht	height of instrument above station. If no instrument height is specified, the height is assumed to be 0.0.
	X	X location in local coordinate system. If the pj= parameter is specified, the X and Y coordinates are assumed to be local grid coordinates and the longitude and latitude will be calculated.
	Y	Y location in local coordinate system (see the note in the "X" description).
	Latitude	Latitude
	Longitude	Longitude
	Elev	station elevation
	Dep_W	water depth
	Dep_I	Ice depth
	Terr_C	Terrain correction (mGals)
	Slope	Local terrain slope in degrees. If specified, this is used to determine the local terrain correction.
Line 4 and on:	The data itself. The first and last rows should be base station readings. Base readings may also be placed anywhere within the data. A base reading may either be indicated by a Type 0 (if a type column is present), a negative station number, a number greater or equal to 90000, or a unique number that matches an entry in the base station database.

Geosoft RAW Data File Sample:

CG-3 Dump Files

If the data is in the Scintrex CG-3 dump format, it will first be converted to a Geosoft RAW data file format. The created RAW data file will have the same name as the new database but with file extension ".raw".

Digital data dumped directly from the Scintrex CG-3 Autograv gravity meter can be converted directly into a GRAVRED compatible format using the CG-3 program. In order to be compatible with the Geosoft system, the survey operators must follow certain guidelines when collecting data.

Survey Procedures

All gravity data must be collected in loops, which start and end at base stations. There can be any number of base station readings in a file, but the first and last reading must be base readings. Base stations can be indicated by using negative station numbers or numbers between 90000 and 99999, as long as you use the same convention throughout a file.

Instrument Height, Elevation and Location

The CG-3 allows the user to enter up to 8 numbers that are output in the dump file and annotated A through H respectively. If present, CG-3 will interpret these numbers as follows:

A: Instrument height above station
B: Station elevation
C: X position
D: Y position
E: ignored
F: ignored
G: ignored
H: ignored

The instrument height in the raw data file is set to 0.0 unless a height is entered as the A value. The elevation is set to a dummy flag (*) unless entered as the B value or found in the reference file. The X and Y positions will be the local coordinates for local mode, or dummied for regional mode unless entered as the C and D values or found in the reference file (using the -rf= parameter). See example 4 below.

We recommend that you use the A parameter of the CG-3 to record the instrument height. The elevation and locations are best provided by merging from a separate location database.

Local Mode

In local mode, the station numbers and line numbers in the dump file are used to determine the X and Y location of each reading. In this mode, you should not use the C and D parameters of the CG-3. If the C and D values are used, they will take precedence.

In local mode, we recommend using numbers between 90000 and 99999 to represent base stations. This allows station numbers to be negative in order to indicate negative line locations. Negative station numbers are replaced by (10000-station) in the station number column (ie. station -10 becomes 10010). For local mode data, the station numbers in the RAW file are for reference only and are not important. Only the station X and Y are used in processing.

Regional Mode

In regional mode, each reading must have a unique station number. We recommend numbering all stations chronologically within a given survey starting from station 1. A separate location database can be used to provide the X and Y coordinates and the station elevations, or these values can be entered as values B, C and D in the CG-3. In regional mode you should always use negative numbers for base stations. Example 3 below is an example of regional mode.

Tide Corrections

The CG-3 can optionally calculate corrections for earth tides. However, GRAVRED also performs this calculation. If earth tides have been removed in the CG-3 data, the corrections will be added back. If this happens, the instrument readings in your gravity survey database will be different than the instrument readings in the CG-3 dump file.

Despite this procedure, we still recommend calculation of the earth tides in the CG-3 because this causes the longitude, latitude and GMT values to be present in the CG-3 dump. These values are necessary for GRAVRED to determine its own earth tides, and the longitude, latitude coordinates are used to locate local surveys. Note that the CG-3 longitudes are positive in the Western hemisphere, whereas Geosoft assumes negative longitudes in the Western Hemisphere. The CG-3 import will change the sign of the stated CG-3 longitude if necessary. Both the CG-3 and Geosoft use positive GMT corrections in the Western Hemisphere. (The GMT difference is added to the stated time to determine true GMT).

Create a CG-3 Dump File

Use the 'Gravity > Import > Download from CG-3' option to download a data file from the CG-3.

CG-5 Dump Files

Support for CG-5 dump files was introduced in v5.1.7. There are a number of different files produced by the CG-5 instrument, but the one supported by this import is the ".txt" file.

CG-6 Dump Files

Support for CG-6 dump files was introduced in 9.4. There are a number of different files produced by the CG-6 instrument, but the one supported by this import is the ".dat file. There are also variations of the CG-6 format, the import recognizes the CG-6 version and imports it accordingly.

*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