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
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
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:
Geosoft RAW Data File Sample:
dt=98/11/14 pj=79.5,43.5,10,200,100 gm=2.5 op=inm in=g105 un=MMM
Stn X Y Time Rdng In_Ht Elev
90010 * * 17:14:45 5090.577 0.0 *
0 0 100 17:14:45 5090.577 0.0 100
2 50 100 17:27:40 5090.239 0.0 112
3 75 100 17:37:40 5091.321 0.0 113
4 100 100 17:47:40 5094.401 0.0 114
5 125 100 17:57:40 5092.482 0.0 115
6 150 100 18:20:40 5088.632 0.0 116
7 175 100 18:31:40 5090.762 0.0 117
8 200 100 19:38:40 5089.472 0.0 118
9 225 100 20:07:40 5089.306 0.0 119
10 250 100 20:17:40 5089.487 0.0 120
11 275 100 20:37:40 5087.500 0.0 121
90010 * * 20:47:40 5090.564 0.0 *
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.
SCINTREX V4.1 AUTOGRAV / Field Mode R4.4
Ser No: 34.
Line: 32. Grid: 0. Job: 1738. Date: 95/02/03 Operator: 0.
GREF.: 0. mGal Tilt x sensit.: 260.6
GCAL.1: 6321.652 Tilt y sensit.: 291.7
GCAL.2: 0. Deg.Latitude: 37.5
TEMPCO.: -0.1246 mGal/mK Deg.Longitude: 6.4
Drift const.: 0.474 GMT Difference: 0.hr
Drift Correction Start Time: 20:10:15 Cal.after x samples: 12
Date: 94/11/21 On-Line Tilt Corrected = "*"
-------------------------------------------------------------------------------
Station Grav. SD. Tilt x Tilt y Temp. E.T.C. Dur # Rej Time
-9999. 4493.006* 0.027 -3. 1. -0.19 -0.060 200 0 07:04:55
5766. 4491.393* 0.040 0. -8. -0.28 -0.076 150 1 09:47:46
5768. 4496.184* 0.028 0. -0. -0.30 -0.071 150 4 10:03:36
5772. 4500.955* 0.026 -3. -3. -0.33 -0.066 150 0 10:15:56
-9999. 4492.952* 0.028 1. 1. -0.31 0.045 150 0 14:06:41
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.
/ Survey name: victor
/ Instrument S/N: 17
/ Client: maiko
/ Operator: Default
/ Date: 2003/ 1/21
/ Time: 08:20:09
/ LONG: 83.900 W
/ LAT: 52.800 N
/ ZONE: 0
/ GMT DIFF.: 5.0
/ CG-5 SETUP PARAMETERS
/ Gref: 0.000
/ Gcal1: 8619.831
/ TiltxS: 528.760
/ TiltyS: 560.254
/ TiltxO: -16.186
/ TiltyO: 24.112
/ Tempco: -0.146
/ Drift: 1.388
/ DriftTime Start: 8 :20:23
/ DriftDate Start: 2003/1 /21
/ CG-5 OPTIONS
/ Tide Correction: YES
/ Cont. Tilt: YES
/ Auto Rejection: YES
/ Terrain Corr.: NO
/ Seismic Filter: YES
/ Raw Data: NO
Line 11.000N
/------LINE-----STATION-----ALT.------GRAV.---SD.--TILTX--TILTY-TEMP---TIDE---DUR-REJ-----TIME----DEC.TIME+DATE--TERRAIN
11.000 9999.000 -22.41 4751.167 0.039 -12.4 14.9 0.83 -0.093 180 71 11:14:20 1100.46754 0.000
11.000 287.000 -18.75 4751.163 0.112 4.9 -6.4 0.82 -0.093 180 19 11:18:24 1100.47036 0.000
11.000 287.000 -22.17 4751.163 0.055 6.8 -7.2 0.81 -0.092 120 5 11:21:37 1100.47259 0.000
11.000 289.000 -17.29 4751.236 0.069 10.7 -2.0 1.28 -0.083 120 1 12:10:35 1100.50654 0.000
11.000 291.000 -18.51 4751.297 0.058 3.6 -13.7 1.19 -0.078 96 1 12:33:13 1100.52223 0.000
11.000 293.000 -20.46 4751.235 0.067 5.1 4.1 1.11 -0.073 120 0 12:55:02 1100.53736 0.000
11.000 295.000 -21.68 4751.365 0.058 -1.2 -5.8 1.15 -0.071 63 4 13:06:24 1100.54524 0.000
11.000 297.000 -22.66 4751.494 0.053 -0.5 -0.6 1.20 -0.068 47 3 13:24:35 1100.55784 0.000
11.000 299.000 -18.51 4751.535 0.062 -1.3 3.2 1.51 -0.063 120 0 13:49:20 1100.57500 0.000
11.000 301.000 -15.58 4751.502 0.067 28.2 -5.4 1.38 -0.056 120 0 15:47:03 1100.65662 0.000
11.000 303.000 -18.75 4751.500 0.065 6.4 -8.2 1.17 -0.056 120 0 16:02:35 1100.66739 0.000
11.000 305.000 -20.70 4751.487 0.054 7.6 -5.2 1.03 -0.057 120 16 16:13:32 1100.67498 0.000
11.000 307.000 -21.19 4751.645 0.068 4.8 7.9 1.09 -0.059 120 12 16:30:14 1100.68656 0.000
11.000 9999.000 -20.70 4751.126 0.048 -26.8 3.5 0.93 -0.069 200 17 17:51:27 1100.74287 0.000
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.
/ CG-6 Survey
/ Survey Name: GEOSOFT_TEST
/ Instrument Serial Number: 000000000000004
/ Created: 2017-08-02 17:58:04
/
/ CG-6 Calibration
/ Operator: RL
/ Gcal1 [mGal]: 7758.3
/ Goff [ADU]: -8388608.0
/ Gref [mGal]: 0.0000
/ X Scale [arc-sec/ADU]: 0.032
/ Y Scale [arc-sec/ADU]: 0.032
/ X Offset [ADU]: -141141.260
/ Y Offset [ADU]: -257854.910
/ Temperature Coefficient [mGal/mK]: -0.1296
/ Temperature Scale [mK/ADU]: -0.000111
/ Drift Rate [mGal/day]: 0.00145
/ Drift Zero Time: 2017-06-30 15:05:56
/ Firmware Version: R-20170705-1
/
/Station Date Time CorrGrav Line StdDev StdErr RawGrav X Y SensorTemp TideCorr TiltCorr TempCorr DriftCorr MeasurDur InstrHeight LatUser LonUser ElevUser LatGPS LonGPS ElevGPS Corrections[drift-temp-na-tide-tilt]
STATION1 2017-08-02 17:58:04 2824.7 1 0.033 0.006 2824.7 8.9 -7.1 726.01 0.022 0.0017 94.1 -0.048 30 0.0 43.79 -79.50 181.40 43.79 -79.50 186.8 00000
STATION1 2017-08-02 17:58:34 2824.7 1 0.015 0.003 2824.7 10.6 -5.7 726.09 0.021 0.0018 94.1 -0.048 30 0.0 43.79 -79.50 181.40 43.79 -79.50 188.4 00000
*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
- Nagy, D.,"The gravitational attraction of a right rectangular prism", Geophysics, vol. 31, no. 2 (1966), pp. 362–371.
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