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GRAVPAC is designed for the convenient entry, display, and processing of land­survey gravity data. It accommodates different survey and timekeeping systems, different types of gravity meters, and a broad range of user­selected options. The data may be entered and processed in one block or in segments as they become available during the course of the survey. The processed data include observed, free­air, and Bouguer gravities and are output as report­ready tables.

GRAVPAC is available for PC computers with these operating systems: MS­DOS, 16­bit Windows and 32­bit Windows.

To initiate GRAVPAC, insert the program disk in an active drive and type GRAVPAC. Alternately, the files can be copied onto a hard disk and GRAVPAC can be run from that drive.

GRAVPAC consists of ten chained programs plus a short data file used to pass information between programs.

Stations refer to surveyed locations at which gravity readings are made. Stations are identified by (up to) six­character alphanumeric names and up to ten­character names from version 3.0. You must supply the elevation and coordinates of each station and can specify a terrain correction. Elevations are in meters or feet. Coordinates can be in:

There must be one or more primary base stations for which absolute gravity is known or assumed. Surveys are normally conducted in tied loops beginning and ending on the base stations and instrument drift is calculated from the base station ties. However, breaks can be specified when a loop cannot be tied or when the meter has been inadvertently bumped resulting in a tare in the drift control. GRAVPAC then processes the data by assuming zero instrument drift between the break and the nearest base station tie.

Secondary bases are stations which will be used as base stations during a survey but whose absolute gravity has not yet been determined. GRAVPAC will find the drift­corrected absolute gravity at secondary bases based on their ties to primary bases. Once their absolute gravity has been adequately established, secondary bases are converted into primary bases through GRAVPAC menu option 1.

Gravimeter dial reading to milliGal conversion factors can be entered as either a single number or as a meter factor table. The meter factor table is then stored on the GRAVPAC program disk for future use. If the electrostatic feedback system is used, the electrostatic factor must also be specified.

Observations are entered as station, date, time, and meter reading. If the electrostatic feedback system is used, both the fixed dial reading and the electrostatic value are entered. If the electrostatic feedback is not used, meter height above surveyed ground level can be variable. This option is used for high­precision surveys or if a short tripod is used to raise the meter above local vegetation. Timekeeping may be in local standard time, daylight savings time, or Greenwich mean time. For the first two options, the local time zone must be specified.

The following steps are used to find the observed gravity at the field stations:

To find free­air gravity, the observed gravity is corrected for theoretical gravity at the station's latitude and elevation. The reference spheroid used to correct for station latitude may be either the 1930 International Gravity Formula or IGSN71 as modified for the 1967 GRS. Elevation is corrected with a free­air gradient of ­0.3086 milliGals/meter (­0.09406 milliGals/foot)

To find Bouguer gravity, the free­air gravity is corrected for the attraction of a flat slab of material between the station and sea level and for the local terrain (if terrain corrections have been entered into the station coordinate file). The Bouguer slab correction is +0.04190 x RO milliGals/meter (+0.01277 x RO milliGals/foot) where RO is the material density in gm/cc. For local site surveys RO should be the mean density of the topographic features within the site, for geodetic surveys it should be the mean density of the continental crust. GRAVPAC finds the Bouguer­gravity for four user­selected densities.

All of the reduced data may be output to a line printer and are written to a readily­accessible disk file. Selected parameters can be written to another file in an X,Y,G ASCII format for direct input to a contouring package (such as SURFER).

GRAVPAC is dimensioned for up to 1000 observations at up to 800 stations. There may be as many as 20 base stations, 20 secondary bases, and 20 breaks in the drift control.

GRAVPAC will also print earth tide tables with calculated values in microgals for every five minutes.

If you have problems or questions please refer to the detailed software manual or contact John Fett at L&R


GMODEL is for interpretive modeling of gravity survey data. Models are constructed from combinations of infinitely­long horizontal polygons, finite­length horizontal polygons, vertical polygons, and spheres.

Gravity may be calculated at select spot locations, along profiles, or over a horizontal surface. For profile calculations observed gravity can be entered and displayed along with the modeled gravity. The user may then either modify the model to improve the fit or print the results. GMODEL will also automatically fit an observed profile to a single surface model such as would represent an alluvium to bedrock interface.

GMODEL is available for PC computers with these operating systems: MS­DOS, 16­bit Windows and 32­bit Windows. GMODEL will not run with the Hercules graphics card.

To initiate GMODEL insert the program disk into an active drive and type GMODEL. Alternately the program GMODEL.EXE can be copied onto a hard disk and GMODEL can be run from that drive.

GMODEL uses an X,Y,Z right­handed coordinate system with the Z­axis (depth) positive downward. Dimensions may be in meters, feet, kilometers, or kilofeet. Horizontal polygons parallel the Y­axis and are defined by the X,Z coordinates of the vertices. For finite­length horizontal polygons the maximum and minimum Y­extents must also be specified. Vertical polygons parallel the Z­axis and are defined by the X,Y coordinates of the vertices plus the depths to the top and bottom of the body. Spheres are defined by the X,Y,Z coordinates of their centers plus their radii. There may be up to 20 bodies each with up to 40 vertices.

The calculated gravity is the downward or +Z component of gravitational attraction. Spot calculations are at select X,Y,Z locations. Profiles are defined by the X,Y,Z coordinates of the end points. Surface calculations are on a horizontal rectangular surface defined by its elevation (Z) and its XMin, XMax YMin, YMax limits. There are 101 calculated values along a profile and 51 x 51 = 2601 values uniformly distributed over a surface. Calculations may be made over, under, on, or within the bodies in the model. Bodies can be anywhere.

Observed values are entered as distance from the start of the profile plus the Bouguer gravity. You may either remove regional trends before the data are entered or elect to have GMODEL find and remove the linear regional which ties the end points directly to the modeled gravity.

Automated inversion is done with an infinitely­long horizontal polygon oriented perpendicular to the profile. There are vertices on the upper surface of the polygon directly beneath each of the observation points. The user specifies fixed tie­depths to the polygon at the ends of the profile plus the density contrast. GMODEL itteratively adjusts the depths to the remaining vertices in a manner that reduces the differences between the observed and modeled gravities. A filter is available to smooth noisy or irregular data.

Maximum use is made of interactive screen displays. The PRINT SCREEN key will result in a hardcopy of a screen display and the print option prints the model parameters, observed gravity, modeled gravity and short BASIC­language program segments that will access the data files. Surface calculations are written to a user designated disk file in a form compatible with the most contouring packages (such as SURFER).

If you have problems or questions please refer to the detailed software manual or contact John Fett at L&R.


TERRAIN is designed for computing near­station terrain corrections. It can calculate the gravitational attraction of irregular topography by use of either Hammer charts or by sloping wedge calculation. The gravity observer should take field notes describing any irregular topography near each gravity observation point. If Hammer charts are selected, a default set of diameters and number of sectors are available or these may be modified. The same is true for calculation using sloping wedges.

The calculated terrain corrections are stored in a file that is compatible with program GRAVPAC.


Infinitely­long horizontal polygons, Talwani, 1973, Computer usage in the computation of gravity anomalies in Bolt, Methods in Computational Physics: Academic Press, pp 343­389

Finite­length horizontal polygons, Rasmussen and Pedersen, 1979, End corrections in potential field modeling: Geophysical Prospecting, v.27, pp 749­760

Vertical polygons, Plouff, 1976, Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections: Geophysics, v.41, pp 727­741

Sloping wedge, Barrows and Fett, 1991, A sloping wedge technique for calculating gravity terrain corrections: Geophysics, v.56, pp 1061­1063

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