# gravmag3d

Compute the gravity/magnetic anomaly of a 3-D body by the method of Okabe

## Synopsis

**gmt gravmag3d** *xyz_file* **-T****v***vert_file* OR **-T****r|s***raw_file* OR **-M****+s***body,params*
[ **-C***density* ]
[ **-E***thickness* ]
[ **-F***xy_file* ]
[ **-G***outgrid* ]
[ **-H***f_dec*/*f_dip*/*m_int*/*m_dec*/*m_dip* ]
[ **-L***z_observation* ]
[ **-S***radius* ]
[ **-Z***level* ]
[ **-V**[*level*] ]
[ **-fg**]
[ **-h**headers ]
[ **-i**flags ]
[ **-o**flags ]
[ **-r**reg ]
[ **--PAR**=*value* ]

**Note**: No space is allowed between the option flag and the associated arguments.

## Description

**gravmag3d** will compute the gravity or magnetic anomaly of a body described by a set of triangles.
The output can either be along a given set of xy locations or on a grid. This method is not particularly
fast but allows computing the anomaly of arbitrarily complex shapes.

## Required Arguments (not all)

**-C***density*Sets body density in SI. This option is mutually exclusive with

**-H**.

**-H***f_dec*/*f_dip*/*m_int*/*m_dec*/*m_dip*Sets parameters for computing a magnetic anomaly. Use

*f_dec*/*f_dip*to set the geomagnetic declination/inclination in degrees.*m_int*/*m_dec*/*m_dip*are the body magnetic intensity declination and inclination.

**-F***xy_file*Provide locations where the anomaly will be computed. Note this option is mutually exclusive with

**-G**.

**-G***outgrid*[=*ID*][**+d***divisor*][**+n***invalid*][**+o***offset*|**a**][**+s***scale*|**a**][:*driver*[*dataType*][**+c***options*]]

Optionally, append =

IDfor writing a specific file format. The following modifiers are supported:

+d- Divide data values by givendivisor[Default is 1].

+n- Replace data values matchinginvalidwith a NaN.

+o- Offset data values by the givenoffset, or appendafor automatic range offset to preserve precision for integer grids [Default is 0].

+s- Scale data values by the givenscale, or appendafor automatic scaling to preserve precision for integer grids [Default is 1].

Note: Any offset is added before any scaling.+saalso sets+oa(unless overridden). To write specific formats via GDAL, use =gdand supplydriver(and optionallydataType) and/or one or more concatenated GDAL-cooptions using+c. See the “Writing grids and images” cookbook section for more details.

**-M+s***body,params*(An alternative to**-Tr**/**-Ts**). Create geometric bodies and compute their grav/mag effect.Select among one or more of the following bodies, where

*x0*&*y0*represent the horizontal coordinates of the body center [default to 0,0 positive up],*npts*is the number of points that a circle is discretized and*n_slices*apply when bodies are made by a pile of slices. For example Spheres and Ellipsoids are made of*2 x n_slices*and Bells have*n_slices*[Default 5]. It is even possible to select more than one body. For example**-M+s***prism,1/1/1/-5/-10/1***+s***sphere,1/-5*computes the effect of a prism and a sphere. Unfortunately there is no current way of selecting distinct densities or magnetic parameters for each body.*bell,height/sx/sy/z0[/x0/y0/n_sig/npts/n_slices]*Gaussian of height*height*with characteristic STDs*sx*and*sy*. The base width (at depth*z0*) is controlled by the number of sigmas (*n_sig*) [Default = 2]*cylinder,rad/height/z0[/x0/y0/npts/n_slices]*Cylinder of radius*rad*and height*height*and base at depth*z0**cone,semi_x/semi_y/height/z0[/x0/y0/npts]*Cone of semi axes*semi_x/semi_y*height*height*and base at depth*z0**ellipsoid,semi_x/semi_y/semi_z/z_center[/x0/y0/npts/n_slices]*Ellipsoid of semi axes*semi_x/semi_y/semi_z*and center depth*z_center**prism,side_x/side_y/side_z/z0[/x0/y0]*Prism of sides*x/y/z*and base at depth*z0**pyramid,side_x/side_y/height/z0[/x0/y0]*Pyramid of sides*x/y*height*height*and base at depth*z0**sphere,rad/z_center[/x0/y0/npts/n_slices]*Sphere of radius*rad*and center at depth*z_center*

**-R***xmin*/*xmax*/*ymin*/*ymax*[**+r**][**+u***unit*]Specify the region of interest. (See full description) (See technical reference).

**-Tv***vert_file*(must have when passing a*xyz_file*) OR**-Tr|s***raw_file*Gives names of a xyz and vertex (

**-Tv***vert_file*) files defining a close surface. The file formats correspond to the output of the triangulate program. The*xyz*file can have 3, 4, 5, 6 or 8 columns. In first case (3 columns) the magnetization (or density) are assumed constant (controlled by**-C**or**-H**). Following cases are: 4 columns -> 4rth col magnetization intensity; 5 columns: mag, mag dip; 6 columns: mag, mag dec, mag dip; 8 columns: field dec, field dip, mag, mag dec, mag dip. When n columns > 3 the third argument of the**-H**option is ignored. A*raw*format (selected by the**-Tr**option) is a file with N rows (one per triangle) and 9 columns corresponding to the*x, y, z*coordinates of each of the three vertex of each triangle. Alternatively, the**-Ts**option indicates that the surface file is in the ASCII STL (Stereo Lithographic) format. These two type of files are used to provide a closed surface.

## Optional Arguments

**-V**[*level*]Select verbosity level [

**w**]. (See full description) (See technical reference).

**-E**[*thickness*]give layer thickness in m [Default = 0 m]. Use this option only when the triangles describe a non-closed surface and you want the anomaly of a constant thickness layer.

**-L**[*z_observation*]sets level of observation [Default = 0]. That is the height (z) at which anomalies are computed.

**-S***radius*search radius in km. Triangle centroids that are further away than

*radius*from current output point will not be taken into account. Use this option to speed up computation at expenses of a less accurate result.

**-Z**[*level*]level of reference plane [Default = 0]. Use this option when the triangles describe a non-closed surface and the volume is defined from each triangle and this reference level. An example will be the hater depth to compute a Bouguer anomaly.

**-fg**Geographic grids (dimensions of longitude, latitude) will be converted to meters via a “Flat Earth” approximation using the current ellipsoid parameters.

**-h**[**i**|**o**][*n*][**+c**][**+d**][**+m***segheader*][**+r***remark*][**+t***title*] (more …)Skip or produce header record(s). Not used with binary data.

**-i***cols*[**+l**][**+d***divisor*][**+s***scale*|**d**|**k**][**+o***offset*][,*…*][,**t**[*word*]] (more …)Select input columns and transformations (0 is first column,

**t**is trailing text, append*word*to read one word only).

**-o***cols*[**+l**][**+d***divisor*][**+s***scale*|**d**|**k**][**+o***offset*][,*…*][,**t**[*word*]] (more …)Select output columns and transformations (0 is first column,

**t**is trailing text, append*word*to write one word only).

**-r**[**g**|**p**] (more …)Set node registration [gridline].

**-^**or just**-**Print a short message about the syntax of the command, then exit (

**Note**: on Windows just use**-**).**-+**or just**+**Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.

**-?**or no argumentsPrint a complete usage (help) message, including the explanation of all options, then exit.

**--PAR**=*value*Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## Grid Distance Units

If the grid does not have meter as the horizontal unit, append **+u***unit* to the input file name to convert from the
specified unit to meter. If your grid is geographic, convert distances to meters by supplying **-fg** instead.

## Examples

To compute the magnetic anomaly of a cube of unit sides located at 5 meters depth and centered at -10,1 in a domain
*-R-15/15/-15/15* with a magnetization of 10 Am with a declination of 10 degrees, inclination of 60 in a magnetic field
with -10 deg of declination and 40 deg of inclination, do:

```
gmt gmtgravmag3d -R-15/15/-15/15 -I1 -H10/60/10/-10/40 -M+sprism,1/1/1/-5/-10/1 -Gcube_mag_anom.grd
```

## See Also

## Reference

Okabe, M., 1979, Analytical expressions for gravity anomalies due to
polyhedral bodies and translation into magnetic anomalies, *Geophysics*,
44, 730-741.