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grdgravmag3d - Compute the gravity effect of a grid by the method of Okabe


grdgravmag3d grdfile_top [grdfile_bot] [ density ] [ thick ] [ xy_file ] [ outgrid ] [
<...> ] [ increment ] [ z_obs ] [ [nn_pad]|[pad_dist]|[<w/e/s/n>] ] [ region ] [ radius ]
[ [level] ] [ level[b|t] ] [ -fg ] [ -x+a|n|-n ]

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


grdgravmag3d will compute the gravity anomaly of a body described by one or (optionally)
two grids 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


grdfile_top [grdfile_bot]
Grid file whose gravity efect is going to be computed. If two grids are provided
then the gravity/magnetic effect of the volume between them is computed.

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

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

Output the gravity anomaly at nodes of this grid file.


To provide the layer thickness in m [Default = 500 m].

-Hf_dec/f_dip/m_int/m_dec/m_dip -H+m<magfile> -Hx|y|z|h|t -H+i|+g|+r|+f|+n
Sets parameters for computation of magnetic anomaly (Can be used multiple times).
f_dec/f_dip -> geomagnetic declination/inclination

m_int/m_dec/m_dip -> body magnetic intensity/declination/inclination

OR for a grid mode
+m<magfile> where 'magfile' is the name of the magnetic intensity file.

To compute a component, specify any of:
x|X|e|E to compute the E-W component.

y|Y|n|N to compute the N-S component.

z|Z to compute the Vertical component.

h|H to compute the Horizontal component.

t|T|f|F to compute the total field.

For a variable inclination and declination use IGRF. Set any of -H+i|+g|+r|+f|+n
to do that

x_inc [and optionally y_inc] is the grid spacing. Optionally, append a suffix
modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s
to indicate arc seconds. If one of the units e, f, k, M, n or u is appended
instead, the increment is assumed to be given in meter, foot, km, Mile, nautical
mile or US survey foot, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the conversion depends on
PROJ_ELLIPSOID). If /y_inc is given but set to 0 it will be reset equal to x_inc;
otherwise it will be converted to degrees latitude. All coordinates: If = is
appended then the corresponding max x (east) or y (north) may be slightly adjusted
to fit exactly the given increment [by default the increment may be adjusted
slightly to fit the given domain]. Finally, instead of giving an increment you may
specify the number of nodes desired by appending + to the supplied integer
argument; the increment is then recalculated from the number of nodes and the
domain. The resulting increment value depends on whether you have selected a
gridline-registered or pixel-registered grid; see App-file-formats for details.
Note: if -Rgrdfile is used then the grid spacing has already been initialized; use
-I to override the values.

Sets level of observation [Default = 0]. That is the height (z) at which anomalies
are computed.


Extend the domain of computation with respect to output -R region.
-Qnn_pad artificially extends the width of the outer rim of cells to have a
fake width of n_pad * dx[/dy].

-Qpad_dist extend the region by west-pad, east+pad, etc.

-Qregion Same syntax as -R.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
Specify the region of interest. Note: this overrides the source grid region
(Default: use same region as input)

Set search radius in km (valid only in the two grids mode OR when -E) [Default = 30
km]. This option serves to speed up the computation by not computing the effect of
prisms that are further away than radius from the current node.

-V[level] (more ...)
Select verbosity level [c].

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 water depth to compute a Bouguer anomaly.
Use -Zb or Zt to close the body at its bottom (for example, to compute the effect
of a dome) or at its top (to compute the effect of a spoon).

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

Choose the number of processors used in multi-threading (Only available with
multi-threading builds).
+a Use all available processors.

n Use n processors (not more than max available off course).

-n Use (all - n) processors.

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation of any
module-specific option (but not the GMT common options), then exits.

-? or no arguments
Print a complete usage (help) message, including the explanation of options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


If the grid does not have meter as the horizontal unit, append +uunit 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.


Suppose you want to compute the gravity effect of the phantom "Sandy Island" together with
its not phantom seamount

gmt grdgravmag3d sandy_bat.grd -C1700 -Z-4300 -fg -I1m -Gsandy_okb.grd -V

To compute the vertical component due to a magnetization stored in mag.grd over a zone
defined by the surface bat.grd, using variable declination and inclination provided the
the IGRF and using 4 processors, do:

gmt grdgravmag3d bat.grd -E10000 -Gcomp_Z.grd -Hz -H+n -H+mmag.grd -x4 -V -S50

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