Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdgravmag3d - Compute the gravity effect of a grid by the method of Okabe
SYNOPSIS
grdgravmag3d grdfile_top [grdfile_bot] [ -Cdensity ] [ -Ethick ] [ -Fxy_file ] [ -Goutgrid ] [ -H<…>
] [ -Iincrement ] [ -Lz_obs ] [ -Q[nn_pad]|[pad_dist]|[<w/e/s/n>] ] [ -Rregion ] [ -Sradius ] [
-V[level] ] [ -Zlevel[b|t] ] [ -fg ] [ -x+a|n|-n ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
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 shapes.
REQUIRED ARGUMENTS
grdfile_top [grdfile_bot]
Grid file whose gravity effect is going to be computed. If two grids are provided then the
gravity/magnetic effect of the volume between them is computed.
-Cdensity
Sets body density in SI. This option is mutually exclusive with -H
-Fxy_file
Provide locations where the anomaly will be computed. Note this option is mutually exclusive with
-G.
-Goutgrid
Output the gravity anomaly at nodes of this grid file.
OPTIONAL ARGUMENTS
-Ethickness
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
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
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 +e 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 +n 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.
-Lz_obs
Sets level of observation [Default = 0]. That is the height (z) at which anomalies are computed.
-Q[nn_pad]|[pad_dist]|[<w/e/s/n>]
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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest. Note: this overrides the source grid region (Default: use same
region as input)
-Sradius
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].
-Zlevel[b|t]
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.
-x+a|n|-n
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 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 exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.
GRID DISTANCE UNITS
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.
EXAMPLES
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
SEE ALSO
gmt, gmtgravmag3d, talwani2d, talwani3d
REFERENCE
Okabe, M., Analytical expressions for gravity anomalies due to polyhedral bodies and translation into
magnetic anomalies, Geophysics, 44, (1979), p 730-741.
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.3 Jan 03, 2018 GRDGRAVMAG3D(1gmt)