grdgravmag3d(1) GMT grdgravmag3d(1)
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<^<i>a|> ] [ -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 vol-
ume 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/declina-
tion/inclination
OR for a grid mode
+m<magfile> where amagfilea 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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: 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 recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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 a|)
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 fur-
ther away than radius from the current node.
-V[level] (more a|)
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 aFlat Eartha 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 explana-
tion 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 aSandy
Islanda 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 dec-
lination 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(1), gmtgravmag3d(1), talwani2d(1), talwani3d(1)
REFERENCE
Okabe, M., Analytical expressions for gravity anomalies due to polyhe-
dral bodies and translation into magnetic anomalies, Geophysics, 44,
(1979), p 730-741.
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdgravmag3d(1)
gmt5 5.4.2 - Generated Wed Jun 28 19:20:11 CDT 2017