grdgravmag3d
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 efect 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][=|+][/yinc[unit][=|+]]
- 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 GMT 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.
- -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)
- -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 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 exits.
- --version
- Print GMT version and exit.
- --show-datadir
- Print full path to GMT share directory and exit.
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
Reference
Okabe, M., Analytical expressions for gravity anomalies due to
polyhedral bodies and translation into magnetic anomalies, Geophysics,
44, (1979), p 730-741.