grdgravmag3d — GMT 5.1.1 documentation

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# grdgravmag3d

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

## Synopsis

grdgravmag3d grdfile_top [grdfile_bot] [ -Cdensity ] [ -D ] [ -Fxy_file ] [ -Goutgrid ] -Ixinc[unit][=|+][/yinc[unit][=|+]] [ -Lz_obs ] [ -Q[nn_pad]|[pad_dist]|[<w/e/s/n>] ] [ -V[level] ] [ -Zlevel ] [ -fg ]

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 efect 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 exlusive with -G.
-Goutgrid
  Output the gravity anomaly at nodes of this grid file.

-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 B. GMT File Formats for details. Note: if -Rgrdfile is used then the grid spacing has already been initialized; use -I to override the values.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
  Specify the region of interest.

## Optional Arguments

-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 artifficially 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 sintax as -R.

-V[level] (more ...)
  Select verbosity level [c].
-Zlevel
  level of reference plane [Default = 0]. Use this option when the triangles describe a non-closed surface and the volume is deffined from each triangle and this reference level. An example will be the whater 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.

-^
  Print a short message about the syntax of the command, then exits (NOTE: on Windows use just -).
+
  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
-?
  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 -M -I1m -Gsandy_okb.grd -V

## See Also

gmt, gmtgravmag3d

## Reference

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

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