Provided by: gmt-common_5.4.5+dfsg-1_all bug


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


       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.


       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 effect 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 +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.

              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.

       -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)

              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
              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.


       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


       gmt, gmtgravmag3d, talwani2d, talwani3d


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


       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe