NAME

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

SYNOPSIS

       grdgravmag3d  grdfile_top  [grdfile_bot]  [  density ] [ thick ] [ xy_file ] [ outgrid ] [
       <...> ] [ increment ] [ z_obs ] [ [nn_pad]|[pad_dist]|[<w/e/s/n>] ] [ region ] [ radius  ]
       [ [level] ] [ level[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 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.

       -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

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

       2015, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe