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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
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