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talwani3d(1)                          GMT                         talwani3d(1)


       talwani3d - Compute free-air, geoid or vertical gravity gradients anom-
       alies over 3-D bodies


       talwani3d [ modeltable ] [  -A ] [  -Drho ] ] [  -Ff|n|v ] [  -Goutfile
       ]  [   -Iincrement  ]  [   -M[h][v]  ] [  -Ntrackfile ] [  -Rregion ] [
       -Zlevel|obsgrid ] [  -V[level] ] [ -bibinary ] [ -dnodata ] [  -eregexp
       ] [ -fg ] [ -iflags ] [ -oflags ] [ -r ] [ -x[[-]n] ]

       Note:  No  space  is allowed between the option flag and the associated


       talwani3d will read the multi-segment modeltable from file or  standard
       input.   This  file contains contours of a 3-D body at different z-lev-
       els, with one contour per segment.  The segment header must contain the
       parameters zlevel rho, which states the z contour level and the density
       of this slice (individual slice densities may be overridden by a  fixed
       density  contrast  given  via  -D).   We  can  compute  anomalies on an
       equidistant grid (by specifying a new grid with -R and -I or provide an
       observation  grid with elevations) or at arbitrary output points speci-
       fied via -N.  Chose from free-air anomalies, vertical gravity  gradient
       anomalies,  or  geoid anomalies.  Options are available to control axes
       units and direction.


              The file describing the horizontal contours of the bodies.  Con-
              tours  will  be  automatically closed if not already closed, and
              repeated vertices will be eliminated.

              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.

       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
              Specify the region of interest.


       -A     The z-axis should be positive upwards [Default is down].

       -Dunit Sets fixed density contrast that overrides any setting in  model
              file, in kg/m^3.

              Specify desired gravitational field component.  Choose between f
              (free-air anomaly) [Default], n (geoid) or v  (vertical  gravity

              Specify  the  name  of the output data (for grids, see GRID FILE
              FORMATS below).  Required when an equidistant  grid  is  implied
              for  output.   If  -N  is  used then output is written to stdout
              unless G specifies an output file.

              Sets units used.  Append h to indicate horizontal distances  are
              in km [m], and append z to indicate vertical distances are in km

              Specifies locations where  we  wish  to  compute  the  predicted
              value.  When this option is used there are no grids and the out-
              put data records are written to stdout.

       -V[level] (more a|)
              Select verbosity level [c].

              Set observation level either as a constant or give the name of a
              grid  with  observation  levels.   If the latter is used the the
              grid determines the output grid region [0].

       -bi[ncols][t] (more a|)
              Select native binary input. [Default is 2 input columns].

       -d[i|o]nodata (more a|)
              Replace input columns that equal nodata  with  NaN  and  do  the
              reverse on output.

       -e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
              Only accept data records that match the given pattern.

       -fg    Geographic  grids  (dimensions  of  longitude, latitude) will be
              converted to km via a aFlat Eartha approximation using the  cur-
              rent ellipsoid parameters.

       -h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
              Skip or produce header record(s). Not used with binary data.

       -icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
              Select input columns and transformations (0 is first column).

       -ocols[,a|] (more a|)
              Select output columns (0 is first column).

       -r (more a|)
              Set pixel node registration [gridline].

       -x[[-]n] (more a|)
              Limit  number of cores used in multi-threaded algorithms (OpenMP

       -:[i|o] (more a|)
              Swap 1st and 2nd column on input and/or output.

       -^ 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.


       For map distance unit, append unit d for arc degree, m for arc  minute,
       and s for arc second, or e for meter [Default], f for foot, k for km, M
       for statute mile, n for nautical mile, and u for  US  survey  foot.  By
       default  we compute such distances using a spherical approximation with
       great circles. Prepend - to a distance (or the unit is no  distance  is
       given) to perform aFlat Eartha calculations (quicker but less accurate)
       or prepend + to perform exact geodesic calculations  (slower  but  more


       To  compute  the  free-air anomalies on a grid over a 3-D body that has
       been contoured and saved to body.txt, using 1.7 g/cm^3 as  the  density
       contrast, try

          gmt talwani3d -R-200/200/-200/200 -I2 body.txt -D1700 -Fg

       To  obtain  the  vertical  gravity  gradient anomaly along the track in
       crossing.txt for the same model, try

          gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fv > vgg_crossing.txt

       Finally, the geoid anomaly along the same track in crossing.txt for the
       same model is written to n_crossing.txt by

          gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fn -Gn_crossing.txt


       Kim,  S.-S.,  and  P. Wessel, 2016, New analytic solutions for modeling
       vertical gravity gradient anomalies, Geochem.  Geophys.  Geosyst.,  17,

       Talwani,  M.,  and  M.  Ewing, 1960, Rapid computation of gravitational
       attraction of three-dimensional bodies of arbitrary shape,  Geophysics,
       25, 203-225.


       gmt.conf(1), gmt(1), grdmath(1), gravfft(1), gmtgravmag3d(1),
       grdgravmag3d(1), talwani2d(1)


       2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe

5.4.2                            Jun 24, 2017                     talwani3d(1)

gmt5 5.4.2 - Generated Thu Jun 29 16:43:09 CDT 2017
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