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talwani3d(1)                          GMT                         talwani3d(1)




NAME

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


SYNOPSIS

       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
       arguments.


DESCRIPTION

       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.


REQUIRED ARGUMENTS

       modeltable
              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.

       -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.

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


OPTIONAL ARGUMENTS

       -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.

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

       -Goutfile
              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.

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

       -Ntrackfile
              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].

       -Zlevel|obsgrid
              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
              required).

       -:[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.


UNITS

       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
       accurate).


EXAMPLES

       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 -G3dgrav.nc 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


REFERENCES

       Kim,  S.-S.,  and  P. Wessel, 2016, New analytic solutions for modeling
       vertical gravity gradient anomalies, Geochem.  Geophys.  Geosyst.,  17,
       http://dx.doi.org/10.1002/2016GC006263.

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


SEE ALSO

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


COPYRIGHT

       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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