NAME

       talwani2d  -  Compute  free-air,  geoid  or  vertical gravity gradients anomalies over 2-D
       bodies

SYNOPSIS

       talwani2d [ modeltable ] [  ] [ rho ] ] [ f|n|v ] [ [h][v] ] [ trackfile ] [ minmax/inc  ]
       [ level[ymin/ymax] ] [ [level] ] [ -bi<binary> ] [ -i<flags> ] [ -o<flags> ] [ -x[[-]n] ]

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

DESCRIPTION

       talwani2d  will  read the multi-segment modeltable from file or standard input.  This file
       contains cross-sections of one or more 2-D bodies, with  one  polygon  per  segment.   The
       segment  header  must contain the parameter rho, which states the the density of this body
       (individual body densities may be overridden by a fixed constant  density  contrast  given
       via -D).  We can compute anomalies on an equidistant lattice (by specifying a lattice with
       -T) or provide arbitrary output points  specified  in  a  file  via  -N.   Choose  between
       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 cross-sectional polygons of one or more bodies.  Polygons  will
              be  automatically  closed  if  not  already  closed,  and repeated vertices will be
              eliminated.

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).

       -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  you cannot use -T* to set an equidistant lattice. The output data records
              are written to stdout.

       -Tminmax/inc
              Specify an equidistant output lattice starting at x = min, with increments inc  and
              ending at x = max.

       -Zlevel[ymin/ymax]
              Set  observation  level  as  a constant [0].  Optionally, and for gravity anomalies
              only, append the finite extent limits of a 2.5-D body.

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

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

       -icols[l][sscale][ooffset][,...] (more ...)
              Select input columns (0 is first column).

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

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

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

       -:[i|o] (more ...)
              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
              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.

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  "Flat Earth" 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 2-D body that has  been  contoured  and
       saved to body.txt, using 1.7 g/cm^3 as the density contrast, try

          gmt talwani2d -T-200/200/2 body.txt -D1700 -Fg > 2dgrav.txt

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

          gmt talwani2d -Ncrossing.txt body.txt -D1700 -Fv > vgg_crossing.txt

       The geoid anomaly for the same setup is given by

          gmt talwani2d -Ncrossing.txt body.txt -D1700 -Fn > n_crossing.txt

NOTES

       1. The 2-D geoid anomaly is a logarithmic potential and  thus  has  no  natural  reference
          level.   We  simply  remove  the  most  negative  (if  density contrast is positive) or
          positive (if density contrast is negative) computed value from  all  values,  rendering
          the  entire  anomaly  positive  (or  negative).  You can use gmtmath to change the zero
          level to suit your needs.

REFERENCES

       Chapman, M. E. (1979), Techniques for interpretation of geoid anomalies, J. Geophys. Res.,
       84(B8),  3793-3801.   Talwani,  M.,  J.  L. Worzel, and M. Landisman (1959), Rapid gravity
       computations for two-dimensional  bodies  with  application  to  the  Mendocino  submarine
       fracture zone, J. Geophys. Res., 64, 49-59.

SEE ALSO

       gmt.conf, gmt, grdmath, gmtmath, gravfft, gmtgravmag3d, grdgravmag3d, talwani3d

COPYRIGHT

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