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

5.2.1                                           January 28, 2016                                 TALWANI2D(1gmt)