NAME

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

SYNOPSIS

       talwani3d [ modeltable ] [  ] [ rho ] ] [ f|n|v ] [ grdfile ] [ increment ] [ [h][v]  ]  [
       trackfile ] [ region ] [ level|obsgrid ] [ [level] ] [ -bi<binary> ] [ -fg ] [ -i<flags> ]
       [ -o<flags> ] [ -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-levels, 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 specified 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.   Contours  will  be
              automatically  closed  if  not  already  closed,  and  repeated  vertices  will  be
              eliminated.

       -Ixinc[unit][=|+][/yinc[unit][=|+]]
              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, nautical
              mile or US survey foot, respectively, and  will  be  converted  to  the  equivalent
              degrees  longitude  at the middle latitude 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 coordinates: If = 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  +  to the supplied integer
              argument; the increment is then recalculated from  the  number  of  nodes  and  the
              domain.  The  resulting  increment  value  depends  on  whether you have selected a
              gridline-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.

       -R[unit]xmin/xmax/ymin/ymax[r] (more ...)
              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).

       -Ggrdfile
              Specify  the  name of the output grid file; see GRID FILE FORMATS below).  Required
              when an equidistant grid is implied for output.  If -N is used then the  output  is
              written to stdout.

       -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 output data records are written to stdout.

       -V[level] (more ...)
              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 ...)
              Select native binary input. [Default is 2 input columns].

       -fg    Geographic  grids (dimensions of longitude, latitude) will be converted to km via a
              "Flat Earth" approximation using the current ellipsoid parameters.

       -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).

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

       -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 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
       returned by

          gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fn > n_crossing.txt

REFERENCES

       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, gmt, grdmath, gravfft, gmtgravmag3d, grdgravmag3d, talwani2d

COPYRIGHT

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