manpagez: man pages & more
man talwani2d(1)
Home | html | info | man
talwani2d(1)                          GMT                         talwani2d(1)




NAME

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


SYNOPSIS

       talwani2d [ modeltable ] [  -A ] [  -Drho  ]  ]  [   -Ff|n[lat]|v  ]  [
       -M[h][v] ] [  -Ntrackfile ] [  -Tminmax/inc ] [  -Zlevel[ymin/ymax] ] [
       -V[level] ] [ -bibinary ] [ -dnodata ] [  -eregexp  ]  [  -iflags  ]  [
       -oflags ] [ -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 speci-
       fied  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 bod-
              ies.  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[lat]|v
              Specify desired gravitational field component.  Choose between f
              (free-air  anomaly)  [Default],  n (geoid, and optionally append
              average latitude for normal gravity reference value [45])  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 std-
              out.

       -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 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.

       -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).

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

       -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 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 anom-
          aly  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.

       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., J. L. Worzel, and M. Landisman, 1959, Rapid gravity compu-
       tations for two-dimensional bodies with application  to  the  Mendocino
       submarine fracture zone, J. Geophys. Res., 64, 49-59.


SEE ALSO

       gmt.conf(5), gmt(1), grdmath(1), gmtmath(1), gravfft(1),
       gmtgravmag3d(1), grdgravmag3d(1), talwani3d(1)


COPYRIGHT

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



5.4.2                            Jun 24, 2017                     talwani2d(1)

gmt5 5.4.2 - Generated Thu Jun 29 16:41:55 CDT 2017
© manpagez.com 2000-2021
Individual documents may contain additional copyright information.