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