talwani2dgmt - Online in the Cloud

PROGRAM:

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.

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