talwani3d(1) GMT talwani3d(1)
NAME
talwani3d - Compute free-air, geoid or vertical gravity gradients anom-
alies over 3-D bodies
SYNOPSIS
talwani3d [ modeltable ] [ -A ] [ -Drho ] ] [ -Ff|n|v ] [ -Goutfile
] [ -Iincrement ] [ -M[h][v] ] [ -Ntrackfile ] [ -Rregion ] [
-Zlevel|obsgrid ] [ -V[level] ] [ -bibinary ] [ -dnodata ] [ -eregexp
] [ -fg ] [ -iflags ] [ -oflags ] [ -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-lev-
els, 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 speci-
fied 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. Con-
tours will be automatically closed if not already closed, and
repeated vertices will be eliminated.
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: If +e 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 +n to
the supplied integer argument; the increment is then recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
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).
-Goutfile
Specify the name of the output data (for grids, see GRID FILE
FORMATS below). Required when an equidistant grid is implied
for output. If -N is used then output is written to stdout
unless G specifies an output file.
-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 out-
put data records are written to stdout.
-V[level] (more a|)
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 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.
-fg Geographic grids (dimensions of longitude, latitude) will be
converted to km via a aFlat Eartha approximation using the cur-
rent ellipsoid parameters.
-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).
-r (more a|)
Set pixel node registration [gridline].
-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 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 written to n_crossing.txt by
gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fn -Gn_crossing.txt
REFERENCES
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., and M. Ewing, 1960, Rapid computation of gravitational
attraction of three-dimensional bodies of arbitrary shape, Geophysics,
25, 203-225.
SEE ALSO
gmt.conf(1), gmt(1), grdmath(1), gravfft(1), gmtgravmag3d(1),
grdgravmag3d(1), talwani2d(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 talwani3d(1)
gmt5 5.4.2 - Generated Thu Jun 29 16:43:09 CDT 2017