talwani3d - Compute free-air, geoid or vertical gravity gradients
anomalies over 3-D bodies
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.
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.
- 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][+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, 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 +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 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.
- -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 output data records are
written to stdout.
- -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].
- -e[~]"pattern"
| -e[~]/regexp/[i] (more ...)
- Only accept data records that match the given pattern.
- -fg
- Geographic grids (dimensions of longitude, latitude) will be converted to
km via a "Flat Earth" approximation using the current ellipsoid
parameters.
- -:[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 just use -).
- -+ 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 all
options, then exits.
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).
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
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.
gmt.conf, gmt, grdmath, gravfft, gmtgravmag3d, grdgravmag3d,
talwani2d
2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F.
Wobbe