talwani3dgmt - Online in the Cloud

PROGRAM:

NAME

talwani3d - Compute free-air, geoid or vertical gravity gradients anomalies over 3-D
bodies

SYNOPSIS

talwani3d [ modeltable ] [ ] [ rho ] ] [ f|n|v ] [ grdfile ] [ increment ] [ [h][v] ] [
trackfile ] [ region ] [ level|obsgrid ] [ [level] ] [ -bi<binary> ] [ -fg ] [ -i<flags> ]
[ -o<flags> ] [ -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-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.

REQUIRED ARGUMENTS

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][=|+][/yinc[unit][=|+]]
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 = 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 + 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.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
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).

-Ggrdfile
Specify the name of the output grid file; see GRID FILE FORMATS below). Required
when an equidistant grid is implied for output. If -N is used then the output is
written to stdout.

-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.

-V[level] (more ...)
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 ...)
Select native binary input. [Default is 2 input columns].

-fg Geographic grids (dimensions of longitude, latitude) will be converted to km via a
"Flat Earth" approximation using the current ellipsoid parameters.

-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).

-r (more ...)
Set pixel node registration [gridline].

-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 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
returned by

gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fn > n_crossing.txt

REFERENCES

Talwani, M., and M. Ewing (1960), Rapid computation of gravitational attraction of
three-dimensional bodies of arbitrary shape, Geophysics, 25(203-225).

Use talwani3dgmt online using onworks.net services