1TALWANI3D(1) GMT TALWANI3D(1)
2
6 talwani3d - Compute free-air, geoid or vertical gravity gradients anom‐
7 alies over 3-D bodies
8
10 talwani3d [ modeltable ] [ -A ] [ -Drho ] ] [ -Ff|n|v ] [ -Goutfile
11 ] [ -Iincrement ] [ -M[h][v] ] [ -Ntrackfile ] [ -Rregion ] [
12 -Zlevel|obsgrid ] [ -V[level] ] [ -bibinary ] [ -dnodata ] [ -eregexp
13 ] [ -fg ] [ -iflags ] [ -oflags ] [ -r ] [ -x[[-]n] ]
14 Note: No space is allowed between the option flag and the associated
16 arguments.
17
19 talwani3d will read the multi-segment modeltable from file or standard
20 input. This file contains contours of a 3-D body at different z-lev‐
21 els, with one contour per segment. The segment header must contain the
22 parameters zlevel rho, which states the z contour level and the density
23 of this slice (individual slice densities may be overridden by a fixed
24 density contrast given via -D). We can compute anomalies on an
25 equidistant grid (by specifying a new grid with -R and -I or provide an
26 observation grid with elevations) or at arbitrary output points speci‐
27 fied via -N. Chose from free-air anomalies, vertical gravity gradient
28 anomalies, or geoid anomalies. Options are available to control axes
29 units and direction.
30
32 modeltable
33 The file describing the horizontal contours of the bodies. Con‐
34 tours will be automatically closed if not already closed, and
35 repeated vertices will be eliminated.
36 -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
38 x_inc [and optionally y_inc] is the grid spacing. Optionally,
39 append a suffix modifier. Geographical (degrees) coordinates:
40 Append m to indicate arc minutes or s to indicate arc seconds.
41 If one of the units e, f, k, M, n or u is appended instead, the
42 increment is assumed to be given in meter, foot, km, Mile, nau‐
43 tical mile or US survey foot, respectively, and will be con‐
44 verted to the equivalent degrees longitude at the middle lati‐
45 tude of the region (the conversion depends on PROJ_ELLIPSOID).
46 If y_inc is given but set to 0 it will be reset equal to x_inc;
47 otherwise it will be converted to degrees latitude. All coordi‐
48 nates: If +e is appended then the corresponding max x (east) or
49 y (north) may be slightly adjusted to fit exactly the given
50 increment [by default the increment may be adjusted slightly to
51 fit the given domain]. Finally, instead of giving an increment
52 you may specify the number of nodes desired by appending +n to
53 the supplied integer argument; the increment is then recalcu‐
54 lated from the number of nodes and the domain. The resulting
55 increment value depends on whether you have selected a grid‐
56 line-registered or pixel-registered grid; see App-file-formats
57 for details. Note: if -Rgrdfile is used then the grid spacing
58 has already been initialized; use -I to override the values.
59 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
61 Specify the region of interest.
62
64 -A The z-axis should be positive upwards [Default is down].
65 -Dunit Sets fixed density contrast that overrides any setting in model
67 file, in kg/m^3.
68 -Ff|n|v
70 Specify desired gravitational field component. Choose between f
71 (free-air anomaly) [Default], n (geoid) or v (vertical gravity
72 gradient).
73 -Goutfile
75 Specify the name of the output data (for grids, see GRID FILE
76 FORMATS below). Required when an equidistant grid is implied
77 for output. If -N is used then output is written to stdout
78 unless G specifies an output file.
79 -M[h][v]
81 Sets units used. Append h to indicate horizontal distances are
82 in km [m], and append z to indicate vertical distances are in km
83 [m].
84 -Ntrackfile
86 Specifies locations where we wish to compute the predicted
87 value. When this option is used there are no grids and the out‐
88 put data records are written to stdout.
89 -V[level] (more ...)
91 Select verbosity level [c].
92 -Zlevel|obsgrid
94 Set observation level either as a constant or give the name of a
95 grid with observation levels. If the latter is used the the
96 grid determines the output grid region [0].
97 -bi[ncols][t] (more ...)
99 Select native binary input. [Default is 2 input columns].
100 -d[i|o]nodata (more ...)
102 Replace input columns that equal nodata with NaN and do the
103 reverse on output.
104 -e[~]"pattern" | -e[~]/regexp/[i] (more ...)
106 Only accept data records that match the given pattern.
107 -fg Geographic grids (dimensions of longitude, latitude) will be
109 converted to km via a "Flat Earth" approximation using the cur‐
110 rent ellipsoid parameters.
111 -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
113 Skip or produce header record(s). Not used with binary data.
114 -icols[+l][+sscale][+ooffset][,...] (more ...)
116 Select input columns and transformations (0 is first column).
117 -ocols[,...] (more ...)
119 Select output columns (0 is first column).
120 -r (more ...)
122 Set pixel node registration [gridline].
123 -x[[-]n] (more ...)
125 Limit number of cores used in multi-threaded algorithms (OpenMP
126 required).
127 -:[i|o] (more ...)
129 Swap 1st and 2nd column on input and/or output.
130 -^ or just -
132 Print a short message about the syntax of the command, then
133 exits (NOTE: on Windows just use -).
134 -+ or just +
136 Print an extensive usage (help) message, including the explana‐
137 tion of any module-specific option (but not the GMT common
138 options), then exits.
139 -? or no arguments
141 Print a complete usage (help) message, including the explanation
142 of all options, then exits.
143
145 For map distance unit, append unit d for arc degree, m for arc minute,
146 and s for arc second, or e for meter [Default], f for foot, k for km, M
147 for statute mile, n for nautical mile, and u for US survey foot. By
148 default we compute such distances using a spherical approximation with
149 great circles. Prepend - to a distance (or the unit is no distance is
150 given) to perform "Flat Earth" calculations (quicker but less accurate)
151 or prepend + to perform exact geodesic calculations (slower but more
152 accurate).
153
155 To compute the free-air anomalies on a grid over a 3-D body that has
156 been contoured and saved to body.txt, using 1.7 g/cm^3 as the density
157 contrast, try
158 gmt talwani3d -R-200/200/-200/200 -I2 -G3dgrav.nc body.txt -D1700 -Fg
160 To obtain the vertical gravity gradient anomaly along the track in
162 crossing.txt for the same model, try
163 gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fv > vgg_crossing.txt
165 Finally, the geoid anomaly along the same track in crossing.txt for the
167 same model is written to n_crossing.txt by
168 gmt talwani3d -Ncrossing.txt body.txt -D1700 -Fn -Gn_crossing.txt
170
172 Kim, S.-S., and P. Wessel, 2016, New analytic solutions for modeling
173 vertical gravity gradient anomalies, Geochem. Geophys. Geosyst., 17,
174 http://dx.doi.org/10.1002/2016GC006263.
175 Talwani, M., and M. Ewing, 1960, Rapid computation of gravitational
177 attraction of three-dimensional bodies of arbitrary shape, Geophysics,
178 25, 203-225.
179
181 gmt.conf, gmt, grdmath, gravfft, gmtgravmag3d, grdgravmag3d, talwani2d
182
184 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
1855.4.5 Feb 24, 2019 TALWANI3D(1)