1GRDGRAVMAG3D(1) GMT GRDGRAVMAG3D(1)
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6 grdgravmag3d - Compute the gravity effect of a grid by the method of
7 Okabe
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10 grdgravmag3d grdfile_top [grdfile_bot] [ -Cdensity ] [ -Ethick ] [
11 -Fxy_file ] [ -Goutgrid ] [ -H<...> ] [ -Iincrement ] [ -Lz_obs ] [
12 -Q[nn_pad]|[pad_dist]|[<w/e/s/n>] ] [ -Rregion ] [ -Sradius ] [
13 -V[level] ] [ -Zlevel[b|t] ] [ -fg ] [ -x+a|n|-n ]
14 Note: No space is allowed between the option flag and the associated
16 arguments.
17
19 grdgravmag3d will compute the gravity anomaly of a body described by
20 one or (optionally) two grids The output can either be along a given
21 set of xy locations or on a grid. This method is not particularly fast
22 but allows computing the anomaly of arbitrarily complex shapes.
23
25 grdfile_top [grdfile_bot]
26 Grid file whose gravity effect is going to be computed. If two
27 grids are provided then the gravity/magnetic effect of the vol‐
28 ume between them is computed.
29 -Cdensity
31 Sets body density in SI. This option is mutually exclusive with
32 -H
33 -Fxy_file
35 Provide locations where the anomaly will be computed. Note this
36 option is mutually exclusive with -G.
37 -Goutgrid
39 Output the gravity anomaly at nodes of this grid file.
40
42 -Ethickness
43 To provide the layer thickness in m [Default = 500 m].
44 -Hf_dec/f_dip/m_int/m_dec/m_dip -H+m<magfile> -Hx|y|z|h|t
46 -H+i|+g|+r|+f|+n
47 Sets parameters for computation of magnetic anomaly (Can be used
48 multiple times).
49 f_dec/f_dip -> geomagnetic declination/inclination
50 m_int/m_dec/m_dip -> body magnetic intensity/declina‐
52 tion/inclination
53 OR for a grid mode
55 +m<magfile> where 'magfile' is the name of the magnetic
56 intensity file.
57 To compute a component, specify any of:
59 x|X|e|E to compute the E-W component.
60 y|Y|n|N to compute the N-S component.
62 z|Z to compute the Vertical component.
64 h|H to compute the Horizontal component.
66 t|T|f|F to compute the total field.
68 For a variable inclination and declination use IGRF. Set any
70 of -H+i|+g|+r|+f|+n to do that
71 -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
73 x_inc [and optionally y_inc] is the grid spacing. Optionally,
74 append a suffix modifier. Geographical (degrees) coordinates:
75 Append m to indicate arc minutes or s to indicate arc seconds.
76 If one of the units e, f, k, M, n or u is appended instead, the
77 increment is assumed to be given in meter, foot, km, Mile, nau‐
78 tical mile or US survey foot, respectively, and will be con‐
79 verted to the equivalent degrees longitude at the middle lati‐
80 tude of the region (the conversion depends on PROJ_ELLIPSOID).
81 If y_inc is given but set to 0 it will be reset equal to x_inc;
82 otherwise it will be converted to degrees latitude. All coordi‐
83 nates: If +e is appended then the corresponding max x (east) or
84 y (north) may be slightly adjusted to fit exactly the given
85 increment [by default the increment may be adjusted slightly to
86 fit the given domain]. Finally, instead of giving an increment
87 you may specify the number of nodes desired by appending +n to
88 the supplied integer argument; the increment is then recalcu‐
89 lated from the number of nodes and the domain. The resulting
90 increment value depends on whether you have selected a grid‐
91 line-registered or pixel-registered grid; see App-file-formats
92 for details. Note: if -Rgrdfile is used then the grid spacing
93 has already been initialized; use -I to override the values.
94 -Lz_obs
96 Sets level of observation [Default = 0]. That is the height (z)
97 at which anomalies are computed.
98 -Q[nn_pad]|[pad_dist]|[<w/e/s/n>]
100 Extend the domain of computation with respect to output -R
102 region.
103 -Qnn_pad artificially extends the width of the outer rim
104 of cells to have a fake width of n_pad * dx[/dy].
105 -Qpad_dist extend the region by west-pad, east+pad, etc.
107 -Qregion Same syntax as -R.
109 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
111 Specify the region of interest. Note: this overrides the source
112 grid region (Default: use same region as input)
113 -Sradius
115 Set search radius in km (valid only in the two grids mode OR
116 when -E) [Default = 30 km]. This option serves to speed up the
117 computation by not computing the effect of prisms that are fur‐
118 ther away than radius from the current node.
119 -V[level] (more ...)
121 Select verbosity level [c].
122 -Zlevel[b|t]
124 level of reference plane [Default = 0]. Use this option when the
125 triangles describe a non-closed surface and the volume is
126 defined from each triangle and this reference level. An example
127 will be the water depth to compute a Bouguer anomaly. Use -Zb or
128 Zt to close the body at its bottom (for example, to compute the
129 effect of a dome) or at its top (to compute the effect of a
130 spoon).
131 -fg Geographic grids (dimensions of longitude, latitude) will be
133 converted to meters via a "Flat Earth" approximation using the
134 current ellipsoid parameters.
135 -x+a|n|-n
137 Choose the number of processors used in multi-threading (Only
138 available with multi-threading builds).
139 +a Use all available processors.
140 n Use n processors (not more than max available off course).
142 -n Use (all - n) processors.
144 -^ or just -
146 Print a short message about the syntax of the command, then
147 exits (NOTE: on Windows just use -).
148 -+ or just +
150 Print an extensive usage (help) message, including the explana‐
151 tion of any module-specific option (but not the GMT common
152 options), then exits.
153 -? or no arguments
155 Print a complete usage (help) message, including the explanation
156 of all options, then exits.
157
159 If the grid does not have meter as the horizontal unit, append +uunit
160 to the input file name to convert from the specified unit to meter. If
161 your grid is geographic, convert distances to meters by supplying -fg
162 instead.
163
165 Suppose you want to compute the gravity effect of the phantom "Sandy
166 Island" together with its not phantom seamount
167 gmt grdgravmag3d sandy_bat.grd -C1700 -Z-4300 -fg -I1m -Gsandy_okb.grd -V
169 To compute the vertical component due to a magnetization stored in
171 mag.grd over a zone defined by the surface bat.grd, using variable dec‐
172 lination and inclination provided the the IGRF and using 4 processors,
173 do:
174 gmt grdgravmag3d bat.grd -E10000 -Gcomp_Z.grd -Hz -H+n -H+mmag.grd -x4 -V -S50
176
178 gmt, gmtgravmag3d, talwani2d, talwani3d
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181 Okabe, M., Analytical expressions for gravity anomalies due to polyhe‐
182 dral bodies and translation into magnetic anomalies, Geophysics, 44,
183 (1979), p 730-741.
184
186 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
1875.4.5 Feb 24, 2019 GRDGRAVMAG3D(1)