Provided by: gmt-common_5.4.5+dfsg-2_all

**NAME**

grdgravmag3d - Compute the gravity effect of a grid by the method of Okabe

**SYNOPSIS**

grdgravmag3dgrdfile_top[grdfile_bot] [-Cdensity] [-Ethick] [-Fxy_file] [-Goutgrid] [-H<...>] [-Iincrement] [-Lz_obs] [-Q[nn_pad]|[pad_dist]|[<w/e/s/n>] ] [-Rregion] [-Sradius] [-V[level] ] [-Zlevel[b|t] ] [-fg] [-x+a|n|-n]Note:No space is allowed between the option flag and the associated arguments.

**DESCRIPTION**

grdgravmag3dwill compute the gravity anomaly of a body described by one or (optionally) two grids The output can either be along a given set of xy locations or on a grid. This method is not particularly fast but allows computing the anomaly of arbitrarily complex shapes.

**REQUIRED** **ARGUMENTS**

grdfile_top[grdfile_bot]Grid file whose gravity effect is going to be computed. If two grids are provided then the gravity/magnetic effect of the volume between them is computed.-CdensitySets body density in SI. This option is mutually exclusive with-H-Fxy_fileProvide locations where the anomaly will be computed. Note this option is mutually exclusive with-G.-GoutgridOutput the gravity anomaly at nodes of this grid file.

**OPTIONAL** **ARGUMENTS**

-EthicknessTo provide the layer thickness in m [Default = 500 m].-Hf_dec/f_dip/m_int/m_dec/m_dip-H+m<magfile>-Hx|y|z|h|t-H+i|+g|+r|+f|+nSets parameters for computation of magnetic anomaly (Can be used multiple times). f_dec/f_dip -> geomagnetic declination/inclination m_int/m_dec/m_dip -> body magnetic intensity/declination/inclination OR for a grid mode +m<magfile> where 'magfile' is the name of the magnetic intensity file. To compute a component, specify any of: x|X|e|E to compute the E-W component. y|Y|n|N to compute the N-S component. z|Z to compute the Vertical component. h|H to compute the Horizontal component. t|T|f|F to compute the total field. For a variable inclination and declination use IGRF. Set any of-H+i|+g|+r|+f|+nto do that-Ixinc[unit][+e|n][/yinc[unit][+e|n]]x_inc[and optionallyy_inc] is the grid spacing. Optionally, append a suffix modifier.Geographical(degrees)coordinates: Appendmto indicate arc minutes orsto indicate arc seconds. If one of the unitse,f,k,M,noruis 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). Ify_incis given but set to 0 it will be reset equal tox_inc; otherwise it will be converted to degrees latitude.Allcoordinates: If+eis appended then the corresponding maxx(east) ory(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 thenumberofnodesdesired by appending+nto 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-Rgrdfileis used then the grid spacing has already been initialized; use-Ito override the values.-Lz_obsSets level of observation [Default = 0]. That is the height (z) at which anomalies are computed.-Q[nn_pad]|[pad_dist]|[<w/e/s/n>] Extend the domain of computation with respect to output-Rregion.-Qnn_padartificially extends the width of the outer rim of cells to have a fake width ofn_pad* dx[/dy].-Qpad_distextend the region by west-pad, east+pad, etc.-QregionSame syntax as-R.-Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...) Specify the region of interest. Note: this overrides the source grid region (Default: use same region as input)-SradiusSet search radius in km (valid only in the two grids mode OR when-E) [Default = 30 km]. This option serves to speed up the computation by not computing the effect of prisms that are further away thanradiusfrom the current node.-V[level] (more ...) Select verbosity level [c].-Zlevel[b|t] level of reference plane [Default = 0]. Use this option when the triangles describe a non-closed surface and the volume is defined from each triangle and this reference level. An example will be the water depth to compute a Bouguer anomaly. Use-ZborZtto close the body at its bottom (for example, to compute the effect of a dome) or at its top (to compute the effect of aspoon).-fgGeographic grids (dimensions of longitude, latitude) will be converted to meters via a "Flat Earth" approximation using the current ellipsoid parameters.-x+a|n|-nChoose the number of processors used in multi-threading (Only available with multi-threading builds).+aUse all available processors.nUse n processors (not more than max available off course).-nUse (all - n) processors.-^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.

**GRID** **DISTANCE** **UNITS**

If the grid does not have meter as the horizontal unit, append+uunitto the input file name to convert from the specified unit to meter. If your grid is geographic, convert distances to meters by supplying-fginstead.

**EXAMPLES**

Suppose you want to compute the gravity effect of the phantom "Sandy Island" together with its not phantom seamount gmt grdgravmag3d sandy_bat.grd -C1700 -Z-4300 -fg -I1m -Gsandy_okb.grd -V To compute the vertical component due to a magnetization stored inmag.grdover a zone defined by the surfacebat.grd, using variable declination and inclination provided the the IGRF and using 4 processors, do: gmt grdgravmag3d bat.grd -E10000 -Gcomp_Z.grd -Hz -H+n -H+mmag.grd -x4 -V -S50

**SEE** **ALSO**

gmt, gmtgravmag3d, talwani2d, talwani3d

**REFERENCE**

Okabe, M., Analytical expressions for gravity anomalies due to polyhedral bodies and translation into magnetic anomalies,Geophysics, 44, (1979), p 730-741.

**COPYRIGHT**

2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe