grdgravmag3d(1) GMT grdgravmag3d(1)

## 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<^<i>a|>] [-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 vol- ume 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/declina- tion/inclination OR for a grid mode +m<magfile> whereamagfileais 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, nau- tical mile or US survey foot, respectively, and will be con- verted to the equivalent degrees longitude at the middle lati- tude 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.Allcoordi-nates: 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 recalcu- lated from the number of nodes and the domain. The resulting increment value depends on whether you have selected a grid- line-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] (morea|) 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 fur- ther away thanradiusfrom the current node.-V[level] (morea|) 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 aaFlat Earthaapproximation 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 explana- tion 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 phantomaSandy Islandatogether 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 dec- lination 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(1),gmtgravmag3d(1),talwani2d(1),talwani3d(1)

## REFERENCE

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

## COPYRIGHT

2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe 5.4.2 Jun 24, 2017 grdgravmag3d(1)

gmt5 5.4.2 - Generated Wed Jun 28 19:20:11 CDT 2017