.. index:: ! grdgravmag3d ************ grdgravmag3d ************ .. only:: not man grdgravmag3d - Compute the gravity effect of a grid by the method of Okabe Synopsis -------- .. include:: ../../common_SYN_OPTs.rst_ **grdgravmag3d** *grdfile_top* [*grdfile_bot*] [ |-C|\ *density* ] [ |-E|\ *thick* ] [ |-F|\ *xy_file* ] [ |-G|\ *outgrid* ] [ |-H|\ *<...>* ] [ |SYN_OPT-I| ] [ |-L|\ *z_obs* ] [ |-Q|\ [\ **n**\ *n_pad*]\ \|\ [*pad_dist*]\ \|\ [] ] [ |SYN_OPT-R| ] [ |-S|\ *radius* ] [ |SYN_OPT-V| ] [ |-Z|\ *level*\ [\ **b**\ \|\ **t**] ] [ **-fg** ] [ **-x**\ *+a|n|-n* ] |No-spaces| Description ----------- **grdgravmag3d** will 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. .. _-C: **-C**\ *density* Sets body density in SI. This option is mutually exclusive with **-H** .. _-F: **-F**\ *xy_file* Provide locations where the anomaly will be computed. Note this option is mutually exclusive with **-G**. .. _-G: **-G**\ *outgrid* Output the gravity anomaly at nodes of this grid file. Optional Arguments ------------------ .. _-E: **-E**\ *thickness* To provide the layer thickness in m [Default = 500 m]. .. _-H: **-H**\ *f_dec/f_dip/m_int/m_dec/m_dip* **-H**\ *+m* **-H**\ *x|y|z|h|t* **-H**\ *+i|+g|+r|+f|+n* Sets 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 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|+n* to do that .. _-I: .. include:: ../../explain_-I.rst_ .. _-L: **-L**\ *z_obs* Sets level of observation [Default = 0]. That is the height (z) at which anomalies are computed. .. _-Q: **-Q**\ [\ **n**\ *n_pad*]\ \|\ [\ *pad_dist*]\ \|\ [] Extend the domain of computation with respect to output **-R** region. **-Qn**\ *n_pad* artificially extends the width of the outer rim of cells to have a fake width of *n_pad* * dx[/dy]. **-Q**\ *pad_dist* extend the region by west-pad, east+pad, etc. **-Q**\ *region* Same syntax as **-R**. .. _-R: .. |Add_-R| replace:: Note: this overrides the source grid region (Default: use same region as input) .. include:: ../../explain_-R.rst_ .. _-S: **-S**\ *radius* Set 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 than *radius* from the current node. .. _-V: .. |Add_-V| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_-V.rst_ .. _-Z: **-Z**\ *level*\ [\ **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 **-Zb** or **Zt** to close the body at its bottom (for example, to compute the effect of a dome) or at its top (to compute the effect of a *spoon*). **-fg** Geographic grids (dimensions of longitude, latitude) will be converted to meters via a "Flat Earth" approximation using the current ellipsoid parameters. **-x**\ *+a|n|-n* Choose the number of processors used in multi-threading (Only available with multi-threading builds). *+a* Use all available processors. *n* Use n processors (not more than max available off course). *-n* Use (all - n) processors. .. include:: ../../explain_help.rst_ Grid Distance Units ------------------- If the grid does not have meter as the horizontal unit, append **+u**\ *unit* to the input file name to convert from the specified unit to meter. If your grid is geographic, convert distances to meters by supplying **-fg** instead. 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 in *mag.grd* over a zone defined by the surface *bat.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 -------- :doc:`gmt `, :doc:`gmtgravmag3d`, :doc:`talwani2d `, :doc:`talwani3d ` Reference --------- Okabe, M., Analytical expressions for gravity anomalies due to polyhedral bodies and translation into magnetic anomalies, *Geophysics*, 44, (1979), p 730-741.