.. index:: ! grdgravmag3d .. include:: ../module_supplements_purpose.rst_ ************ grdgravmag3d ************ |grdgravmag3d_purpose| Synopsis -------- .. include:: ../../common_SYN_OPTs.rst_ **gmt grdgravmag3d** *grdfile_top* [*grdfile_bot*] [ |-C|\ *density* ] [ |-E|\ *thickness* ] [ |-F|\ *xy_file* ] [ |-G|\ *outgrid* ] [ |-H|\ *args* ] [ |SYN_OPT-I| ] [ |-L|\ *z_obs* ] [ |-Q|\ [**n**\ *n_pad*]\|\ [*pad_dist*]\|\ [*region*] ] [ |SYN_OPT-R| ] [ |-S|\ *radius* ] [ |SYN_OPT-V| ] [ |-Z|\ *level*\ [**b**\|\ **t**] ] [ |SYN_OPT-f| ] [ **-x**\ *+a|n|-n* ] [ |SYN_OPT--| ] |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. Append either a constant or the name of a grid file with variable densities. 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: .. |Add_outgrid| replace:: Give the name of the output grid file. .. include:: /explain_grd_inout.rst_ :start-after: outgrid-syntax-begins :end-before: outgrid-syntax-ends 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**\ *magfile* **-Hx**\|\ **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 *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. If we want to compute the magnetic anomalies over a large region where the ambient magnetic field can no longer be assumed to be constant we can set variable inclinations and declinations via 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*]\|\ [*region*] 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|. .. |Add_-R| replace:: **Note**: This overrides the source grid region (Default: use same region as input) |Add_-R_links| .. include:: ../../explain_-R.rst_ :start-after: **Syntax** :end-before: **Description** .. _-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. .. |Add_-V| replace:: |Add_-V_links| .. include:: /explain_-V.rst_ :start-after: **Syntax** :end-before: **Description** .. _-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*). |SYN_OPT-f| 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 |SYN_OPT-f| 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 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., 1979, Analytical expressions for gravity anomalies due to polyhedral bodies and translation into magnetic anomalies, *Geophysics*, 44, 730-741.