This is the command grdgravmag3dgmt that can be run in the OnWorks free hosting provider using one of our multiple free online workstations such as Ubuntu Online, Fedora Online, Windows online emulator or MAC OS online emulator

**PROGRAM:**

**NAME**

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

**SYNOPSIS**

**grdgravmag3d**

__grdfile_top__[

__grdfile_bot__] [

__density__] [

__thick__] [

__xy_file__] [

__outgrid__] [

__<...>__] [

__increment__] [

__z_obs__] [ [

**n**

__n_pad__]|[

__pad_dist__]|[<w/e/s/n>] ] [

__region__] [

__radius__]

[ [

__level__] ] [

__level__[

**b**|

**t**] ] [

**-fg**] [

**-x**

__+a|n|-n__]

**Note:**No space is allowed between the option flag and the associated arguments.

**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 efect is going to be computed. If two grids are provided

then the gravity/magnetic effect of the volume between them is computed.

**-C**

__density__

Sets body density in SI. This option is mutually exclusive with

**-H**

**-F**

__xy_file__

Provide locations where the anomaly will be computed. Note this option is mutually

exclusive with

**-G**.

**-G**

__outgrid__

Output the gravity anomaly at nodes of this grid file.

**OPTIONAL** **ARGUMENTS**

**-E**

__thickness__

To provide the layer thickness in m [Default = 500 m].

**-H**

__f_dec/f_dip/m_int/m_dec/m_dip__

**-H**

__+m<magfile>__

**-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<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|+n__

to do that

**-I**

__xinc__

**[**

__unit__

**][=|+][/**

__yinc__

**[**

__unit__

**][=|+]]**

__x_inc__[and optionally

__y_inc__] is the grid spacing. Optionally, append a suffix

modifier.

**Geographical**

**(degrees)**

**coordinates**: Append

**m**to indicate arc minutes or

**s**

to indicate arc seconds. If one of the units

**e**,

**f**,

**k**,

**M**,

**n**or

**u**is 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). If /

__y_inc__is given but set to 0 it will be reset equal to

__x_inc__;

otherwise it will be converted to degrees latitude.

**All**

**coordinates**: If

**=**is

appended then the corresponding max

__x__(

__east__) or

__y__(

__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 the

__number__

__of__

__nodes__desired by appending

**+**to 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

**-R**

__grdfile__is used then the grid spacing has already been initialized; use

**-I**to override the values.

**-L**

__z_obs__

Sets level of observation [Default = 0]. That is the height (z) at which anomalies

are computed.

**-Q[n**

__n_pad__

**]|[**

__pad_dist__

**]|[<w/e/s/n>]**

**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[**

__unit__

**]**

__xmin__

**/**

__xmax__

**/**

__ymin__

**/**

__ymax__

**[r]**

**(more**

**...)**

Specify the region of interest. Note: this overrides the source grid region

(Default: use same region as input)

**-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[**

__level__

**]**

**(more**

**...)**

Select verbosity level [c].

**-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.

**-^**

**or**

**just**

**-**

Print a short message about the syntax of the command, then exits (NOTE: on Windows

use just

**-**).

**-+**

**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 options, then

exits.

**--version**

Print GMT version and exit.

**--show-datadir**

Print full path to GMT share directory and exit.

**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

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