talwani3d

talwani3d(cmd0::String="", arg1=nothing; kwargs...)

Compute geopotential anomalies over 3-D bodies by the method of Talwani.

(Warning: Manual translate by Claude. Needs revision)

Description

talwani3d will read a multi-segment table from file (or a GMTdataset). This file contains horizontal contours of a 3-D body at different z-levels, with one contour per segment. Each segment header must contain the parameters zlevel density, which states the z level of the contour and the density of this slice (optionally, individual slice densities may be overridden by a fixed density contrast given via density).

We can compute anomalies on an equidistant grid (by specifying a new grid with region and inc or provide an observation grid with desired elevations) or at arbitrary output points specified via track. Choose between free-air anomalies, vertical gravity gradient anomalies, or geoid anomalies. Options are available to control axes units and direction. For theory, see Kim and Wessel [2016] and Talwani and Ewing [1960].

Required Arguments

  • intable : – The file or GMTdataset describing the horizontal contours of the bodies. Contours will be automatically closed if not already closed, and repeated vertices will be eliminated. The segment header for each slice must contain the pair zlevel density, i.e., the depth level of the slice and a density contrast in kg/m³ or g/cm³.

  • I or inc or increment or spacing : – inc=x_inc | inc=(x_inc, y_inc) | inc=“xinc[+e|n][/yinc[+e|n]]”
    Specify the grid increments or the block sizes. More at spacing

  • R or region or limits : – limits=(xmin, xmax, ymin, ymax) | limits=(BB=(xmin, xmax, ymin, ymax),) | limits=(LLUR=(xmin, xmax, ymin, ymax),units=“unit”) | …more
    Specify the region of interest. More at limits. For perspective view view, optionally add zmin,zmax. This option may be used to indicate the range used for the 3-D axes. You may ask for a larger w/e/s/n region to have more room between the image and the axes.

Optional Arguments

  • A or z_up : – z_up=true
    The z-axis should be positive upwards [Default is down].

  • D or density : – density=value
    Sets a fixed density contrast that overrides any individual slice settings in the model file, in kg/m³ or g/cm³.

  • F or field : – field=:faa | field=:geoid | field=:vgg | field=(geoid=true, lat=value)
    Specify desired gravitational field component. Choose between:

    • :faa or :f - free-air anomaly [Default]
    • :geoid or :n - geoid anomaly; optionally specify latitude for normal gravity reference value via field=(geoid=true, lat=value) [Default is mid-grid or mid-profile]
    • :vgg or :v - vertical gravity gradient

  • G or outgrid or outfile : – outgrid=“filename”
    Specify the name of the output grid file. Required when an equidistant grid is implied for output. If track is used then output is written to stdout unless outfile specifies an output file.

  • M or units : – units=:h | units=:v | units=:hv | units=“hv”
    Sets distance units used. Use :h to indicate that horizontal distances are in km [m], and :v to indicate vertical distances are in km [m]. Combine as :hv or “hv” for both.

  • N or track : – track=“filename” | track=dataset
    Specifies individual (x, y[, z]) locations where we wish to compute the predicted value. When this option is used there are no grids and the output data records are written to stdout. If trackfile has 3 columns we take the z value as our observation level; this may be overridden via level.

  • V or verbose : – verbose=true | verbose=level
    Select verbosity level. More at verbose

  • Z or level : – level=value | level=“obsgrid”
    Set observation level, either as a constant or variable by giving the name of a grid with observation levels. If the latter is used then this grid determines the output grid region [0].

  • bo or binary_out : – binary_out=??
    Select native binary format for table output. More at

  • f or colinfo : – colinfo=??
    Specify the data types of input and/or output columns (time or geographical data). More at

Examples

To compute the free-air anomalies on a grid over a 3-D body that has been contoured and saved to body3d.txt, using 1700 kg/m³ as the fixed density contrast, with horizontal distances in km and vertical distances in meters:

using GMT
G = talwani3d("body3d.txt", region=(-200,200,-200,200), inc=2, units=:h,
              outgrid="3dgrav.nc", density=1700, field=:faa)

To obtain the vertical gravity gradient anomaly along the track in crossing.txt for the same model:

using GMT
D = talwani3d("body3d.txt", track="crossing.txt", units=:h, density=1700, field=:vgg)

To compute the geoid anomaly along the same track for the same model at 30S:

using GMT
D = talwani3d("body3d.txt", track="crossing.txt", units=:h, density=1700,
              field=(geoid=true, lat=-30), outfile="n_crossing.txt")

References

Kim, S.-S., and P. Wessel, 2016, New analytic solutions for modeling vertical gravity gradient anomalies, Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006263.

Talwani, M., and M. Ewing, 1960, Rapid computation of gravitational attraction of three-dimensional bodies of arbitrary shape, Geophysics, 25, 203-225.

See Also