talwani2d

talwani2d(cmd0::String="", arg1=nothing; kwargs...)

Compute geopotential anomalies over 2-D bodies by the method of Talwani.

(Warning: Manual translate by Claude. Needs revision)

Description

talwani2d will read a multi-segment table from file (or a GMTdataset). This file contains cross-sections of one or more 2-D bodies, with one polygon per segment. The segment header must contain the parameter density, which states the density of this body (individual body densities may be overridden by a fixed constant density contrast given via density).

We can compute anomalies on an equidistant lattice (by specifying a lattice with range) or provide arbitrary output points specified in a file via track. Choose between free-air anomalies, vertical gravity gradient anomalies, or geoid anomalies. Options are available to control axes units and direction.

Required Arguments

  • intable : – One or more files or a GMTdataset describing cross-sectional polygons of one or more bodies. Polygons will be automatically closed if not already closed, and repeated vertices will be eliminated. The segment header for each body must contain a density parameter in kg/m³ or g/cm³; see density for overriding this value.

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 per-body 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 45]
    • :vgg or :v - vertical gravity gradient

  • 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 locations where we wish to compute the predicted value. When this option is used you cannot use range to set an equidistant lattice. The output data records are written to stdout.

  • T or range : – range=(min, max, inc) | range=“min/max/inc”
    Specify an equidistant output lattice. Cannot be used with track.

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

  • Z or level : – level=value | level=“level/ymin/ymax”
    Set a constant observation level [0]. Optionally, and for gravity anomalies only (field=:faa), append the finite extent limits of a 2.5-D body.

  • bi or binary_in : – binary_in=??
    Select native binary format for primary table input. More at

Notes

The 2-D geoid anomaly is a logarithmic potential and thus has no natural reference level. We simply remove the most negative (if density contrast is positive) or positive (if density contrast is negative) computed value from all values, rendering the entire anomaly positive (or negative). You can use gmtmath to change the zero level.

Examples

To compute the free-air anomalies on an equidistant profile over a 2-D body that has been contoured and saved to body2d.txt, using 1700 kg/m³ as a constant density contrast, with all distances in meters:

using GMT
D = talwani2d("body2d.txt", range=(-200,200,2), density=1700, field=:faa)

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

using GMT
D = talwani2d("body2d.txt", track="crossing.txt", density=1700, field=:vgg)

The geoid anomaly for the same setup, evaluated at 60N:

using GMT
D = talwani2d("body2d.txt", track="crossing.txt", density=1700, field=(geoid=true, lat=60))

References

Chapman, M. E., 1979, Techniques for interpretation of geoid anomalies, J. Geophys. Res., 84(B8), 3793-3801.

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

Rasmussen, R., and L. B. Pedersen, 1979, End corrections in potential field modeling, Geophys. Prospect., 27, 749-760.

Talwani, M., J. L. Worzel, and M. Landisman, 1959, Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone, J. Geophys. Res., 64, 49-59.

See Also