talwani2d(1) GMT talwani2d(1)

## NAME

talwani2d - Compute free-air, geoid or vertical gravity gradients anom- alies over 2-D bodies

## SYNOPSIS

talwani2d[modeltable] [-A] [-Drho] ] [-Ff|n[lat]|v] [-M[h][v] ] [-Ntrackfile] [-Tminmax/inc] [-Zlevel[ymin/ymax] ] [-V[level] ] [-bibinary ] [-dnodata ] [-eregexp ] [-iflags ] [-oflags ] [-x[[-]n] ]Note:No space is allowed between the option flag and the associated arguments.

## DESCRIPTION

talwani2dwill read the multi-segmentmodeltablefrom file or standard input. This file contains cross-sections of one or more 2-D bodies, with one polygon per segment. The segment header must contain the parameterrho, which states the the density of this body (individual body densities may be overridden by a fixed constant density contrast given via-D). We can compute anomalies on an equidistant lattice (by specifying a lattice with-T) or provide arbitrary output points speci- fied in a file via-N. Choose between free-air anomalies, vertical gravity gradient anomalies, or geoid anomalies. Options are available to control axes units and direction.

## REQUIRED ARGUMENTS

modeltableThe file describing cross-sectional polygons of one or more bod- ies. Polygons will be automatically closed if not already closed, and repeated vertices will be eliminated.

## OPTIONAL ARGUMENTS

-AThez-axis should be positive upwards [Default is down].-DunitSets fixed density contrast that overrides any setting in model file, in kg/m^3.-Ff|n[lat]|vSpecify desired gravitational field component. Choose betweenf(free-air anomaly) [Default],n(geoid, and optionally append average latitude for normal gravity reference value [45]) orv(vertical gravity gradient).-M[h][v] Sets units used. Appendhto indicate horizontal distances are in km [m], and appendzto indicate vertical distances are in km [m].-NtrackfileSpecifies locations where we wish to compute the predicted value. When this option is used you cannot use-Tto set an equidistant lattice. The output data records are written to std- out.-Tminmax/incSpecify an equidistant output lattice starting atx=min, with incrementsincand ending atx=max.-Zlevel[ymin/ymax] Set observation level as a constant [0]. Optionally, and for gravity anomalies only, append the finite extent limits of a 2.5-D body.-bi[ncols][t] (morea|) Select native binary input. [Default is 2 input columns].-d[i|o]nodata(morea|) Replace input columns that equalnodatawith NaN and do the reverse on output.-e[~]^<i>apattern^<i>a|-e[~]/regexp/[i] (morea|) Only accept data records that match the given pattern.-h[i|o][n][+c][+d][+rremark][+rtitle] (morea|) Skip or produce header record(s). Not used with binary data.-icols[+l][+sscale][+ooffset][,^<i>a|] (morea|) Select input columns and transformations (0 is first column).-ocols[,a|] (morea|) Select output columns (0 is first column).-V[level] (morea|) Select verbosity level [c].-x[[-]n] (morea|) Limit number of cores used in multi-threaded algorithms (OpenMP required).-:[i|o] (morea|) Swap 1st and 2nd column on input and/or output.-^or just-Print a short message about the syntax of the command, then exits (NOTE: on Windows just use-).-+or just+Print an extensive usage (help) message, including the explana- tion 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 all options, then exits.

## UNITS

For map distance unit, appendunitdfor arc degree,mfor arc minute, andsfor arc second, orefor meter [Default],ffor foot,kfor km,Mfor statute mile,nfor nautical mile, andufor US survey foot. By default we compute such distances using a spherical approximation with great circles. Prepend-to a distance (or the unit is no distance is given) to performaFlat Earthacalculations (quicker but less accurate) or prepend+to perform exact geodesic calculations (slower but more accurate).

## EXAMPLES

To compute the free-air anomalies on a grid over a 2-D body that has been contoured and saved to body.txt, using 1.7 g/cm^3 as the density contrast, try gmt talwani2d -T-200/200/2 body.txt -D1700 -Fg > 2dgrav.txt To obtain the vertical gravity gradient anomaly along the track in crossing.txt for the same model, try gmt talwani2d -Ncrossing.txt body.txt -D1700 -Fv > vgg_crossing.txt The geoid anomaly for the same setup is given by gmt talwani2d -Ncrossing.txt body.txt -D1700 -Fn > n_crossing.txt

## NOTES

1. 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 anom- aly positive (or negative). You can use gmtmath to change the zero level to suit your needs.

## 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,. Talwani, M., J. L. Worzel, and M. Landisman, 1959, Rapid gravity compu- tations for two-dimensional bodies with application to the Mendocino submarine fracture zone,http://dx.doi.org/10.1002/2016GC006263J.Geophys.Res.,64, 49-59.

## SEE ALSO

gmt.conf(5),gmt(1),grdmath(1),gmtmath(1),gravfft(1),gmtgravmag3d(1),grdgravmag3d(1),talwani3d(1)

## COPYRIGHT

2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe 5.4.2 Jun 24, 2017 talwani2d(1)

gmt5 5.4.2 - Generated Thu Jun 29 16:41:55 CDT 2017