1.. index:: ! talwani2d 2.. include:: ../module_supplements_purpose.rst_ 3 4********* 5talwani2d 6********* 7 8|talwani2d_purpose| 9 10Synopsis 11-------- 12 13.. include:: ../../common_SYN_OPTs.rst_ 14 15**gmt talwani2d** [ *table* ] 16[ |-A| ] [ |-D|\ *density* ] ] 17[ |-F|\ **f**\|\ **n**\ [*lat*]\|\ **v** ] 18[ |-M|\ [**h**]\ [**v**] ] 19[ |-N|\ *trackfile* ] 20[ |-T|\ *min*\ /*max*\ /\ *inc*\ [**+i**\|\ **n**]\ \|\ *file*\|\ *list* ] 21[ |-Z|\ *level*\ [*ymin*\ /*ymax*] ] 22[ |SYN_OPT-V| ] 23[ |SYN_OPT-bi| ] 24[ |SYN_OPT-d| ] 25[ |SYN_OPT-e| ] 26[ |SYN_OPT-h| ] 27[ |SYN_OPT-i| ] 28[ |SYN_OPT-o| ] 29[ |SYN_OPT-x| ] 30[ |SYN_OPT--| ] 31 32|No-spaces| 33 34Description 35----------- 36 37**talwani2d** will read the multi-segment *table* from file (or standard input). 38This file contains cross-sections of one or more 2-D bodies, with one polygon 39per segment. The segment header must contain the parameter *density*, which 40states the the density of this body (individual body 41densities may be overridden by a fixed constant density contrast given via an optional **-D**). 42We can compute anomalies on an equidistant lattice (by specifying a lattice with 43**-T**) or provide arbitrary output points specified in a file via **-N**. 44Choose between free-air anomalies, vertical gravity gradient anomalies, or geoid anomalies. 45Options are available to control axes units and direction. 46 47 48Required Arguments 49------------------ 50 51*table* 52 One or more ASCII files describing cross-sectional polygons of one or more bodies. Polygons 53 will be automatically closed if not already closed, and repeated vertices will 54 be eliminated. The segment header for each body will be examined for a density 55 parameter in kg/m^3; see **-D** for overriding this value. If no *table* is given then we 56 read standard input. 57 58Optional Arguments 59------------------ 60 61.. _-A: 62 63**-A** 64 The *z*-axis should be positive upwards [Default is down]. 65 66.. _-D: 67 68**-D**\ *density* 69 Sets a fixed density contrast that overrides any per-body settings in the model file, in kg/m^3. 70 71.. _-F: 72 73**-F**\ **f**\|\ **n**\ [*lat*]\|\ **v** 74 Specify desired gravitational field component. Choose between **f** (free-air anomaly) [Default], 75 **n** (geoid; optionally append average latitude for normal gravity reference value [45]) 76 or **v** (vertical gravity gradient). 77 78.. _-M: 79 80**-M**\ [**h**]\ [**v**] 81 Sets distance units used. Append **h** to indicate horizontal distances are in km [m], 82 and append **z** to indicate vertical distances are in km [m]. 83 84.. _-N: 85 86**-N**\ *trackfile* 87 Specifies locations where we wish to compute the predicted value. When this option 88 is used you cannot use **-T** to set an equidistant lattice. The output data records are written to 89 standard output (see **-bo** for binary output). 90 91.. _-T: 92 93**-T**\ *min*\ /*max*\ /\ *inc*\ [**+i**\|\ **n**]\ \|\ *file*\|\ *list* 94 Specify an equidistant output lattice. 95 For details on array creation, see `Generate 1D Array`_. 96 97.. |Add_-V| replace:: |Add_-V_links| 98.. include:: /explain_-V.rst_ 99 :start-after: **Syntax** 100 :end-before: **Description** 101 102.. _-Z: 103 104**-Z**\ *level*\ [*ymin*\ /*ymax*] 105 Set a constant observation level [0]. Optionally, and for gravity anomalies only (**-Ff**), 106 append the finite extent limits of a 2.5-D body. 107 108.. |Add_-bi| replace:: [Default is 2 input columns]. 109.. include:: ../../explain_-bi.rst_ 110 111.. |Add_-d| unicode:: 0x20 .. just an invisible code 112.. include:: ../../explain_-d.rst_ 113 114.. |Add_-e| unicode:: 0x20 .. just an invisible code 115.. include:: ../../explain_-e.rst_ 116 117.. |Add_-h| replace:: Not used with binary data. 118.. include:: ../../explain_-h.rst_ 119 120.. include:: ../../explain_-icols.rst_ 121 122.. include:: ../../explain_-ocols.rst_ 123 124.. include:: ../../explain_core.rst_ 125 126.. include:: ../../explain_colon.rst_ 127 128.. include:: ../../explain_help.rst_ 129 130.. include:: ../../explain_distunits.rst_ 131 132.. include:: ../../explain_array.rst_ 133 134Examples 135-------- 136 137To compute the free-air anomalies on an equidistant profile over a 2-D body that has been contoured 138and saved to body2d.txt, using 1700 kg/m^3 as a constant density contrast, with all distances in meters, 139try 140 141:: 142 143 gmt talwani2d -T-200/200/2 body2d.txt -D1700 -Ff > 2dgrav.txt 144 145To obtain the vertical gravity gradient anomaly along the track given by the file crossing.txt 146for the same model, try 147 148:: 149 150 gmt talwani2d -Ncrossing.txt body2d.txt -D1700 -Fv > vgg_crossing.txt 151 152 153The geoid anomaly for the same setup, evaluated at 60N, is given by 154 155:: 156 157 gmt talwani2d -Ncrossing.txt body2d.txt -D1700 -Fn60 > n_crossing.txt 158 159 160Notes 161----- 162 163#. The 2-D geoid anomaly is a logarithmic potential and thus has no natural 164 reference level. We simply remove the most negative (if density contrast 165 is positive) or positive (if density contrast is negative) computed value 166 from all values, rendering the entire anomaly positive (or negative). You 167 can use :doc:`gmtmath </gmtmath>` to change the zero level to suit your needs. 168 169References 170---------- 171 172Rasmussen, R., and L. B. Pedersen (1979), End corrections in potential field modeling, 173*Geophys. Prospect., 27*, 749-760. 174 175Chapman, M. E., 1979, Techniques for interpretation of geoid anomalies, 176*J. Geophys. Res., 84(B8)*, 3793-3801. 177 178Kim, S.-S., and P. Wessel, 2016, New analytic solutions for modeling vertical 179gravity gradient anomalies, *Geochem. Geophys. Geosyst., 17*, 180`http://dx.doi.org/10.1002/2016GC006263 <http://dx.doi.org/10.1002/2016GC006263>`_. 181 182Talwani, M., J. L. Worzel, and M. Landisman, 1959, Rapid gravity computations for 183two-dimensional bodies with application to the Mendocino submarine fracture zone, 184*J. Geophys. Res., 64*, 49-59. 185 186See Also 187-------- 188 189:doc:`gmt.conf </gmt.conf>`, :doc:`gmt </gmt>`, 190:doc:`grdmath </grdmath>`, :doc:`gmtmath </gmtmath>`, 191:doc:`gravfft </supplements/potential/gravfft>`, 192:doc:`gmtgravmag3d </supplements/potential/gmtgravmag3d>`, 193:doc:`grdgravmag3d </supplements/potential/grdgravmag3d>`, 194:doc:`talwani3d </supplements/potential/talwani3d>` 195