Provided by: gmt-common_5.4.5+dfsg-1_all bug

NAME

       gravfft  -  Compute  gravitational  attraction  of  3-D  surfaces  in  the  wavenumber (or
       frequency) domain

SYNOPSIS

       gravfft  ingrid  [   ingrid2   ]    -Goutfile   [    -Cn/wavelength/mean_depth/tbw   ]   [
       -Ddensity|rhogrid ] [  -En_terms ] [  -F[f[+]|g|v|n|e] ] [  -Iw|b|c|t |k ] [  -Nparams ] [
       -Q ] [  -Tte/rl/rm/rw[/ri][+m] ] [  -V[level] ] [  -Wwd] [  -Zzm[zl] ] [ -fg ]

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

DESCRIPTION

       gravfft can be used into three main modes. Mode 1: Simply compute the geopotential due  to
       the  surface  given in the topo.grd file.  Requires a density contrast (-D) and possibly a
       different observation level (-W).  It will take the 2-D forward FFT of the  grid  and  use
       the  full  Parker's  method  up  to  the  chosen  terms.  Mode 2: Compute the geopotential
       response due to flexure of the topography file. It will take the 2-D forward  FFT  of  the
       grid  and  use  the   full  Parker's  method  applied  to the chosen isostatic model.  The
       available models are the "loading from top", or elastic plate model, and the "loading from
       below"  which accounts for the plate's response to a sub-surface load (appropriate for hot
       spot modeling - if you believe them). In both cases, the model parameters are set with  -T
       and  -Z options. Mode 3: compute the admittance or coherence between two grids. The output
       is the average in the radial direction.  Optionally, the  model  admittance  may  also  be
       calculated.  The  horizontal  dimensions  of  the  grdfiles  are  assumed to be in meters.
       Geographical grids may be used by specifying the -fg option that scales degrees to meters.
       If  you have grids with dimensions in km, you could change this to meters using grdedit or
       scale the output with grdmath.  Given the  number  of  choices  this  program  offers,  is
       difficult  to  state  what are options and what are required arguments. It depends on what
       you are doing; see the examples for further guidance.

REQUIRED ARGUMENTS

       ingrid 2-D binary grid file to be  operated  on.  (See  GRID  FILE  FORMATS  below).   For
              cross-spectral operations, also give the second grid file ingrd2.

       -Goutfile
              Specify  the  name of the output grid file or the 1-D spectrum table (see -E). (See
              GRID FILE FORMATS below).

OPTIONAL ARGUMENTS

       -Cn/wavelength/mean_depth/tbw
              Compute only the theoretical admittance curves of the selected model  and  exit.  n
              and  wavelength  are  used  to compute (n * wavelength) the total profile length in
              meters. mean_depth is the mean water depth. Append dataflags (one or two) of tbw in
              any order. t = use "from top" model, b = use "from below" model. Optionally specify
              w to write wavelength instead of frequency.

       -Ddensity|rhogrid
              Sets density contrast across surface. Used, for example,  to  compute  the  gravity
              attraction  of the water layer that can later be combined with the free-air anomaly
              to get the Bouguer anomaly. In this case do not use -T.  It  also  implicitly  sets
              -N+h.   Alternatively,  specify  a  co-registered  grid with density contrasts if a
              variable density contrast is required.

       -En_terms
              Number of terms used in Parker expansion (limit is 10, otherwise terms depending on
              n will blow out the program) [Default = 3]

       -F[f[+]|g|v|n|e]
              Specify desired geopotential field: compute geoid rather than gravity
                 f  =  Free-air  anomalies (mGal) [Default].  Append + to add in the slab implied
                 when removing the mean value from the topography.  This requires zero topography
                 to mean no mass anomaly.

                 g = Geoid anomalies (m).

                 v = Vertical Gravity Gradient (VGG; 1 Eotvos = 0.1 mGal/km).

                 e = East deflections of the vertical (micro-radian).

                 n = North deflections of the vertical (micro-radian).

       -Iw|b|c|t |k
              Use   ingrd2   and   ingrd1   (a   grid  with  topography/bathymetry)  to  estimate
              admittance|coherence and write it to stdout (-G ignored if set). This  grid  should
              contain  gravity  or  geoid  for  the  same  region  of  ingrd1.  Default  computes
              admittance. Output contains 3 or  4  columns.  Frequency  (wavelength),  admittance
              (coherence)  one  sigma error bar and, optionally, a theoretical admittance. Append
              dataflags (one to three) from w|b|c|t.  w writes wavelength instead of  wavenumber,
              k selects km for wavelength unit [m], c computes coherence instead of admittance, b
              writes a fourth column with "loading from  below"  theoretical  admittance,  and  t
              writes a fourth column with "elastic plate" theoretical admittance.

       -N[a|f|m|r|s|nx/ny][+a|[+d|h|l][+e|n|m][+twidth][+v][+w[suffix]][+z[p]]
              Choose  or  inquire  about  suitable  grid  dimensions  for  FFT  and  set optional
              parameters. Control the FFT dimension:
                 -Na lets the FFT select dimensions yielding the most accurate result.

                 -Nf will force the FFT to use the actual dimensions of the data.

                 -Nm lets the FFT select dimensions using the least work memory.

                 -Nr lets the FFT select dimensions yielding the most rapid calculation.

                 -Ns will present a list of optional dimensions, then exit.

                 -Nnx/ny will do FFT on array size nx/ny (must be >=  grid  file  size).  Default
                 chooses  dimensions  >=  data  which  optimize speed and accuracy of FFT. If FFT
                 dimensions > grid file dimensions, data are extended and tapered to zero.

              Control detrending of data: Append modifiers for removing a linear trend:
                 +d: Detrend data, i.e. remove best-fitting linear trend [Default].

                 +a: Only remove mean value.

                 +h: Only remove mid value, i.e. 0.5 * (max + min).

                 +l: Leave data alone.

              Control extension and tapering of data: Use modifiers to control how the  extension
              and tapering are to be performed:
                 +e extends the grid by imposing edge-point symmetry [Default],

                 +m extends the grid by imposing edge mirror symmetry

                 +n turns off data extension.

                 Tapering  is  performed  from the data edge to the FFT grid edge [100%].  Change
                 this percentage via +twidth. When +n is  in  effect,  the  tapering  is  applied
                 instead to the data margins as no extension is available [0%].

                 Control  messages  being  reported:  +v  will  report suitable dimensions during
                 processing.

              Control writing of temporary results: For detailed investigation you can write  the
              intermediate  grid  being  passed  to  the forward FFT; this is likely to have been
              detrended,  extended  by  point-symmetry  along  all  edges,  and  tapered.  Append
              +w[suffix] from which output file name(s) will be created (i.e., ingrid_prefix.ext)
              [tapered], where ext is your file extension. Finally, you may save the complex grid
              produced  by  the  forward  FFT  by  appending +z. By default we write the real and
              imaginary components to ingrid_real.ext  and  ingrid_imag.ext.  Append  p  to  save
              instead  the  polar  form  of  magnitude  and  phase  to  files  ingrid_mag.ext and
              ingrid_phase.ext.

       -Q     Writes out a grid with the flexural topography (with z positive up)  whose  average
              depth  was  set  by -Zzm and model parameters by -T (and output by -G). That is the
              "gravimetric Moho". -Q implicitly sets -N+h

       -S     Computes predicted gravity or geoid grid due to a subplate  load  produced  by  the
              current  bathymetry  and  the  theoretical  model. The necessary parameters are set
              within -T and -Z options. The number of powers in Parker expansion is restricted to
              1.  See an example further down.

       -Tte/rl/rm/rw[/ri][+m]
              Compute the isostatic compensation from the topography load (input grid file) on an
              elastic plate of thickness te. Also append densities for load,  mantle,  water  and
              infill  in  SI units. If ri is not provided it defaults to rl.  Give average mantle
              depth via -Z. If the elastic thickness is > 1e10 it  will  be  interpreted  as  the
              flexural  rigidity  (by  default  it  is  computed  from  te  and  Young  modulus).
              Optionally, append +m to write a grid with the Moho's geopotential effect (see  -F)
              from  model  selected  by  -T.   If te = 0 then the Airy response is returned. -T+m
              implicitly sets -N+h

       -Wwd   Set water depth (or observation height) relative to topography [0].   Append  k  to
              indicate km.

       -Zzm[zl]
              Moho  [and  swell]  average  compensation  depths  (in  meters positive dows -- the
              depth). For the "load from top" model you only have to  provide  zm,  but  for  the
              "loading from below" don't forget zl.

       -V[level] (more ...)
              Select verbosity level [c].

       -fg    Geographic  grids  (dimensions  of longitude, latitude) will be converted to meters
              via a "Flat Earth" approximation using the current ellipsoid parameters.

       -^ 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  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  all  options,
              then exits.

GRID FILE FORMATS

       By  default  GMT  writes  out grid as single precision floats in a COARDS-complaint netCDF
       file format. However, GMT is able to produce grid files in many other commonly  used  grid
       file formats and also facilitates so called "packing" of grids, writing out floating point
       data as 1- or 2-byte integers. (more ...)

GRID DISTANCE UNITS

       If the grid does not have meter as the horizontal unit, append +uunit 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.

CONSIDERATIONS

       netCDF COARDS grids will automatically  be  recognized  as  geographic.  For  other  grids
       geographical  grids  were you want to convert degrees into meters, select -fg. If the data
       are close to either pole, you should consider projecting the grid file onto a  rectangular
       coordinate system using grdproject.

PLATE FLEXURE

       The  FFT  solution  to elastic plate flexure requires the infill density to equal the load
       density.  This is typically only true directly beneath  the  load;  beyond  the  load  the
       infill tends to be lower-density sediments or even water (or air).  Wessel [2001] proposed
       an approximation that allows for the specification of an infill density different from the
       load  density  while  still  allowing for an FFT solution. Basically, the plate flexure is
       solved for using the infill density as the effective load density but the  amplitudes  are
       adjusted  by  a factor A = sqrt ((rm - ri)/(rm - rl)), which is the theoretical difference
       in  amplitude  due  to  a  point  load  using  the  two  different  load  densities.   The
       approximation  is  very  good  but  breaks  down  for  large loads on weak plates, a fairy
       uncommon situation.

EXAMPLES

       To compute the effect of the water layer above the bat.grd bathymetry using 2700 and  1035
       for  the  densities of crust and water and writing the result on water_g.grd (computing up
       to the fourth power of bathymetry in Parker expansion):

              gmt gravfft bat.grd -D1665 -Gwater_g.grd -E4

       Now subtract it from your free-air anomaly faa.grd and you will get the  Bouguer  anomaly.
       You  may  wonder  why  we  are  subtracting and not adding.  After all the Bouguer anomaly
       pretends to correct the mass deficiency presented by the water layer,  so  we  should  add
       because  water  is  less  dense than the rocks below. The answer relies on the way gravity
       effects are computed by the Parker's method and practical aspects of using the FFT.

              gmt grdmath faa.grd water_g.grd SUB = bouguer.grd

       Want an MBA anomaly? Well compute  the  crust  mantle  contribution  and  add  it  to  the
       sea-bottom  anomaly.  Assuming  a  6 km thick crust of density 2700 and a mantle with 3300
       density we could repeat the command used to compute the water  layer  anomaly,  using  600
       (3300  -  2700) as the density contrast. But we now have a problem because we need to know
       the mean Moho depth. That is when the scale/offset that can be appended to the grid's name
       comes  in  hand. Notice that we didn't need to do that before because mean water depth was
       computed directly from data (notice also the negative sign of the offset due to  the  fact
       that z is positive up):

              gmt gravfft bat.grd=nf/1/-6000 -D600 -Gmoho_g.grd

       Now, subtract it from the Bouguer to obtain the MBA anomaly. That is:

              gmt grdmath bouguer.grd moho_g.grd SUB = mba.grd

       To  compute the Moho gravity effect of an elastic plate bat.grd with Te = 7 km, density of
       2700, over a mantle of density 3300, at an average depth of 9 km

              gmt gravfft bat.grd -Gelastic.grd -T7000/2700/3300/1035+m -Z9000

       If you add now the sea-bottom and Moho's effects, you will get the full  gravity  response
       of your isostatic model. We will use here only the first term in Parker expansion.

              gmt gravfft bat.grd -D1665 -Gwater_g.grd -E1
              gmt gravfft bat.grd -Gelastic.grd -T7000/2700/3300/1035+m -Z9000 -E1
              gmt grdmath water_g.grd elastic.grd ADD = model.grd

       The  same  result  can be obtained directly by the next command. However, PAY ATTENTION to
       the following. I don't yet know if it's because of a bug or due to  some  limitation,  but
       the  fact is that the following and the previous commands only give the same result if -E1
       is used.  For higher powers of bathymetry in Parker  expansion,  only  the  above  example
       seams to give the correct result.

              gmt gravfft bat.grd -Gmodel.grd -T7000/2700/3300/1035 -Z9000 -E1

       And  what  would  be  the  geoid anomaly produced by a load at 50 km depth, below a region
       whose bathymetry is given by bat.grd, a Moho at 9 km  depth  and  the  same  densities  as
       before?

              gmt gravfft topo.grd -Gswell_geoid.grd -T7000/2700/3300/1035 -Fg -Z9000/50000 -S -E1

       To  compute  the  admittance  between the topo.grd bathymetry and faa.grd free-air anomaly
       grid using the elastic plate model of a crust of 6 km mean thickness with 10 km  effective
       elastic thickness in a region of 3 km mean water depth:

              gmt gravfft topo.grd faa.grd -It -T10000/2700/3300/1035 -Z9000

       To  compute  the  admittance between the topo.grd bathymetry and geoid.grd geoid grid with
       the "loading from below" (LFB) model with the same as above and sub-surface load at 40 km,
       but assuming now the grids are in geographic and we want wavelengths instead of frequency:

              gmt gravfft topo.grd geoid.grd -Ibw -T10000/2700/3300/1035 -Z9000/40000 -fg

       To  compute the gravity theoretical admittance of a LFB along a 2000 km long profile using
       the same parameters as above

              gmt gravfft -C400/5000/3000/b -T10000/2700/3300/1035 -Z9000/40000

REFERENCES

       Luis, J.F. and M.C. Neves. 2006, The isostatic compensation of the Azores  Plateau:  a  3D
       admittance  and coherence analysis. J. Geothermal Volc. Res. Volume 156, Issues 1-2, Pages
       10-22, http://dx.doi.org/10.1016/j.jvolgeores.2006.03.010

       Parker, R. L., 1972, The rapid  calculation  of  potential  anomalies,  Geophys.  J.,  31,
       447-455.

       Wessel.  P.,  2001,  Global  distribution  of seamounts inferred from gridded Geosat/ERS-1
       altimetry,        J.        Geophys.         Res.,         106(B9),         19,431-19,441,
       http://dx.doi.org/10.1029/2000JB000083

SEE ALSO

       gmt, grdfft, grdmath, grdproject

COPYRIGHT

       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe