GRDFFT
Section: Misc. Reference Manual Pages (l)
Updated: DATE
Index
Return to Main Contents
NAME
grdfft - Perform mathematical operations on grdfiles in the frequency domain
SYNOPSIS
grdfft in_grdfile -Gout_grdfile [ -Aazimuth ]
[ -Czlevel ] [ -D[scale] ] [ -E[x|y][w] ]
[ -F[x|y]lc/lp/hp/hc ] [ -I[scale] ] [ -L ] [ -M ]
[ -Sscale ] [ -Tte/rl/rm/rw/ri ] [ -V ]
DESCRIPTION
grdfft will take the 2-D forward Fast Fourier Transform and perform one or more mathematical
operations in the frequency domain before transforming back to the space domain. An option
is provided to scale the data before writing the new values to an output file. The
horizontal dimensions of the grdfiles are assumed to be in meters. Geographical grids
may be used by specifying the -M option that scales degrees to meters. If you
have grdfiles with dimensions in km, you could change this to meters using grdedit
or scale the output with grdmath.
No space between the option flag and the associated arguments. Use upper case for the
option flags and lower case for modifiers.
- in_grdfile
-
2-D binary grd file to be operated on.
- -G
-
Specify the name of the output grd file.
OPTIONS
- -A
-
Take the directional derivative in the azimuth direction measured in degrees CW from north.
- -C
-
Upward (for zlevel > 0) or downward (for zlevel < 0) continue the field zlevel meters.
- -D
-
Differentiate the field, i.e., take d(field)/dz. This is equivalent to multiplying by kr in the frequency
domain (kr is radial wave number). Append a scale to multiply by (kr * scale) instead. Append g to
indicate that your data are geoid heights in meters and output should be gravity anomalies in mGal.
[Default is no scale].
- -E
-
Estimate power spectrum in the radial direction. Place x or y immediately after -E to
compute the spectrum in the x or y direction instead. No grdfile is created; f (i.e., frequency or wave number),
power[f], and 1 standard deviation in power[f] are written to stdout.
Append w to write wavelength instead of frequency.
- -F
-
Filter the data. Place x or y immediately after -F to filter x or y direction only; default is isotropic.
Specify four wavelengths in correct units (see -M) to design a bandpass filter; wavelengths greater than
lc or less than hc will be cut, wavelengths greater than lp and less than hp will be
passed, and wavelengths in between will be cosine-tapered. E.g., -F1000000/250000/50000/10000 -M
will bandpass, cutting wavelengths > 1000 km and < 10 km, passing wavelengths between 250 km and 50 km. To make
a highpass or lowpass filter, give hyphens (-) for hp/hc or lc/lp. E.g., -Fx-/-/50/10
will lowpass X, passing wavelengths > 50 and rejecting wavelengths < 10. -Fy1000/250/-/- will highpass Y,
passing wavelengths < 250 and rejecting wavelengths > 1000.
- -I
-
Integrate the field, i.e., compute integral_over_z (field * dz). This is equivalent to divide by kr
in the frequency domain (kr is radial wave number). Append a scale to divide by (kr * scale) instead.
Append g to indicate that your data set is gravity anomalies in mGal and output should be geoid
heights in meters. [Default is no scale].
- -L
-
Leave trend alone. By default, a linear trend will be removed prior to the transform.
- -M
-
Map units. Choose this option if your grdfile is a geographical grid and you want to convert
degrees into meters. If the data are close to either pole, you should consider
projecting the grdfile onto a rectangular coordinate system using grdproject.
- -S
-
Multiply each element by scale in the space domain (after the frequency domain operations).
[Default is 1.0].
- -T
-
Compute the isostatic compensation from the topography load (input grdfile) on an elastic plate
of thickness te. Also append densities for load, mantle, water, and infill in SI units.
If te == 0 then the Airy response is returned. -T implicitly sets -L.
EXAMPLES
To upward continue the sea-level magnetic anomalies in the file mag_0.grd to a level 800 m above
sealevel, try
grdfft mag_0.grd -C800 -V -Gmag_800.grd
To transform geoid heights in m (geoid.grd) on a geographical grid to free-air gravity anomalies in mGal,
do
grdfft geoid.grd -Dg -M -V -Ggrav.grd
To transform gravity anomalies in mGal (faa.grd) to deflections of the vertical (in micro-radians) in the
038 direction, we must first integrate gravity to get geoid, then take the directional derivative, and
finally scale radians to micro-radians:
grdfft faa.grd -Ig -A38 -S1e6 -V -Gdefl_38.grd
Second vertical derivatives of gravity anomalies are related to the curvature of the field. We can
compute these as mGal/m^2 by differentiating twice:
grdfft gravity.grd -D -D -V -Ggrav_2nd_derivative.grd
The first order gravity anomaly (in mGal) due to the compensating surface caused
by the topography load topo.grd (in m) on a 20 km thick elastic plate, assumed to
be 4 km beneath the observation level can be computed as
grdfft topo.grd -T20000/2800/3330/1030/2300 -C4000 -S0.022 -Gcomp_faa.grd
where 0.022 is the scale needed for the first term in Parker's expansion for
computing gravity from topography (= 2 * PI * G * (rhom - rhol)).
SEE ALSO
gmt, grdedit, grdmath, grdproject
#include "refs.i"
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- EXAMPLES
-
- SEE ALSO
-
This document was created by
man2html,
using the manual pages.
Time: 01:25:44 GMT, February 16, 2023