grdfft(1) GMT grdfft(1)

## NAME

grdfft - Do mathematical operations on grids in the wavenumber (or fre- quency) domain

## SYNOPSIS

grdfftingrid[ingrid2] [-Goutfile|table] [-Aazimuth] [-Czlevel] [-D[scale|g] ] [-E[r|x|y][+w[k]][+n] ] [-F[r|x|y]params] [-I[scale|g] ] [-Nparams] [-Sscale] [-V[level] ] [-fg]Note:No space is allowed between the option flag and the associated arguments.

## DESCRIPTION

grdfftwill take the 2-D forward Fast Fourier Transform and perform one or more mathematical operations in the frequency domain before trans- forming 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 grid are assumed to be in meters. Geographical grids may be used by specifying the-fgoption 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.

## REQUIRED ARGUMENTS

ingrid2-D binary grid file to be operated on. (See GRID FILE FORMATS below). For cross-spectral operations, also give the second grid fileingrd2.-GoutfileSpecify the name of the output grid file or the 1-D spectrum ta- ble (see-E). (See GRID FILE FORMATS below).

## OPTIONAL ARGUMENTS

-AazimuthTake the directional derivative in theazimuthdirection mea- sured in degrees CW from north.-CzlevelUpward (forzlevel> 0) or downward (forzlevel< 0) continue the fieldzlevelmeters.-D[scale|g] Differentiate the field, i.e., take d(field)/dz. This is equiva- lent to multiplying by kr in the frequency domain (kr is radial wave number). Append a scale to multiply by (kr *scale) instead. Alternatively, appendgto indicate that your data are geoid heights in meters and output should be gravity anomalies in mGal. [Default is no scale].-E[r|x|y][+w[k]][+n] Estimate power spectrum in the radial direction [r]. Placexoryimmediately after-Eto compute the spectrum in the x or y direction instead. No grid file is created. If one grid is given then f (i.e., frequency or wave number), power[f], and 1 stan- dard deviation in power[f] are written to the file set by-G[stdout]. If two grids are given we write f and 8 quantities: Xpower[f], Ypower[f], coherent power[f], noise power[f], phase[f], admittance[f], gain[f], coherency[f]. Each quantity is followed by its own 1-std dev error estimate, hence the out- put is 17 columns wide. Give+wto write wavelength instead of frequency, and if your grid is geographic you may further appendkto scale wavelengths from meter [Default] to km. Finally, the spectrum is obtained by summing over several frequencies. Append+nto normalize so that the mean spectral values per fre- quency are reported instead.-F[r|x|y]paramsFilter the data. Placexoryimmediately after-Fto filterxorydirection only; default is isotropic [r]. Choose between a cosine-tapered band-pass, a Gaussian band-pass filter, or a But- terworth band-pass filter.Cosine-taper:Specify four wavelengthslc/lp/hp/hcin correct units (see-fg) to design a bandpass filter: wavelengths greater thanlcor less thanhcwill be cut, wavelengths greater thanlpand less thanhpwill be passed, and wavelengths in between will be cosine-tapered. E.g.,-F1000000/250000/50000/10000-fgwill 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 (-) forhp/hcorlc/lp. E.g.,-Fx-/-/50/10 will lowpassx, passing wavelengths > 50 and rejecting wavelengths < 10.-Fy1000/250/-/- will highpassy, passing wavelengths < 250 and rejecting wavelengths > 1000.Gaussianband-pass:Appendlo/hi, the two wavelengths in correct units (see-fg) to design a bandpass filter. At the given wave- lengths the Gaussian filter weights will be 0.5. To make a highpass or lowpass filter, give a hyphen (-) for thehiorlowavelength, respectively. E.g.,-F-/30 will low- pass the data using a Gaussian filter with half-weight at 30, while-F400/- will highpass the data.Butterworthband-pass:Appendlo/hi/order, the two wavelengths in correct units (see-fg) and the filter order (an integer) to design a bandpass filter. At the given cut-off wavelengths the Butterworth filter weights will be 0.707 (i.e., the power spectrum will therefore be reduced by 0.5). To make a highpass or lowpass filter, give a hyphen (-) for thehiorlowavelength, respectively. E.g.,-F-/30/2 will low- pass the data using a 2nd-order Butterworth filter, with half-weight at 30, while-F400/-/2 will highpass the data.-Goutfile|tableFilename for output netCDF grid file OR 1-D data table (see-E). This is optional for -E (spectrum written to stdout) but manda- tory for all other options that require a grid output.-I[scale|g] 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. Alternatively, appendgto indicate that your data set is gravity anomalies in mGal and output should be geoid heights in meters. [Default is no scale].-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:-Nalets the FFT select dimensions yielding the most accurate result.-Nfwill force the FFT to use the actual dimensions of the data.-Nmlets the FFT select dimensions using the least work mem- ory.-Nrlets the FFT select dimensions yielding the most rapid calculation.-Nswill present a list of optional dimensions, then exit.-Nnx/nywill do FFT on array sizenx/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 lin- ear 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:+eextends the grid by imposing edge-point symmetry [Default],+mextends the grid by imposing edge mirror symmetry+nturns off data extension. Tapering is performed from the data edge to the FFT grid edge [100%]. Change this percentage via+twidth. When+nis in effect, the tapering is applied instead to the data margins as no extension is available [0%]. Control messages being reported:+vwill 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], whereextis 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 toingrid_real.extandingrid_imag.ext. Appendpto save instead the polar form of magnitude and phase to filesingrid_mag.extandingrid_phase.ext.-SscaleMultiply each element byscalein the space domain (after the frequency domain operations). [Default is 1.0].-V[level] (morea|) Select verbosity level [c].-fgGeographic grids (dimensions of longitude, latitude) will be converted to meters via aaFlat Earthaapproximation 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 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.

## 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 calledapackingaof grids, writing out floating point data as 1- or 2-byte integers. (morea|)

## GRID DISTANCE UNITS

If the grid does not have meter as the horizontal unit, append+uunitto the input file name to convert from the specified unit to meter. If your grid is geographic, convert distances to meters by supplying-fginstead.

## 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

## NORMALIZATION OF SPECTRUM

By default, the power spectrum returned by-Esimply sums the contribu- tions from frequencies that are part of the output frequency. Forx- ory-spectra this means summing the power across the other frequency dimension, while for the radial spectrum it means summing up power within each annulus of widthdelta_q, the radial frequency (q) spacing. A consequence of this summing is that the radial spectrum of a white noise process will give a linear radial power spectrum that is propor- tional toq. Appendingnwill instead compute the mean power per out- put frequency and in this case the white noise process will have a white radial spectrum as well.

## EXAMPLES

To upward continue the sea-level magnetic anomalies in the file mag_0.nc to a level 800 m above sealevel: gmt grdfft mag_0.nc -C800 -V -Gmag_800.nc To transform geoid heights in m (geoid.nc) on a geographical grid to free-air gravity anomalies in mGal: gmt grdfft geoid.nc -Dg -V -Ggrav.nc To transform gravity anomalies in mGal (faa.nc) to deflections of the vertical (in micro-radians) in the 038 direction, we must first inte- grate gravity to get geoid, then take the directional derivative, and finally scale radians to micro-radians: gmt grdfft faa.nc -Ig -A38 -S1e6 -V -Gdefl_38.nc Second vertical derivatives of gravity anomalies are related to the curvature of the field. We can compute these as mGal/m^2 by differenti- ating twice: gmt grdfft gravity.nc -D -D -V -Ggrav_2nd_derivative.nc To compute cross-spectral estimates for co-registered bathymetry and gravity grids, and report result as functions of wavelengths in km, try gmt grdfft bathymetry.nc gravity.grd -E+wk -fg -V > cross_spectra.txt To examine the pre-FFT grid after detrending, point-symmetry reflec- tion, and tapering has been applied, as well as saving the real and imaginary components of the raw spectrum of the data in topo.nc, try gmt grdfft topo.nc -N+w+z -fg -V You can now make plots of the data in topo_taper.nc, topo_real.nc, and topo_imag.nc.

## SEE ALSO

gmt(1),grdedit(1),grdfilter(1),grdmath(1),grdproject(1),gravfft(1)

## COPYRIGHT

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

gmt5 5.4.2 - Generated Wed Jun 28 18:25:25 CDT 2017