(37) Spectral coherence between gravity and bathymetry grids

The next script produces the plot in Figure. We demonstrate how grdfft is used to compute the spectral coherence between two data sets, here multibeam bathymetry and satellite-derived gravity. The grids are detrended and tapered before the Fourier transform is computed; the intermediate plots show the grids being extended and padded to a suitable dimension.

#!/bin/bash
#               GMT EXAMPLE 37
#               $Id$
#
# Purpose:      Illustrate 2-D FFT and coherence between gravity and bathymetry grids
# GMT modules:  psbasemap, psxy, makecpt, grdfft, grdimage, grdinfo
# Unix progs:   rm
#
ps=example_37.ps

# Testing gmt grdfft coherence calculation with Karen Marks example data
# Prefix of two .nc files

G=grav.V18.par.surf.1km.sq
T=mb.par.surf.1km.sq
gmt set FONT_TITLE 14p GMT_FFT kiss

gmt makecpt -Crainbow -T-5000/-3000 > z.cpt
gmt makecpt -Crainbow -T-50/25 > g.cpt
gmt grdinfo $T.nc -Ib > bbox
scl=1.4e-5
sclkm=1.4e-2
gmt grdimage $T.nc -I+a0+nt1 -Jx${scl}i -Cz.cpt -P -K -X1.474i -Y1i > $ps
gmt psbasemap -R-84/75/-78/81 -Jx${sclkm}i -O -K -Ba -BWSne+t"Multibeam bathymetry" >> $ps
gmt grdimage $G.nc -I+a0+nt1 -Jx${scl}i -Cg.cpt -O -K -X3.25i >> $ps
gmt psbasemap -R-84/75/-78/81 -Jx${sclkm}i -O -K -Ba -BWSne+t"Satellite gravity" >> $ps

gmt grdfft $T.nc $G.nc -E+wk -N192/192+d+wtmp > cross.txt

gmt makecpt -Crainbow -T-1500/1500 > z.cpt
gmt makecpt -Crainbow -T-40/40 > g.cpt

gmt grdimage ${T}_tmp.nc -I+a0+nt1 -Jx${scl}i -Cz.cpt -O -K -X-3.474i -Y3i >> $ps
gmt psxy -R${T}_tmp.nc -J bbox -O -K -L -W0.5p,- >> $ps
gmt psbasemap -R-100/91/-94/97 -Jx${sclkm}i -O -K -Ba -BWSne+t"Detrended and extended" >> $ps

gmt grdimage ${G}_tmp.nc -I+a0+nt1 -Jx${scl}i -Cg.cpt -O -K -X3.25i >> $ps
gmt psxy -R${G}_tmp.nc -J bbox -O -K -L -W0.5p,- >> $ps
gmt psbasemap -R-100/91/-94/97 -Jx${sclkm}i -O -K -Ba -BWSne+t"Detrended and extended" >> $ps

gmt set FONT_TITLE 24p
gmt psxy -R2/160/0/1 -JX-6il/2.5i -Bxa2f3g3+u" km" -Byafg0.5+l"Coherency@+2@+" -BWsNe+t"Coherency between gravity and bathymetry" -O -K -X-3.25i -Y3.3i cross.txt -i0,15 -W0.5p >> $ps
gmt psxy -R -J cross.txt -O -K -i0,15,16 -Sc0.075i -Gred -W0.25p -Ey >> $ps
gmt psxy -R -J -O -T >> $ps
rm -f cross.txt *_tmp.nc ?.cpt bbox

Spectral coherence between gravity and bathymetry grids.