[Scilab-users] FFT - upward continuation of a 2D grid

[arctica1963]

Hi Rafael,

Thanks for the pointers. Still having some issues getting it to work on real
data. Upward continuation, should act like a low-pass and give a smoother
view of the data.

dimx=size(xt);
dimy=size(yt);

nx=dimx(1);
ny=dimy(1);

dxy = 0.016666666;
dkx=1/(nx*dxy);
dky=1/(ny*dxy);

kx=dkx*(-(nx/2)+1:nx/2);
ky=dky*(-(ny/2)+1:ny/2);

[KX,KY]=meshgrid(kx,ky);
k=sqrt(KX.^2 + KY.^2);

uc=4000; // continue up 4000m

//fft_Boug_corr=fft2(Boug_corr);
//shift_f=fftshift(fft_Boug_corr); 
//fft_clean=clean(shift_f);

fft_clean=clean(fftshift(fft2(Boug_corr))); // merge commands

Fup=fft_clean.*exp(-k*uc);

F_real=real(ifft(Fup)); 

It does generate output, but certainly not as expected for upward
continuation of data. Not sure what I'm missing, assuming I have done the
wavenumber thing correct. Normally I would do this via GMT, but want to get
it going under Scilab for convenience.

Clearly, before doing a proper fft, we need to pad the data to expand the
dimensions over which the fft is done to avoid edge effects. Just running it
with the grid dimensions should still produce a recognisable result.

Matbe testing with a synthetic grid would help.

Cheers
Lester



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