function [S,f,k] = fk(d,dt,dx,L);%FK: compute FK spectrum of a seismic gather.%%  [S,f,k] = fk(d,dt,dx,L);%%  IN   d:      data (traces in columns) %       dt:     time interval%       dx:     spatial increment between traces %       L:      apply spectral smoothing using a separable%               2D Hamming window of LxL samples%%  OUT  S:      FK spectrum%       f:      freq axis in Hz%       k:      wave-number spectrum in cylces/m (if dx is in meters)%%  Note: when plotting spectra (S)  use log(S) or S.^alpha (alpha=0.1-0.3) to%  increase the visibility of small events or background %%  Example:%%         [d,t,h] = linear_events;   %         [S,f,k] = fk(d,0.004,5,9); subplot(221);wigb(d,1,h,t);%                                    xlabel('offset [m]'); ylabel('t [s]');%                                    subplot(222);imagesc(k,f,S);%                                    xlabel('k [c/m]'); ylabel('f [Hz]');%%  Author(s): M.D.Sacchi (sacchi@phys.ualberta.ca)%  Copyright 1988-2005 SeismicLab%  Revision: 1.2  Date: Ago/2005%%  Signal Analysis and Imaging Group (SAIG)%  Department of Physics, UofA%[nt,nx]=size(d); nk = 4*(2^nextpow2(nx)); nf = 4*(2^nextpow2(nt)); S = fftshift( abs(fft2(d,nf,nk)) ); H = hamming(L)*hamming(L)'; S = conv2(S,H,'same'); S = S(nf/2:nf,:);  f = (0:1:nf/2)/nf/dt; k = (-nk/2+1:1:nk/2)/nk/dx;