kernel_buursink_2k Computes 2D Sensitivity kernel based on 1st order EM scattering theory
  
  See 
    Buursink et al. 2008. Crosshole radar velocity tomography 
                          with finite-frequency Fresnel. Geophys J. Int.
                          (172) 117;
 
   CALL : 
      % specify a source trace (dt, wf_trace):
      [kernel,L,L1_all,L2_all]=kernel_buursink_2d(model,x,z,S,R,dt,wf_trace);
      % Use a ricker wavelet with center frequency 'f0'
      [kernel,L,L1_all,L2_all]=kernel_buursink_2d(model,x,z,S,R,f0));
 
 
  Knud Cordua, 2009, 
  Thomas Mejer Hansen (small edits, 2009)