2D frechet kernel, First Fresnel Zone 
 
  See Jensen, Jacobsen, Christensen-Dalsgaard (2000) Solar Physics 192.
 
  Call :
  S=munk_fresnel_2d(T,dt,alpha,As,Ar,K);
 
  T : dominant period
  dt : 
  alpha : degree of cancellation 
  As : Amplitude fo the wavefield propagating from the source
  Ar : Amplitude fo the wavefield propagating from the receiver
  K : normalization factor