SUVELAN_USEL - compute stacking velocity panel for cdp gathers	     
		using the UnNormalized Selective CrossCorrelation Sum	     

 suvelan_usel <stdin >stdout [optional parameters]			     

 Optional Parameters:							     
 nx=tr.cdpt              number of traces in cdp			     
 dx=tr.d2                offset increment				     
 nv=50                   number of velocities				     
 dv=50.0                 velocity sampling interval			     
 fv=1500.0               first velocity				     
 tau=0.5                 threshold for significance values                  
 smute=1.5               samples with NMO stretch exceeding smute are zeroed
 dtratio=5               ratio of output to input time sampling intervals   
 nsmooth=dtratio*2+1     length of smoothing window                         
 verbose=0               =1 for diagnostic print on stderr		     
 pwr=1.0                 semblance value to the power      		     

 Notes:								     
 UnNormalized Selective CrossCorrelation sum: is based on the coherence     
 measure known as crosscorrelation sum. The difference is that the selective
 approach sum only crosscorrelation pairs with relatively large differential
 moveout, thus increasing the resolving power in the velocity spectra       
 compared to that achieved by conventional methods.  The selection is made  
 using a parabolic approximation of the differential moveout and imposing a 
 threshold for those differential moveouts.				     

 That threshold is the parameter tau in this program, which varies between  
 0 to 1.  A value of tau=0, means conventional crosscorrelation sum is      
 applied implying that all crosscorrelations are included in the sum. In    
 contrast, a value of tau=1 (not recomended) means that only the            
 crosscorrelation formed by the trace pair involving the shortest and longest
 offset is included in the sum. Intermediate values will produce percentages
 of the crosscorrelations included in the sum that will be shown in the     
 screen before computing the velocity spectra. Typical values for tau are   
 between 0.2 and 0.6, producing approximated percentages of crosscorrelations
 summed between 60% and 20%. The higher the value of tau the lower the     
 percentage and higher the increase in the resolving power of velocity	      
 spectra.								      

 Keeping the percentage of crosscorrelations included in the sum between 20%
 and 60% will increase resolution and avoid the precense of artifacts in the
 results.  In data contaminated by random noise or statics distortions is    
 recomended to mantaing the percentage of crosscorrelations included in the  
 sum above 25%.  After computing the velocity spectra one might want to     
 adjust the level and number of contours before velocity picking.  	      

 
 Credits:  CWP: Valmore Celis, Sept 2002
 
 Based on the original code: suvelan_.c 
    Colorado School of Mines:  Dave Hale c. 1989


 References: 
 Neidell, N.S., and Taner, M.T., 1971, Semblance and other coherency
             measures for multichannel data: Geophysics, 36, 498-509.
 Celis, V. T., 2002, Selective-correlation velocity analysis: CSM thesis.

 Trace header fields accessed:  ns, dt, delrt, offset, cdp
 Trace header fields modified:  ns, dt, offset, cdp