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 Optional Parameters:							

 nt=501        	number of time samples				

 dt=0.004      	time sampling interval (sec)			

 ft=0.0        	first time (sec)				

 fpeak=0.2/dt		peak frequency of symmetric Ricker wavelet (Hz)	

 nxg=			number of receivers of input traces		

 dxg=15		receiver sampling interval (m)			

 fxg=0.0		first receiver (m)				

 nxd=5         	skipped number of receivers			

 nxs=1			number of offsets				

 dxs=50		shot sampling interval (m)			

 fxs=0.0		first shot (m)				

 dx=50         	x sampling interval (m)				

 fx=0.         	first x sample (m)				

 dz=50         	z sampling interval (m)				

 nxb=nx/2    	band width centered at midpoint (see note)	

 nxc=0         hozizontal range in which velocity is changed	

 nzc=0         vertical range in which velocity is changed	

 pert=0        =1 calculate time correction from v_p[nx][nz]	

 vpfile        file containing slowness perturbation array v_p[nx][nz]	

 ref="1:1,2;4,2"	reflector(s):  "amplitude:x1,z1;x2,z2;x3,z3;...

 smooth=0		=1 for smooth (piecewise cubic spline) reflectors

 ls=0			=1 for line source; =0 for point source		

 tmin=10.0*dt		minimum time of interest (sec)			

 ndpfz=5		number of diffractors per Fresnel zone		

 cable=1		roll reciever spread with shot			

 			=0 static reciever spread			

 verbose=0		=1 to print some useful information		



 Notes:								

 This algorithm is based on formula (58) in Geo. Pros. 34, 686-703,	

 by N. Bleistein.							



 Traveltime and amplitude are calculated by finite difference which	

 is done only in one of every NXD receivers; in skipped receivers, 	

 interpolation is used to calculate traveltime and amplitude.		", 

 For each receiver, traveltime and amplitude are calculated in the 	

 horizontal range of (xg-nxb*dx, xg+nxb*dx). Velocity is changed by 	

 constant extropolation in two upper trianglar corners whose width is 	

 nxc*dx and height is nzc*dz.						



 Eikonal equation will fail to solve if there is a polar turned ray.	

 In this case, the program shows the related geometric information. 	

 There are three ways to remove the turned rays: smoothing velocity, 	

 reducing nxb, and increaing nxc and nzc (if the turned ray occurs  	

 in shallow areas). To prevent traveltime distortion from an over-	

 smoothed velocity, traveltime is corrected based on the slowness 	

 perturbation.								



 More than one ref (reflector) may be specified.			

 Note that reflectors are encoded as quoted strings, with an optional	

 reflector amplitude: preceding the x,z coordinates of each reflector.	

 Default amplitude is 1.0 if amplitude: part of the string is omitted.	







	Author: Zhenyue Liu, 07/20/92, Center for Wave Phenomena

		Many subroutines borrowed from Dave Hale's program: SUSYNLV



		Trino Salinas, 07/30/96, fixed a bug in the geometry

		setting to allow the spread move with the shots.