README for VplusZ computes the velocity and depth estimation

Note:
This code performs velocity and depth estimation after horizon-based
prestack depth migration.

Reference:
 This program is design for postmigration velocity analysis.  It is
 bsed on Zhaobo Meng's thesis, chapter 5.

 "Tetrahedral based earth models, ray tracing in tetrahedral models,
 and analytical migration velocity analysis"--Zhaobo Meng,
 Ph. D. thesis, Center for Wave Phenomena, Department of
 Mathematical and Computer Sciences,
 Colorado School of Mines, 1999.

 (1) vplusz converts the traveltime misfits (offset??.picks)
 to depth misfits that PSDM generates; then multiply the results
 using the velocity at the velocity at the base grid.  The final results
 are added to the base grid to get the final depth grid.  Thus, now,
 different offsets correspond to different depth grids.

 For vplusz, not all CIGs are used (because there may not enough data.
 Only those CIGs from nxrtbeg to nxrtend and from nyrtbeg to nyrtend are
 used.  Similarly, not all offsets are used, user can specify how many 
 offsets to use and what offsets to use.  They are specified by 
 isort(1), isort(2),..., isort(npicks).

Instructions for running vplusz are located near the top of vpluszf.f

John Stockwell	 | jstockwe@mines.edu
Center for Wave Phenomena (The Home of Seismic Un*x)
Colorado School of Mines
Golden, CO 80401 | http://www.cwp.mines.edu/cwpcodes
voice: (303) 273-3049  |    fax: (303) 273-3478.
