Generally, time to depth conversion using a simple velocity function will result in a mis-tie due to lateral velocity changes in the overburden.  In processing we would say that is a “depth problem”.  A typical example is the case of a salt dome where sediments with a velocity of 10,000 feet per second rest unconformably against an intrusive salt with a velocity of 20,000 feet per second.  Time migration is not adequate in this case due the limiting assumption of slowly changing lateral velocities.  Depth migration, however, improves imaging by iteratively ray tracing through a velocity model until the CDP gathers are flat thus providing the clearest stack possible.  The land data shown in this paper exhibit a shallow, high velocity layer of interbedded evaporates and anhydrites overlaying relatively lower velocity sediments.  This high velocity, near surface inversion creates the ray path distortions similar to the offshore case of extreme lateral velocity changes. 

The presentation starts with a look at the PSDM data and the ray bending caused by a near surface velocity inversion. (Figure 1).  A quick look at a few wells confirms the near surface high velocity inversion.  The wells also confirm the velocity and the complexity of not just anhydrite but slower salts within the anhydrite rock and their changing thickness.  Both the anhydrites and salts are higher velocity than the underlying shales and sands in the immediate Delaware section.  It is this zone immediately below the high velocity layers that is improved from a depth imaging.  As we move deeper in the section from the Delaware group into the Bone Spring Group and eventually to the Wolfcamp section, the problem begins to heal itself.  Unfortunately, depending on the thickness and complexity of the velocity changes from above, the Bone Spring Group through the top of the Wolfcamp section can be affected. 

Speaker Biography: Bruce Karr, FairfieldNodal
Bruce Karr, Technical Sales Manager for Fairfield Seismic Technologies, has worked for FairfieldNodal since 1994 as a Geophysicist.  Mr. Karr’s processing expertise includes 3D and 4D multi-component land data, with particular focus on geophysical problems including long wave length statics, spectral enhancement, noise, depth-time issues, multi-component data and field technology. 
 
Mr. Karr received a BS in Geophysical Engineering and a minor in Geology from the Colorado School of Mines in 1988, and began his career with GSI shortly after graduation.  After two years of field work in Saudi Arabia, and after GSI was purchased by Halliburton, Mr. Karr was transferred to Midland, Texas, where he began processing seismic data.  By the early 1990s, West Texas was a prolific region for 3D surveys, and Mr. Karr learned his trade on 3D projects in the Midland and Delaware basins.  Five years later, after Halliburton sold their geophysical services, Mr. Karr moved to Denver to begin work for Golden Geophysical, which was later purchased by Fairfield Industries. 
 
Mr. Karr has worked with students and professors in partnership with the Colorado School of Mines Reservoir Characterization Project (RCP), Golden, Colorado; the Bureau of Economic Geology Consortium (EGL), Austin, Texas; and the Kansas Geological Survey (KGS), Lawrence, Kansas. Mr. Karr’s latest projects have focused on the Permian Basin, Mid-continent and Rockies.  Most recently, Mr. Karr has produced or coproduced a number of papers and presentations concentrating on his areas of expertise in solving geophysical and geologic problems.  As a member of the FairfieldNodal team, Mr. Karr uses his knowledge to help clients resolve complex land project challenges.