JEFFERY, ROBERT W., Forcenergy Inc., Anchorage, AK; and DAVID R. JEFFERIS, Dynamic Graphics Inc., Bakersfield, CA

Abstract: 3-D Time-to-Depth Conversion Beneath a Deep Marine Channel: Finding the Depth Error Model Using a Pragmatic Approach

From a work flow point of view, a 3-D geologic model of an exploration prospect in Cook Inlet, Alaska was initially interpreted in time-domain using a 3-D seismic interpretation system, then assembled into a 3-D geologic model in time, reviewed, revised, and vertically rescaled to depth-domain using a 3-D geological modeling system.

Following the assemblage into a 3-D geologic framework in time-domain, the results of the interpretation were verified against the working conceptual model of the geophysicist as being reasonable. Since only a single checkshot survey was available, and a deep marine channel was directly above the prospect, the project geophysicist knew that the 3-D average velocity model could not possibly honor reality. A "back-of-the-envelope" calculation predicted the amount of error that would be realized from applying a single linear velocity function over the 3-D space. This calculation took into consideration the location of the checkshot survey, channel geometry, and geologic structure of the prospect.

Once the geological model was in depth, an error analysis was performed using the few wells that had been drilled in the area. Using various 2-D gridding algorithms and 3-D visualization, the different error models were reviewed to find one that matched the error model developed by the project geophysicist. It became apparent that the best solution would be to use the geostatistical technique of kriging with an external drift; using the channel bathymetry as the external drift. This error model was then applied to the depth model to derive the final 3-D geological model.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California