WROLSTAD, K. H., F. AMINZADEH, and C. K. von KAHRS, Unocal S&T, Brea, CA

ABSTRACT: Method for Analysis and Display of Elastic Synthetic Shot Gathers

Elastic synthetic shot gathers contain a very complex superposition of compressional waves, shear waves, multiples, and converted waves. A method has been developed to compute surface synthetic shot gathers or VSP gathers, display model parameters, and provide an analysis of the model to aid in identifying events. This modeling interpretation tool generates fast synthetic elastic seismograms for group of linear surface or borehole receivers using a surface point source in models with horizontal layers. An associated display includes a plot of interval compressional and shear velocities, densities, and Poisson's ratios annotated in depth and time, event travel time curves in the time-offset domain or for VSP modeling in the depth-time domain, and a scattergram of events in the time-rms velocity domain. The travel time curves and points or events in the velocity scattergram are cross-referenced to a listing of ray path histories for precise event identification. The event amplitudes are indicated by circle diameters and event type by color code. Elastic synthetic traces are computed with a ray trace program which is necessary for event ray path history storage. Computation time is decreased by specification of an amplitude threshold beyond which further event splitting is no longer allowed. Tests show that the ray tracing program is 100-200 times faster than wave equation methods with comparable quality of output, except that frequency-dependent thin-layer transmission effects would not be modeled accurately and head waves and surface waves are not present in the ray tr cing program. Amplitudes are computed at each boundary using the Zoeppritz equations with no approximations.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.