Interpretive Lessons from Three-Dimensional Finite-Difference Seismic Models

Irshad R. Mufti

Finite-difference seismic modeling is commonly done by treating the earth as a two-dimensional medium and exciting the model with a line source. However, a significant amount of reflected energy reaching the surface originates from the subsurface features that are not confined to the vertical plane corresponding to the direction of the seismic profile. Moreover, the drop in amplitude with distance due to a line source is quite different from that due to a point source. These problems have been investigated, with special reference to their impact on the interpretation of data, by computing a series of shot records and seismic sections from three-dimensional finite-difference models involving a point source, and then comparing them with similar data based on the correspondi g two-dimensional models. The results obtained from three-dimensional models provide not only additional information, such as sideswipes, but they are also significantly different from the two-dimensional results. Among other things, anomalously strong events that look like bright spots are identified as originating from a single interface possessing a perfectly uniform reflectivity and are caused by the focusing of the reflected energy in a three-dimensional environment. Thus, lateral variations in amplitude that are commonly attributed to changes in lithology are strongly influenced by the geometric changes in the subsurface reflectors along the third, and commonly ignored, spatial dimension. Such interpretation pitfalls can be avoided by a simultaneous analysis of data along mutually ntersecting seismic lines.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.