PART I: Modeling Theory and Practice
The goal of modeling is to elicit the seismic response of a subsurface structure. The plausibility of that structure is gauged by how well the form of the simulated reflection matches observed reflections. Correctly interpreting and mapping reflection structure, as opposed to depth structure, is therefore critical to the modeler’s ability to assess his results and derive the proper structural solution.
This chapter examines the meaning of reflection maps, shows how a reflection is organized in three dimensions, and explains how the map is constructed from various manifestations of the reflection, including diffractions and sideswipe. It also shows how the reflection map can be migrated in three dimensions, using inverse rays, to derive a depth structure which properly accounts for raypath bending and sideswipe. The procedure is termed map migration.
Figures & Tables
Seismic interpretation apparently is becoming primarily a geologic rather than a geophysical skill. This observation has been true from the moment seismic reflection data were displayed as a continuous record with the intention of creating an image of subsurface structure. The imaging advances that have occurred in the past two decades only reinforce the tendency. More effective migration algorithms making use of faster and less expensive computers, as well as high-fold and, in particular, 3-D data all serve to make the seismic picture better. As the image increasingly reveals more geology, the geologic skills become more crucial to the task of extracting the information made available. As seismic artifacts such as multiples, sideswipe, and raypath distortion effects are successively eliminated from the image, the geophysical sophistication of the interpreter becomes increasingly less important. At first glance it would seem that these tendencies can only intensify as these technological trends continue.
And yet the depiction of complex structures remains elusive. Migration programs have been developed that can manage the severe raypath bending attendant with complex structures. Moreover, the ever decreasing costs of computation make the application of these programs increasingly more feasible. Unfortunately, to benefit from these imaging approaches requires, a priori, an increasingly more precise definition of the velocity field which often is, in itself, an expression of geologic structure. Therefore, before we can create the image, we require an understanding of what the image is supposed to show. This circumstance implies that the preparation of the seismic image has become, and will likely remain, inextricably bound up with its interpretation.