In preceding chapters, we considered interpretation from the complementary standpoints of seismic fundamentals and data quality, leading naturally to discussions of correlation concepts and procedures. This chapter addresses additional topics and issues you probably will encounter in the normal course of work as a seismic interpreter. Ranging from technical fields such as 4D seismic and seismic modeling to philosophical concepts such as interpretive judgment and the interpretation paradox, the following sections are intended to raise your awareness of these topics as building blocks in the foundation of your career. Also included in this chapter are several sections on professional development, the substance of which is based solely on the experience of the author.
In the modern workstation environment, gridding and contouring interpreted 2D or 3D seismic data are accomplished routinely and relatively quickly as automated processes. This is in sharp contrast to the previous generation of interpretation of exclusively 2D seismic data on paper sections, in which gridding, the measurement of two-way reflection times to picked horizons at specific user-defined points, and contouring consumed a considerably greater fraction of an interpreter's time and were dramatically more subjective and dependent on the interpreter's individual skills and experience. Gridding of interpreted seismic data, even 3D data which are inherently “gridded” by the nature of their acquisition and processing, is now done primarily for two reasons: to facilitate automated contouring and to manipulate picked horizons (e.g., calculate the thickness of the interval between two horizons).
Figures & Tables
First Steps in Seismic Interpretation
Accurate interpretation of geophysical data — in particular, reflection seismic data — is one of the most important elements of a successful oil and gas exploration program. Despite technological advances in data acquisition and processing and the regular use of powerful computers and sophisticated software applications, you still face a tremendous challenge each time you begin to reconstruct the geologic story contained in a grid or volume of seismic data — that is, to interpret the data. On occasion, this interpretive tale can be clearly told; but most of the time, each page of each chapter is slowly turned, and rarely is the full meaning of the story completely understood.
Where the correlation of one reflection record with another is very easy, little needs to be said. Almost anyone can understand such a correlation. On the other hand, this is a rare occurrence. The usual thing is for the correlation to be so difficult as to be impossible. It is for this reason that correlation procedure can hardly be described in words (Dix, 1952).
Although Dix is speaking about the correlation of individual reflection records, which were used routinely before the advent of continuous common-depth-point (CDP) profiling, he clearly recognized the essence of interpretation as the considered extraction of geologic information from indirect geophysical measurements. His words are no less relevant and applicable now than they were 60 years ago, even in view of the high standards of data quality made possible by advances in seismic acquisition and processing, to say nothing of accompanying developments in interpretation technology. In the modern interpretation environment, you still face correlations that are “so difficult as to be impossible“ because these correlations define the frontiers of opportunity, the ones posing the sternest challenges and ultimately leading to the greatest rewards.
The primary aim of this book is to describe Dix's correlation procedure in terms of the science, data, tools, and techniques now used in seismic interpretation in the oil and gas industry. As an individual geoscientist, you develop and apply your own approach and style when interpreting seismic data. You continually revise and refine correlation procedures during the course of your career and expand them as you complete different interpretation projects. With experience, you learn to check and recheck the validity of your procedures to fully understand the rules of evidence that govern their use:
You must have a good understanding of seismic acquisition and processing principles as well as fundamentals of geology before beginning to collect interpretive evidence and solve interpretation problems correctly.