This chapter describes the basic correlation procedures used in a typical interpretation project, beginning with how to start an interpretation and then discussing fundamentals of the two main correlation techniques (loop tying and jump correlation). The section on correlations in depth-migration projects provides helpful information and guidance for handling aspects of interpretation that are specific to working with depth-migrated data, especially for building velocity models. The discussion of visualization emphasizes the importance of this procedure for validating correlations and communicating interpretation results. The chapter closes with a summary of individual processes and an example of a generic interpretation workflow.
After checking to be sure that all data (seismic, well, cultural, potential field, etc.) for your project are in hand, take time to review the data before beginning correlations. You often make many important observations with a minimum of bias at this early stage of a project. Here are the steps you should follow:
You select horizons for interpretation and mapping early in a project; some horizons may be more geologically significant or more obvious than others, and in some projects there is neither time nor need for picking the entire set of common dip families that make up an individual seismic line or a whole data set. These choices, which often involve correlation with other control such as well information by way of well ties, are determined by the exploration objectives for the project, the geologic complexity of the study area, and the quality of the available seismic data. On occasion, you modify the number of horizons and faults that you interpret during a project as you learn more about the study area. Within the limits of project objectives and available time, there is no point picking horizons (boundaries) and faults that do not contribute meaningfully to building the geologic history of an area. Although there might be many interesting features and surfaces visible in your data, attempting to identify and track all of them can compromise the objectives of an interpretation, destroy interpretive focus, and waste time and resources.
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.