Model 4: Migration
In this chapter, we develop tools that migrate data and that display migrated data.
In Section 9.2.2, we will use migration program sustolt. Here, we examine its parameters (except verbose and tmpdir).
The first two parameters, cdpmin and cdpmax, are required and have no default values. They allow us to apply DMO to a specified range of input CDPs. We will always set these minimum and maximum values to include all our input CDPs. In other words, we will always let sustolt apply DMO to all input CDPs. DMO, dip moveout correction, is NMO corrected for dipping beds in prestack data. Because we are migrating stacked traces, range refers to the CDPs that will be migrated.
The third parameter, dxcdp, is required and has no default value. To determine the CDP bin distance of Model 4, we created a stacking chart similar to the one shown in Sheriff (2002). Figure 9.1 is a subset of the acquisition geometry described in Section 6.3, but it includes enough geometry to let us calculate the CDP bin distance. Model 4 was acquired with a split-spread line of 60 receivers, receivers spaced 50 meters apart, with line moves 50 meters between shots. The split-spread line of Figure 9.1 has only 10 receivers, but receiver spacing and line movement are the same as for Model 4. The diagonal line of Common Midpoint Direction cuts the diagonal of 50 meter x 50 meter boxes. Therefore,
Since we will migrate stacked data, we will ignore parameter noffmix.
Figures & Tables
Our objective is to introduce you to the fundamentals of seismic data processing with a learn-by-doing approach. We do this with Seismic Un*x (SU), a free software package maintained and distributed by the Center for Wave Phenomena (CWP) at the Colorado School of Mines (CSM). At the outset, we want to express our gratitude to John Stockwell of the CWP for his expert counsel.
SU runs on several operating systems, including Unix, Microsoft Windows, and Apple Macintosh. However, we discuss SU only on Unix.
Detailed discussion of wave propagation, convolution, cross- and auto-correlation, Fourier transforms, semblance, and migration are too advanced for this Primer. Instead, we suggest you refer to other publications of the Society of Exploration Geophysicists, such as “Digital Processing of Geophysical Data – A Review” by Roy O. Lindseth and one of the two books by Ozdogan Yilmaz: “Seismic Data Processing,” 1987 and “Seismic Data Analysis,” 2001.
Our goal is to give you the experience and tools to continue exploring the concepts of seismic data processing on your own.
This Primer covers all processing steps necessary to produce a time migrated section from a 2-D seismic line. We use three sources of input data:
Synthetic data generated by SU;
Real shot gathers from the Oz Yilmaz collection at the Colorado School of Mines (ftp://ftp.cwp.mines.edu/pub/data); and
Real 2-D marine lines provided courtesy of Prof. Greg Moore of the University of Hawaii: the “Nankai” data set and the “Taiwan” data set.
The University of Texas, the University of Tulsa, and the University of Tokyo collected the Nankai data. The U.S. National Science Foundation and the government of Japan funded acquisition of the Nankai data.
The University of Hawaii, San Jose State University, and National Taiwan University collected the Taiwan data. The U.S. National Science Foundation and the National Science Council of Taiwan funded acquisition of the Taiwan data.
Chapters 1–3 introduce the Unix system and Seismic Un*x.
Chapters 4–5 build three simple models (complexity slowly increases) and acquire a 2-D line over each model. (These chapters may be skipped if you are only interested in processing.)
Chapters 6–9 build a model based on the previous three, acquire a 2-D line over that model, and process the line through migration.
Chapters 10–11 start with a real 2-D seismic line of shot gathers (Nankai) and process it through migration.
Chapters 12–13 and 15–16 start with a real 2-D line of shot gathers (Taiwan) and process it through migration.