Model 4: Sort, Velocity Analysis
From the previous chapter, we have a 2-D line of seismic data. In this chapter, we:
sort the shot gathers to common midpoint (CMP) gathers and
perform velocity analysis on selected CMP gathers.
Because our synthetic data are noise-free, we can proceed quickly from shot gathers to migration. If we wanted to, we could use suaddnoise to add noise to the data; this is often useful. You should consider adding some noise to the data set, then processing it using the examples we provide in this chapter and the next.
Let's discuss “common-depth-point” (CDP) and “common midpoint” (CMP) (Sheriff, 2002). We try to avoid the term CDP because there is no common (same) point at the reflector if the reflector dips. On the other hand, common-midpoints almost always exist because they are defined by the geometric midpoint between sources and receivers. Because SU does not have a cmp key, we reluctantly use the cdp key.
Script sort2cmp.sh does two jobs:
We use program suchw (Change Header Word) to create header (key) cdp and assign values to it. Using the equation below, values of cdp (key1) are computed from gx (key2) and sx (key3).
Scalars a, b, c, and d need to be determined by sketching the geometry. Our geometry dictates a = 1525, b =1, c = 1, and d = 50. Therefore,
We use program susort to sort the traces based on primary sort key cdp and secondary sort key offset. A secondary sort is the order within the
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.