Three Simple Models: Acquire 2-D Lines
In the previous chapter, we developed three simple models and used them to generate single shot gathers and images of rays and wavefronts. In this chapter, we use the same models to acquired 2-D lines of seismic data.
Script acql.sh acquires seismic data over model modell.dat, generated by script modell.sh (Section 4.2). The survey layout:
40 shots (line 28)
Shots are equally spaced at 50m intervals (line 32 and line 35)
60 split-spread traces are recorded from each shot location (line 42)
Geophone spacing is 50m (line 46 and line 49)
Geophone offsets range from −1475 m to +1475 m (line 52)
Shot locations range from 2 km to 3.95 km (line 68)
Geophone locations range from 0.525 km to 5.425 km (line 68)
The final data set has 2400 traces (40 shots x 60 geophones per shot). The generation of this data set took about half an hour on a Sun UltraSPARC III with four processors.
Lines 61 and 62 write variable values to the screen. Notice that line 4 is commented out. We find that once a script is perfected, the messages from line 4 interfere with reading the values printed to the screen by lines 61 and 62.
Previously (Section 4.5), we discussed variables nangle (the number of rays or angles that emanate from the source), fangle (the first angle of the fan of rays that emanate from the source), and langte (the last angle of the fan of rays that emanate from 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.