Taiwan Data: Examine, Zero Traces, Re-examine
Our third data set is another 2-D line of seismic data provided by Prof. Greg Moore of the University of Hawaii. This “Taiwan” data set was collected near the coast of Taiwan in 1995 (Berndt & Moore, 1999) by the University of Hawaii, San Jose State University, and National Taiwan University. The size of our shot gather file, Tshot.su, is 411 Mbytes. (We have only one Taiwan data set, the original shot gathers.) This data set is more difficult to process than the Nankai data set.
Below is the surange output of Tshot.su.
Keys tracl and tracr number the 25344 traces (although, they use different starting numbers).
Key fldr tells us there are 176 shot gathers (975-800+1).
Since the number of samples per trace (ns) is 3999 and the trace sample interval (dt) is 4000 microseconds, the trace length is 16 seconds (15.996 seconds)
Notice that the Taiwan shot gather trace headers already contain cdp values.
This data set has many acquisition key values such as source x-coordinate (sx), receiver x-coordinate (gx), receiver y-coordinate (gy), and source depth (sdepth).
Figure 12.1 is a chart of the number of traces per shot gather. (We describe the program, sukeycount, which generated the data for the figure in Appendix C.) The chart shows that every shot gather has 144 traces.
We do not want to process 16 seconds of data. Let's use the first 5 seconds:
The size of our windowed shot gather file is 133 Mbytes.
As we discussed in Section 10.2
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.