Nankai Data: Examine, Resample, Sort, Gain
We are going to process a real 2-D line of seismic data provided by Prof. Greg Moore of the University of Hawaii. The data were collected near the coast of Japan, over the Nankai trough, where the Philippines plate is subducting beneath Eurasia. The “Nankai” data were collected by the University of Texas, the University of Tulsa, and the University of Tokyo. Based on this data set, a paper was published (Moore, et al., 1990) in which this line is called NT62-8.
The following Nankai seismic files are available:
Below is the surange output of Nshots.su.
Keys tracl and tracr number the 19057 traces (although they use different starting numbers).
Key fldr tells us there are 326 shot gathers (2012-1687+1).
Since the number of samples per trace (ns) is 5500 and the trace sample interval (dt) is 2000 microseconds, the trace length is 11 seconds (5500 samples/trace x 0.002 seconds/sample)
Notice that the Nankai shot gather trace headers already contain cdp values.
Below is a chart of the number of traces per shot gather (Figure 10.1). (We describe the program, sukeycount, which generated the data for the figure in Appendix C.) The first shot gather, 1687, has one trace, then the number of traces per gather increases to a maximum of 69 at gather 1735. This is constant through gather 1965. After gather 1965, the number of traces per gather steadily decreases to 1 at the last gather, 2012.
Below is the surange output of Nstack.su. This stack file (Figure
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.