Trace Headers and Windowing Data
When seismic traces are in SEG-Y format, the SU trace format, and many other formats, the beginning of every trace, the trace header, has information about the trace. You can think of these as slots of information above the data part of the trace. The data part is the time-amplitude series that we see in a seismic display.
Trace header information might include the trace number and the offset of the trace (for shot or CMP gathers). In an SU seismic data set, the number of trace header slots is the same to ensure that every trace in a data set has the same length (in terms of bytes of storage).
SU doesn't call them headers; it calls them keys. The following table lists some SU keys.
You can use program surange to learn which keys are in a data set and the range of their values (the largest and smallest values). Now that you have run script myplot.sh (Section 2.4), use program surange to examine the trace headers of the .seismic file. Enter:
The output (screen display) is:
This synthetic data set has 32 traces. Only five of its keys (trace headers) have non-zero values. The minimum value of trad and tracr is 1; the maximum value of trad and tracr is 32. This is not surprising since there are 32 traces in the data set. We can suppose that the traces are numbered in sequence, 1 to 32. Surprisingly, all traces have the same offset: 400.
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.