Taiwan: Gain-Filter, Filter-Gain
We now have 5 seconds of Taiwan data and we zeroed the bad hydrophones. In this chapter we will
apply spherical divergence correction, then band-pass filter the data. We will also
apply a band-pass filter, then apply spherical divergence correction. We will compare gathers from these two paths and consider whether one path is better.
To determine which exponential gain to apply to the line of CMPs, we will apply gain to two shot gathers: 820 and 930̤ We window them from the 2-D line:
We will determine a tpow value for sugain by using script igain (Section 10.5), option T. Below is the user area of igain.sh changed for the Taiwan data.
The script creates three displays:
a wiggle plot of the file,
a decibel (dB) ximage plot of the user-selected trace(s), and
an amplitude graph of the user-selected trace(s). Remember:
The script applies gain to the entire file, but only the user-selected trace or traces are used to create the second and third displays.
The user is asked to supply the name of a key (offset, tracr, etc.) and corresponding key minimum and key maximum values for the second and third displays.
We selected key tracf and supplied tracf values 92 93 for both shot gathers, based on our displays in Chapter 12.
After several trials, we decide to use a spherical divergence correction of 1.8 (Figures 13.1 and 13.2).
We apply spherical divergence correction with the command below.
To select frequencies
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.