Nankai: Velocity Analysis, NMO, Stack, Migration
Below is the user area of iva.sh changed for the Nankai data. As stated in Section 10.4.2, we will do velocity analysis on every 25th CMP starting with CMP 933 (lines 13-16). We change the perc value to 95 (line 29), an accommodation for real data. Semblance values go from 1000 m/s (line 38) to 7000 m/s (nvs*dvs + fvs). CVS panels range from 1000 m/s to 6000 m/s (lines 41-44).
By looking at the Nankai stack, Figure 10.3 left, we know the water layer is approximately 6 seconds (two-way time) and we think there is not much reflection character below 7.8 seconds. The left side of Figure 11.1 is the velan of CMP 933. It appears that the time for picking events is restricted to between 6 seconds and 8 seconds.
Instead of making velocity picks in a narrow space on the screen, we window the file of CMPs: and use this as the input to iva.sh
We chose the full time (tmax=11) because on CMP 933 we see events around 9.5 seconds and we don't want to miss these and other potentially later events.
The result of using this new input file is shown on the right side of Figure 11.1. This makes better use of the screen.
Because we are not picking from zero time, we will have to modify the output pick file, adding zero time to the beginning of all the tnmo series and adding the water velocity to the beginning of all the vnmo
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.