next up previous contents
Next: SEGYREAD - Getting SEG-Y Up: Reading and Writing Data Previous: Reading and Writing Data   Contents

The SEGY format and the SU data format

The data format that is expected by all programs in the CWP/SU package whose names begin with the letters `su' (with the exception of the program subset), consists of ``SEGY traces written in the native binary format of the machine you are running the programs on.'' To understand what this phrase means, we must understand what the SEGY standard is.

In the early 1980's, the most common data storage format was SEG-Y. This is the Society of Exploration Geophysicists Y format which is described in the SEG's publication Digital Tape Standards. The format is still widely used, today, though there is no guarantee that the format is used ``by the book.''

The SEGY data format consists of 3 parts. The first part is a 3200 byte EBCDIC card image header which contains 40 cards (i.e. 40 lines of text with 80 characters per line) worth of text data describing the tape. The second part is a 400 byte binary header containing information about the contents of the tape reel. The third portion of the SEG-Y format consists of the actual seismic traces. Each trace has a 240 byte trace header. The data follow, written in one of 4 possible 32 formats in IBM floating point notation as defined in IBM Form GA 22-6821. (Note, this ``IBM format'' is not the common IEEE format found on modern IBM PC's.)

The SU data format is based on the trace portion of the SEGY format. The primary difference between the SEGY traces and SU traces is that the data portion of the SU format are floats, written in the native binary float format of the machine you are running SU on. SU data consists of the SEGY traces only! The ebcdic and binary reel headers are not preserved in the SU format, so simply redirecting in a SEGY file will not work with any SU program.

To convert SEGY data into a form that can be used by SU programs, you will need to use segyread.


next up previous contents
Next: SEGYREAD - Getting SEG-Y Up: Reading and Writing Data Previous: Reading and Writing Data   Contents
John Stockwell 2007-04-10