GeoSyn can export integrated time data (traces, wavelets, wells, rocs, etc.) in SEGY format. The format specification discussed here is a derivative of the SEGY 9-track standard. Because there is no official MS DOS standard, some liberties have been taken with respect to the original specifications. For a discussion of the 9-track SEGY format, refer to "Recommended Standards for Digital Tape Formats" K.M. Barry, D.A. Cavers, C.W. Kneale: Geophysics, Vol. 40 No. 2, p. 344-352.
A SEGY file consists of three sections or blocks.
The first 3600 bytes of the file is the File Identification Header and consists of a 3200-byte ASCII card image block followed by a 400-byte binary block. The 3200-byte ASCII section represents a deviation from the 9-track SEGY standard, which specifies that this section be in EBCDIC format. Both are text formats, but use of the ASCII format enables a user to read this portion of the SEGY file in any PC-based text editor. This deviation from the standard has no detrimental effects because most programs rely on the binary data for hard information.
The ASCII section consists of 40 lines of 80 characters each. Each line starts with a C and the line number. The following is an abridged example of the 3200-byte ASCII header for a typical GeoSyn SEGY file and describes the well name, sample rate, data type, and byte order. This same information is encoded in the 400-byte binary section.
C1 GeoSyn Synthetic Seismogram
C2 ABC OIL AND GAS 11-11-11-11W1
C3 Date: 11:35 AM Fri Dec 01, 1995¯
C4 Sample Rate: 2.00 (ms)
C5 Data Type: Integer(2 byte)
C6 Byte Swap: Yes
C7
.....
C36
C37
C38
C39
C40
The 400-byte binary file header follows the ASCII file header and is situated on bytes 3201 to 3600. GeoSyn does not require or make use of most of the space.
The following bytes are used:
3213 to 3214 Number of byte traces per record =1
3217 to 3218 Sample interval in microseconds
3221 to 3222 Number of samples per data trace
3225 to 3226 Data sample format code (will be either 1 or 3)
1=floating point (4bytes)
2=fixed point (4 bytes)
3=fixed point (2 bytes)
4=fixed point with gain code (4 bytes)
3261 to 3262 Total number of traces per line =1
3269 to 3270 Start time of data trace (can be negative in the case of a wavelet)
The trace header is a 240-byte binary section that precedes each section of trace data. GeoSyn only writes one data curve, so there is one trace header in the file. The trace header occupies bytes 3601 to 3840. The bytes used by GeoSyn are tabulated below. Note that some values are 4 bytes long instead of 2-byte integer.
The following bytes are used:
3601 to 3604 Trace Number.
3629 to 3630 Trace identification code 1=seismic data, 2=dead trace.
3715 to 3716 Number of samples.
3717 to 3718 Sample interval in microseconds.
Trace data is stored in binary format and follows the 240-byte trace header. The binary data may be stored in one of two formats, 16-bit integer or IEEE floating point. The 16-bit integer format requires 2 bytes per amplitude value, whereas the floating point format requires 4 bytes.
The data area follows the trace header and consists of a number of 2- or 4-byte numbers representing the amplitude data of the trace or wavelet. Note that the number of samples in this record will exceed the number of samples in the synthetic if a trace is being saved, because the roll out portion of the trace below the bottom of the log is included. The number of extra samples will equal the length of the trace plus half the length of the wavelet operator.