Version: 3.2
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The LEA Seismic Data Object is an LEA Object designed to access seismic data. It will recognise standard SEG-Y disk files as well as Lynx's internal TR format used with the LEA vectorising software. The seismic object is implemented by the DLL SEISCOM.DLL.
Although not all seismic data file formats can contain these attributes, they are common to all seismic data. For file formats where an attribute is not present, the value will be null and will be displayed as blank.
Attributes are defined in the INI file SEISOBJ.INI
The LEA Seismic Data Object can read and write the following file formats:
There is no widely accepted convention for a file extension for SEG-Y files. Commonly used extensions are .sgy or any extension matching the wildcard .sg?.
The original SEG-Y standard was defined by the Society of Exploration Geophysicists as a seismic data interchange format for tapes, but has been widely adopted for disk storage of seismic data.
SEG-Y is strictly defined to use big-endian binary data storage and IBM floating-point numbers, whereas PCs use little-endian binary data storage and IEEE floating-point numbers. This means that for use on a PC, all values read from a SEG-Y file must be byte-swapped. This is why most LEA applications that perform seismic processing prefer the Lynx Trace File format (see below), which is essentially a byte-swapped version of SEG-Y.
The structure of a SEG-Y tape is as below, but note that the official SEG-Y tape standard does not support multiple lines on a single tape:
Block Description 3200 bytes Reel Identification Header (Part 1)
EBCDIC coded comments
This section is used to store textual information such as the line and survey name and processing sequence.IBG Inter-Block Gap 400 bytes Reel Identification Header (Part 2)
Binary coded information
The following infomation must be present if the file is to be recognised as SEG-Y:
Byte Position Description 5-8 Line number 9-12 Reel number 13-14 Total number of traces per record 17-18 Sample interval (microseconds) 21-22 Number of samples per trace 25-26 Sample format code:
1=FLOAT-4 (32-bit floating point)
3=FIXED-2 (16-bit integer)
5=BYTE-1 (8-bit integer)27-28 Traces per CDP 55-56 Units code (=1, metres)
SEG-Y files written by LEA applications may also contain the following extra information if written as SEG-Y LYNX variant:
Byte Position Description 13-14 Number of traces in file (instead of total number of traces per record) 61-62 Lynx variant marker:
Hexadecimal AAAA63-64 Shotpoint number of first trace
(IBM floating-point value)67-70 Shotpoint increment
(IBM floating-point value)IBG 240 bytes Trace Header for trace 1
Each trace header will contain as a minimum the following information:SEG-Y files written by LEA applications may also contain the following additional information in trace headers:
Byte Position Description 1-4 Trace sequence number in line 5-8 Trace sequence number on reel 17-20 Shotpoint number (rounded to nearest integer) 21-24 CDP ensemble number 29-30 Trace identification code
1=live trace109-110 Recording time delay (milliseconds)
(=data start time)115-116 Number of samples in this trace 117-118 Sample interval (microseconds) in this trace
SEG-Y STANDARD variant:SEG-Y LYNX variant: As for STANDARD with the following additional information:
Byte Position Description 9-12 Trace sequence number within line 13-16 Trace sequence number within line 69-70 Scalar to apply to elevations (=1) 71-72 Scalar to apply to coordinates (=1) 73-76 X coordinate 77-80 Y coordinate 81-84 X coordinate 85-88 Y coordinate 89-90 Coordinate units = 1 (metres)
Byte Position Description 25-28 Trace number within CDP ensemble (=1) 91-96 Scale factor applied to trace values (Turbo Pascal 6-byte real type) - used for 8-bit (BYTE-1) sample format only 181-184 Shotpoint number (IBM floating point) n Trace Data for trace 1 IBG 240 bytes Trace Header for trace 2 n Trace Data for trace 2 IBG ... 240 bytes Trace Header for last trace n Trace Data for last trace EOF End of File marker On a multi-line tape, this pattern is repeated for each seismic line. The last seismic line on the tape is followed by two EOF markers.
When written as a disk file, the convention is to have the same number of samples in each trace (so the value in bytes 21-22 of the binary header is the same as the value in bytes 115-116 of each trace header. This allows for random access to traces, since disk files have no intrinsic blocking.
The internal format of a Lynx Trace File follows the general structure of a SEG-Y disk file. Lynx Trace Files use native Intel byte-ordering (little-endian), native number formats (ie IEEE instead of IBM to represent floating-point values), and native character-encoding (the 3200-byte text header at the start of the file is stored as ASCII)
Lynx Trace Files usually have a file extension .tr? (where ? is 0-9 or a-z) and are used mainly by LEA seismic vectorising and processing applications.
The default viewer for seismic files is SEISVIEW
Listed below are the error messages that may be generated by a LEA Seismic Data Object .
Error opening file
Error creating new file
Error reading file
Error writing file/disk full
Filestream not initialised
Filestream is read-only
Unknown seismic format
Trace number does not exist
Illegal trace number
See also
[ The Lynx Exploration
Archive Object ]