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The SEG-Y (sometimes SEG Y) file format is one of several standards developed by the Society of Exploration Geophysicists (SEG) for storing geophysical data. It is an open standard, and is controlled by the SEG Technical Standards Committee, a non-profit organization.

History
The format was originally developed in 1973 to store single-line seismic reflection digital data on magnetic tapes. The specification was published in 1975.

The format and its name evolved from the SEG "Ex" or Exchange Tape Format. However, since its release, there have been significant advancements in geophysical data acquisition, such as 3-dimensional seismic techniques and high speed, high capacity recording.

The most recent revision of the SEG-Y format was published in 2023, named the rev 2.1 specification. It still features certain legacies of the original format (referred as rev 0), such as an optional SEG-Y tape label, the main 3200 byte textual EBCDIC character encoded tape header and a 400 byte binary header.

Data structure


This image shows the byte stream structure of a SEG-Y file, with rev 1 Extended Textual File Header records. A SEG-Y file begins with an optional 128 byte SEG-Y tape label, followed by a 3200 byte Textual File Header and 400 Binary File header. These are followed by zero or more Extended Textual File Header records. The remaining file consists of a number of Data Trace records, which are each preceded by a 240-byte Standard Trace Header, and optional 240-byte Trace Header Extensions.

Textual File Header
The Textual File Header consists of 40 lines of text, providing a human readable description of the data contained in the file. The information contained is free form and is not well defined. The Extended Textual File Headers were defined in SEG-Y rev 2 and provide a defined structure for this data, allowing it to be stored in a machine-readable way.

The Extended Textual File Headers are 3200 byte records used to record information about the SEG-Y file, such as trace header mappings.

Since the first SEG-Y standard was published, many companies dealing with seismic data have produced variants of the SEG-Y standard which have run contrary to the aims of defining a standard for universal interchange, thus generally causing confusion and delay when data received by a company in expected SEG-Y format turns out to be a variant of that format. Initially, many of these derived from the fact that the format was based on the de facto standard of using IBM computers for digital processing where character data was coded in EBCDIC and number data in IBM Floating Point, whereas processing systems in use quickly evolved based on ASCII character and IEEE number representations.

Even before the SEG-Y standard was agreed and published, earlier seismic data format standards published by the SEG such as SEG-A, SEG-B and SEG-C were modified by seismic acquisition companies, though not to the wide extent that the SEG-Y format has been modified by seismic acquisition and processing companies and oil companies using their own inhouse software.

As magnetic tape technology developed, the original SEG-Y format using individual small data blocks for each distinct seismic trace became very inefficient in terms of tape performance so the first and subsequent revisions allowed for larger tape data blocks containing many individual traces.

There have been many suggestions for including different kinds of metadata within the standard over the years, since when the standard was first proposed the processes of acquisition and processing were technologically much simpler.

For example geographical positioning information either real world or relative wasn't stored in the trace header at acquisition or final processing whereas it is routine today.

However, the relative simplicity of the SEG-Y format has meant that it has worked well for interchange of seismic data allowing anyone to read and understand data recorded in the 1970s on half inch magnetic tape as easily as reading it on modern tape media or from a disk file, as long as the standard has been adhered to.

While SEG-Y format data is still stored on magnetic tape as a permanent archive, SEG-Y data is increasingly stored as disk files for online and near-line ease of access, and later format revisions allow for character and number data to be stored in native system representations as ASCII and IEEE.