SEG-Y versus SEG-D seismic data Format explained

In geophysical field surveying, data is collected and stored in various formats to simplify data processing and exchanging (data transfer). However in Seismic geophysics, It is recommended to collect seismic data in industry standard format so as to reduce ambiguity as most seismic data involves complex processing. There are two (2) industry standard formats for handling seismic data, SEG-D and SEG-Y. This post will give you an idea on these two format handle seismic data,

The great format is Industry - standard which was first developed in 1973, and published in 1975 by Society of Exploration Geophysicists (SEG).

i/ SEG-D Format

It stores acquired seismic field data either in multiplexed or demultiplexed form. We can say that it is raw data that was collected from geophysical sensors with no source applied. The seismic data is in unprocessed format and implies the original seismic data that was collected from the seismic recording system.

This means a different traces will have the same sample at which each samples from different traces are at time sequence such as at a certain sampling interval. Say first sample (S1) is contained from each trace A, B, C to last trace. Then another sample (S2) also contained from each trace, A, B, C to last trace. But sample S1 and S2 are in time sequence (at sampling interval).

In Large scale surveys sampling interval may be 2ms or 4ms for a total Two way travel (TWT) time of 6 - 7 sec (Reynold, 1997).

a/ Multiplexed data : Are stored in a series of scans per block. It is a form of grouped data.

b/ De- multiplexed data

Are stored in sequence of a header record followed by a number of de- multiplexed data records and then a single end file mark. It is a form of scattered/ ungrouped data.

Note: Terms Multiplexing can simply be defined as the process of combining many (multi) input data and producing a single channel data while Demultiplexing is just the reverse of Multiplexing, you have a single data and break it down into its components data. These terms are frequently utilized in telecommunication during signal transmission, such as many signals with different frequencies are combined into a single signal to simplify their transmission in a single channeled path,This is Multiplexing. As this single signal arrived at a point where it separated back into its original multi signals to simplify distribution through many channels, this is demultiplexing.

However in seismic geophysics Demultiplexing can be defined as the process of separating all samples to produce a time sequence for each geophone. For Example if you have geophones P,Q,R,S and samples 1,2,3,4. Each geophone can record as follows P1Q1R1S1P2Q2R2S2....

Then after demultiplexing our record would be P1P2P3P4 , Q1Q2Q3Q4, and so on. You can look at the diagram in figure 1 below to get a simple insight.

Figure 1: Simple demultiplexing process.

So In multiplexing you reverse the demultiplexing that shown in the figure 1 above.

ii/ SEG-Y Format

Is the trace sequential (or demultiplexed ) format used for storage of processed seismic data or partially processed data from field processing crew. SEG-Y format obtained after running the various seismic quality control. Most Seismic interpreters are working with this type of format.

A simple format of SEG-Y, contains Number of bytes and Header value descriptions. Number of bytes are separated into a certain interval. Example for 240 bytes, it means from 1 - 240, but broken sections can be 1 - 4, 5 - 9, and so on up to the highest byte value. Each byte interval has a specific header value description.

This means that all number of samples for a certain trace are in one block. Say trace 1 contain all samples, the same in trace 2 contains all samples, and so on.

The following are advantages of using seismic data stored in SEG-Y format.

- It enhances data transfer from one contractor to another hence SEG-Y is used for Data exchange.

- Manual inspection to identify unsuitable recording such as Data quality control

- Integration of positional and survey geometry information.

Thank you for reading this!

Reference.

- Hagelund, Rune, Stewart A. Levin, eds (2017), SEG-Y, revision,2.0 , Data exchange format, Tulsa, OK. SEG.

- Barry, K.M; Cavers, D.A,; Kneale, C.W. (1975), Recommended standards for digital tape formats. Geophysics, 40 (2); 344 - 352.

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