Page 252 - Acquisition and Processing of Marine Seismic Data

5.2 DATA LOADING 243

5.2 DATA LOADING it is processed SegY data. This header is in
ASCII format and is filled by field engineers
The input process of the raw field data vol- during the data acquisition or by data proces-
ume into the processing system is termed data sors at the end of the processing. The second
loading. Raw seismic data is recorded in specific block is a 400 byte–long binary header and con-
binary data formats defined by the Society of sists of general information of identical param-
Exploration Geophysicists (SEG). This specific eters for all channels, such as number of
format is read by the processing system and channels per shot, sampling rate, number of
the data becomes ready to process after the samples per trace, etc. The last header block
data loading step. This specific data format is known as the TRACE header, which is
includes not only the digitized seismic reflection located just before the very first samples of each
amplitude samples, but also vital information recording channel. Each TRACE header is 240
related to the data, such as shot number bytes long and every single trace in a SegY seis-
(FFID), sampling rate, number of samples per mic data volume has its own TRACE header,
trace, number of channels, etc., and the blocks which stores trace-specific substantial informa-
of these subsidiary areas inside of the data are tion about the trace it belongs to, such as chan-
termed headers. nel number of the trace, shot number that the
Various seismic data format types used for trace resides in, offset distance or coordinates
different purposes are defined by SEG today. of the trace, etc. (Fig. 5.3A).
For instance, seismic data for engineering pur- On the seismic market today, the SegD format
poses collected by engineering seismographs is preferred for raw seismic data recording dur-
are generally recorded using a format known ing the acquisition because it has huge header
as Seg2, while raw seismic reflection data both spaces to store the field data parameters.
from land and marine surveys are commonly Fig. 5.3B shows a schematic diagram of a SegD
recorded in SegD format. However, the most file structure consisting of only three seismic
common seismic data format is known as the traces. The SegD format can be grouped into
SegY data format, which can be recognized by six blocks including the headers: (i) GENERAL
any kind of seismic data processing as well as header(s), (ii) CHANNEL SET header,
interpretation software on the market. SegY is (iii) EXTENDED header(s), (iv) EXTERNAL
a seismic data exchange format that allows seis- header(s), (v) TRACE headers, and (vi) ampli-
mic data to circulate among data processing sys- tude values of the seismic data. Unlike the SegY
tems of different contractors. The SegY format file format, the number of EXTENDED and
has three different header blocks of different EXTERNAL headers in SegD format can be
sizes to record the subsidiary information along extended up to 1000 header blocks, 32 bytes
with the seismic amplitude samples. each, which ultimately makes the SegD format
Fig. 5.3A shows a schematic diagram of a suitable for acquisition.
SegY file structure consisting of only three seis- Seismic data is normally recorded either in
mic traces. The file structure can be grouped SegD or in the internal format of the seismic
into four blocks: (i) EBCDIC header, acquisition system. However, seismic data pro-
(ii) BINARY header, (iii) TRACE headers, and cessing software commonly has proprietary
(iv) amplitude values of the seismic data. The seismic data formats, known as internal format,
first block is a 3200 byte–long EBCDIC header which is normally much more practical to use
consisting of 80 columns and 40 rows, which during the processing. Therefore, data loading
stores either field parameters if it is raw SegY also includes a format conversion from raw data
data or processing sequence and parameters if format into the processing software’s internal