This format description is designed to assist users of AGSO digital seismic data. In general the standard descriptions have been adhered to as specified by Barry et. al. (Recommended Standardsfor Digital Tape Formats, Geophysics, vol 40, No 2 (April 1975) pp 344-352).
Trace header mnemonics for many of the entries are as per used by CogniSeis in their DISCO
software package, others have been defined by AGSO.
Variations to the standard trace header descriptions (bytes 1-180) are as follows. TRACED has expanded codes which are applicable to unstacked data eg. reformatted field data. Static corrections and delays are defined to account for the field acquisition system.
In addition to the standard entries a number of optional entries are also used. Some of these arecreated by DISCO and others relate to AGSO data acquisition and processing. The most importantof these are SHOT or SPN.
Please note that for AGSO data released prior to 1993, a pseudo shot point number was used (SPN)
as opposed to processing shot number (SHOT). The SPN was an assigned number generated from
the stack data set and usually started at 100. This number has no direct relationship to the original field shot number (FFID) or processing shot number (SHOT) and may be considered to be a re-sequenced CDP. Therefore the first SPN on a line may not occur at fall fold but the first live stack
trace. To relate this vintage data to original field data either 141-11) if present, must be used or time values from the headers must be used. Its use was historical and primarily for the purposes of
producing shot point maps which could be related to a section. Its use has been discontinued from 01993 and replaced by SHOT which in most cases will be identical to FFID. Exceptions occur when
the line has been merged for acquisition or processing reasons and SHOT numbers have been re-sequenced to obtain a continuous numbering sequence.
The convention used for the SHOT annotation position is the mid-point of the source and the firstactive channel. Therefore the ship antenna position, which is Rig Seismic's firing navigation referencepoint, is corrected to relate to this point for all final navigation data.
Field data polarity is maintained throughout the processing sequence. The convention for AGSOdata is that a compression wave is plotted as negative.
Some data sets may include bathymetry and geophysical data such as magnetics and gravity. These are also corrected to the mid-point location.
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