INTAS-IA-96-03 - Moscow, July'98
MOSCOW - Spetsgeofizika
seismic Lab.
|
Time-signal processing:
considerations on
the SEG-Y format utilisation
|
by Hermann Zeyen [1]
SEG-Y
is a format for storage of reflection seismic data, i.e. traces recorded
near to the shot point (small offset). The DSS data, instead, have often
offsets of several hundreds of km. This implies that the first signal of
a shot arrives at increasing time delays with increasing distances. Usually,
one can suppose that the velocity of the first arrivals is 8.0-8.5 km/s
out to distances of at least 1000 km. So, e.g. for a distance of 400 km
from the shot point, the first 50 s after the shot are of no interest and
should not be stored.
SEG-Y accounts for the possibility that the first recorded sample of
a trace is not from the exact shot time, by defining a "delay recording
time" in the trace headers (bytes 109-110) which is defined as milliseconds
after shot-time, and stored in a INTEGER*2 number. An INTEGER*2 number
cannot be larger than 32767 value, and therefore the maximum delay time
allowed is 32.767 seconds. In the example above, one would not need to
store the first 50000 ms (=50 s). So, there is a problem in the DSS data
processing, and long-range DSS data cannot be stored in SEG-Y format without
making certain modifications in the headers.
The following options can be envisaged:
-
Store the delay-time in milliseconds, but in an INTEGER*4 value. In this
case the datum has to be stored between bytes 181 and 240 of the trace
headers, which are free for optional use. In the original place (109-110)
one could store the delay-time in samples (usually this number is 4-20
times smaller than milliseconds);
-
Store the delay-time as time after the reduced travel-time. In this case,
one should store the used reduction velocity somewhere between bytes 181
and 240 (it may be different for every trace);
-
Make no modifications to the original SEG-Y file and add leading zeros
in the traces, which else would start later than c. 32 seconds (makes the
file evidently larger).
Option 1. has the following advantage: it is very easy to change
the Synapse software, no need to do further changes later on: just take
the difference between the given shot-time (to be introduced manually during
the digitisation process) and the absolute-time of the first sample, which
is calculated during the digitisation process. In this contest, you have
to note that SEG-Y allows storage of the recording-time of every trace,
but only to the second !!! There is no space foreseen for the milliseconds
of the absolute-time of the first sample. These can only be defined in
relation to the shot-time (i.e. the "delay recording time", bytes 109-110).
The disadvantage of this option is that standard commercial seismic software
cannot treat these data with additional information in the later bytes
(perhaps it would be possible to redefine variables also in such programs,
but it's not clear).
Option 2. has the following advantage: commercial software would easily
treat this kind of data, since the reduced travel-time display would be
the natural way of presenting these data in a commercial software environment.
The disadvantage is that it needs quite some new treatment in the Synapse
system. In order to use the mentioned advantage for commercial software,
the starting point of every trace should be at the same time relative to
an applied reduction velocity. The digitisation is, however, done in blocks
of c. 48 traces, which after the process start all at the same absolute
time (which is however different for all the blocks of seismograms). So,
one should remove the data which were digitised before the reduced time
and possibly has to add data in some traces to fill them to the desired
reduced initial time.
Option 3. has evidently the advantage that nothing has to be changed
in the original SEG-Y format. The disadvantages are also clear: larger
files for very long distances and additional programming to fill the traces
to start with the first sample not later than 32767 ms after the shot-time.
[1]
Geophysical Institute, Uppsala University
after September 1st 1998: Dept. des Sciences de
la Terre,
Villavägen 16, S-752 36 Uppsala, Sweden
Uni. Paris-Sud, Bat. 504
F-991405 ORSAY Cedex