Page 135 - Acquisition and Processing of Marine Seismic Data

126 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.79 (A) Run-in and (B) run-out distances used to keep the streamers straight before and after each line change. Shoot-
ing starts at run-in distance before, and stops at run-out distance after the seismic profile to collect full-fold data along the
actual preplot line.

3
taking the acquisition fold from its minimum array of 1000 in generates only 10 times higher
value in the fold taper zones to its nominal of signal amplitude than an air gun of only 1 in 3
full-fold value in the shooting direction, and volume. Therefore, the most effective way to
generally equals to the streamer length. Simi- increase the signal amplitude, and hence the
larly, run-out is defined as the distance required overall signal penetration, is to use several air
for taking the acquisition fold from its nominal guns simultaneously instead of increasing the
value to minimum in the fold taper zone, and volume of each single air gun.
generally equals to one half of the streamer Air gun array design is important to obtain a
length. proper seismic signal of high amplitude and
peak-to-bubble ratio (PBR) by suitably design-
2.5.1.10 Air Gun and Array Parameters ing the volumes and positions of each individual
Effect of the air gun array parameters on the air gun or gun cluster within the array
quality of the acquired data is explained in (Section 2.2.6). PBR increases as the volume of
Section 2.2 in detail. The volume of each individ- the gun and air pressure increase. Signal ampli-
ual air gun is inversely proportional to the fre- tude is directly proportional to the air pressure
quency content of the generated signal. Small- applied to the guns, and 2000 100 psi air pres-
size air guns produce higher frequency seismic sure is standard for the seismic acquisition
signals, which provide higher resolution, but industry today. Capacity of the air compressors
lower penetration. As a general rule, the resul- is also important. The compressors should
tant seismic signal amplitude is approximately deliver compressed air to the gun array(s) of
proportional to the cubic root of the total volume specified total volume at time periods deter-
of an air gun array, which means that an air gun mined by the shot interval and vessel speed