Page 57 - Acquisition and Processing of Marine Seismic Data

48 2. MARINE SEISMIC DATA ACQUISITION

streamers. Modern seismic systems have 480–960 no need to place a separate transmission line
active channels per streamer. The center of each within the streamer for each recording channel
hydrophone group is appointed as the exact loca- as is the case for analog streamers. This allows
tion of that specific recording channel, and the us to manufacture streamers with significantly
term group interval, instead of channel interval, reduced diameters, and it has become possible
denotes the distance between channels in marine to spool longer streamers onto the reels of the
seismic acquisition. Group intervals of modern vessels. Increasing the overall length of a digital
streamers are typically 3.125 or 6.25 m for high- streamer is quite easy: It is generally sufficient to
resolution surveys, and it is generally 12.5 m in append new active sections and digitizers to the
case of a conventional 2D or 3D seismic survey tail of an existing spread, in some cases, along
for hydrocarbon exploration. Thus, a streamer with an update in the recording software. Fur-
section of 100 m consists of 16 or 8 channels for thermore, digital streamers allow us to redesign
6.25 and 12.5 m group intervals, respectively. In the group interval of the streamer according to
practice, smaller group intervals are preferred the requirements of the survey by an option
to keep the horizontal resolution higher and to termed trace summing. With this option, it is
prevent the spatial aliasing. possible to increase the group interval of the dig-
Streamers can be grouped as analog and dig- ital streamer by a suitable channel setting in the
ital types. Analog streamers transmit the seismic recording software without physically modify-
amplitudes as electric signals to the recorders, ing the existing streamer configuration. Trace
and the data is digitized by a 24-bit analog-to- summing can be performed in the order of 1:2,
digital (A/D) converter in the recording equip- 1:3, or 1:4. For the 1:2 case for instance, two
ment. Today, small-size analog streamers are neighboring channels are grouped to form a sin-
still in common use to acquire commonly single gle combined channel simply by adding the out-
trace high-resolution seismic data for site survey puts of the adjacent channels. Similarly, in order
purposes. Modern streamers used for explora- to form 1:3 and 1:4 order trace summing, three
tion and reservoir development purposes by and four neighboring channels are grouped into
the hydrocarbon industry and academia are dig- a single channel, respectively. Consequently,
ital streamers in which the perceived signal is group interval and the number of channels
digitized within the streamer and transmitted involved in the streamer change accordingly.
to the recorder in digital form. The digitizing For instance, in the case of a 960-channel digital
process is maintained by digitizers or modules streamer with a 6.25 m group interval, the num-
mounted between the active streamer sections ber of total recording traces is reduced to 480,
at regular intervals, and there is no need for an and the group interval is increased to 12.5 m
additional A/D converter at the recorder. when a trace summing of 1:2 is used.
Depending on the streamer configuration used, Today, three types of streamers are used in
one digitizer per 16–60 channels is located on the seismic exploration:
streamer. These are placed between two active
• Fluid-filled streamers
sections so that they digitize the analog signals
• Gel-filled streamers
from the hydrophones of both sections, and set
• Solid streamers
up a two-way telemetry along the streamer by
transmitting the digitized signal to the recorder. Fluid-filled streamers are considered to be the
Digital streamers have several advantages as traditional seismic streamers, having been used
compared to the analog ones. The diameter of in the seismic industry for more than 50 years
digital streamers is much smaller, since, in trans- (Fig. 2.9A). These streamers are filled with a non-
ferring the digital data to the recorder, there is conductive buoyant fluid, generally low-density