Page 54 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 45

form is known as acceleration canceling hydro- and vibrations from the tail buoys. A stretch sec-
phone, and it ultimately cancels out the streamer tion is basically very similar to an active
acceleration when the voltage wirings are con- streamer section, but the strength members of
nected in parallel. The pressure waves from the stretch sections are generally constructed
reflected signals, however, generate a positive from a more elastic material. Even though they
voltage on the surfaces of both elements, and are quite efficient to suppress the aforemen-
hence strengthen each other. Fig. 2.5C shows a tioned noise types, especially those lower than
hydrophone placed in a fluid-filled streamer. 8 Hz, they may exhibit issues regarding environ-
Several hydrophones are located at regular mental and safety situations since they are pro-
intervals along the length of a long tow cable duced as fluid-filled sections. In addition to the
termed a streamer. The diameter of modern active and stretch sections, modern streamer
streamers is approximately 2 in., and their total layouts contain several other components such
length ranges from a few meters for single- as lead-in cables, digitizing modules, intercon-
channel acquisition to 12 km for conventional nects, tail buoy power units, auxiliary channel
hydrocarbon exploration. Lengths of the box, etc. They also include several external
streamers have gradually increased over time devices mostly for positioning purposes, such
depending on the type and depth of the subsur- as depth controller birds, streamer steering
face target. Considering the total length of the units, acoustic rangers, etc. Power required for
streamers for one single spread, a typical 3D ves- the operation of these systems is provided by
sel can tow more than 100 km of active streamer the wirings inside of the streamer or the batteries
section for a single 3D survey. included in the external devices.
Streamers are manufactured as appendable The manufacturing of streamers and hydro-
bidirectional sections of 75, 100, or 150 m, phones is quite complex work, and there are sev-
known as active sections so that repair, mainte- eral innovative companies that produce low-
nance and replacement of a damaged streamer noise streamers and hydrophones worldwide
section can easily be managed. Several active today:
sections are conjoined end-to-end to form a com- • Sercel (1C and 3C Digital solid streamers)
plete seismic streamer of many kilometers. Dur- • ION Geophysical (Digital solid streamers)
ing data recording, streamers are towed at a • HydroScience Technologies Inc. (Digital solid
predetermined depth in the water, typically at streamers)
7–15 m below the sea surface for conventional
• Geometrics (Analog fluid-filled/solid
acquisition for hydrocarbons, and at 3–5 m for
streamers)
high-resolution surveys. Once deployed, it is
• Teledyne (Hydrophones, digital/analog
quite common for the streamers to remain in
fluid-filled streamers)
the water for several weeks to several months,
depending on the size of the project. Fig. 2.6 shows configurations of the most
Streamers end up with dead sections, gener- widely used digital streamers in the market:
ally 50–100 m long, termed stretch sections, the Sentinel streamer of Sercel, DigiStreamer of
which have the same structure as the active sec- ION Geophysical, and SSCT streamer of Hydro-
tions but host no hydrophones. Stretch sections Science Technologies Inc.
isolate the active streamer sections from the Fig. 2.7 shows a sketch of the contents and a
strum noise originating from the movement of photo of a typical solid streamer section. An
the water around the lead-in cables, mechanical active section consists of hydrophones, an exter-
cable noise from the fluctuations in tow speed, nal jacket, wiring and connectors, buoyancy
operational noise from the vessel machinery, spacers, bird communication coils commonly