Page 58 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 49

FIG. 2.9 Photos of (A) fluid-filled digital, (B) solid digital streamers.

kerosene, to enable cable buoyancy in the seawa- streamer skin. Axial vibrations of the streamer
ter. Although fluid-filled streamers are outdated travel along the cable as pressure fluctuations
today, they are still in use, especially along with resulting in the streamer jacket contracting and
small-scale seismic systems used by academia bulging out. The bulge waves induce noise into
and with the single-channel acquisitions for site the hydrophones of the fluid-filled streamers.
surveys. This kind of streamers suffers from Moreover, these streamers are also quite sen-
several significant issues. One of the most sitive to the ripples at the surface of the ocean.
important problems with the fluid-filled High-amplitude acoustic noise at the low-
streamers is safety, since the kerosene is toxic frequency band originating from the ocean sur-
and easily flammable, which may cause envi- face ripples radiates along the fluid filling the
ronmental hazards and safety or health issues streamer, which ultimately means that the
onboard in case of possible damage in the jacket fluid-filled streamers must be towed at relatively
of the streamer encountered during the tow and higher depths to avoid the ocean surface noise.
deployment. Furthermore, such damage to the This situation, however, adversely affects the
skin results in salty seawater intrusion into the seismicdataresolution,resultinginthecollection
housing, which may result in serious issues with of seismic data with much narrower frequency
the electronic components of the streamer and band, since the depth of the streamer is an impor-
typically results in a spike-like noise appearing tant parameter directly affecting the resolution
as extreme amplitude bursts distributed ran- of the seismic data due to the ghost signal inter-
domly along the channels within the damaged ference (Section 2.5.1.2).
streamer sections. Therefore, fluid-filled An approach to overcoming the aforemen-
streamers require frequent maintenance, which tioned shortcomings of the fluid-filled streamers
may ultimately cause downtimes during the is to use a soft and flexible gel material produced
acquisition. from polyurethane to fill in the streamer housing.
Fluid-filled streamers also suffer from noise Gel-filled streamers have several advantages as
originated by bulge waves, which are formed compared to fluid-filled streamers. First of all,
by the interactions among the members in the they are more durable than fluid-filled streamers
streamer housing, such as kerosene itself, wiring, and they do not induce environmental, safety
buoyancy spacers, or strength members, and the and health issues. Since the gel is not liquid, no