Page 59 - Acquisition and Processing of Marine Seismic Data

50 2. MARINE SEISMIC DATA ACQUISITION

water invasion occurs if the outer skin is rup- sea swell (Fig. 2.10B). This enables us to tow
tured. In addition, gel-filled streamers do not suf- the solid streamers at shallower depths, result-
fer from bulge waves as do fluid-filled streamers. ing in a relatively wider frequency band and
However, they are sensitive to the noise occur- hence much higher-resolution seismic data,
ring along the streamer skin due to the turbulent and acquisition can continue at relatively rough
flow, and the hydrophones are mounted into spe- seas. In solid streamers, generally one strength
cific cells to isolate them from the noise originat- member is located at the center of the cable
ing from the cable motion in the water. and the hydrophone is isolated from strength
The total lifetime of the gel-filled streamers member to reduce the noise. In addition, they
was not very long in the industry, and they have are not affected by the bulge waves and also less
been quickly replaced by solid streamers over sensitive to bird noise during the tow. They
the last decade (Fig. 2.9B). Today, almost all of require less maintenance and they are environ-
the seismic acquisition service companies use mentally friendly since they do not contain
the new technology solid streamers in hydrocar- kerosene.
bon exploration, since they have significant In towed streamer acquisition, downgoing
advantages over both the fluid- and gel-filled energy reflected back from the sea surface is
streamers. The filling fluid is replaced by flexible termed ghost and it occurs both at the source
polymer foam in solid streamers (Fig. 2.10A), and receiver sides (Section 2.2.3). Ghost reflec-
and therefore they are more robust and resistant tions cause constructive and destructive inter-
to external damage; they require smaller mainte- ference at the reflected signals, and results in
nance costs; and they do not present environ- the attenuation of the amplitudes of certain fre-
mental, safety and health issues due to the quency bands of the spectrum. These narrow
rupture of the outer streamer skin. Their buoy- bands in the spectrum are termed ghost notches
ancy is much more consistent between the and they significantly limit the higher frequency
active sections as compared to the fluid-filled end of the amplitude spectrum and severely
streamers. In addition, one of the most impor- reduce the resolution of the data, thus reducing
tant advantages of solid streamers is that they the available bandwidth. Removal of ghost sig-
are less sensitive to the noise originating from nals from the recorded data provides higher

FIG. 2.10 (A) Internal structure of Sercel Sentinel solid streamer, and (B) RMS noise amplitudes for solid and fluid-filled
streamers in moderate sea conditions measured using a fluid-solid combined streamer. After Dowle, R., 2006. Solid streamer
noise reduction principles. SEG Technical Program Expanded Abstracts 2006, pp. 85–89.