Page 94 - Acquisition and Processing of Marine Seismic Data

2.3 3D MARINE SEISMIC ACQUISITION 85

FIG. 2.41 Different 3D streamer steering devices. (A) Q-Fin (Schlumberger), (B) Nautilus (Sercel), (C) eBird (Kongsberg),
and (D) DigiFIN (ION Geophysical).

surface currents, although it perfectly preserves fan mode acquisition (Section 2.3.7) by auto-
the separation between the streamers. matic steering using sophisticated computer
Today, there are four different 3D streamer algorithms. Steering devices also provide more
steeringdevicesavailableinthemarket (Fig.2.41): reliable 4D data acquisition with a better repli-
cating streamer shape for subsequent surveys
• Q-fin (Schlumberger)
for reservoir monitoring.
• Nautilus (Sercel)
• eBird (Kongsberg)
• DigiFIN (ION Geophysical) 2.3.3 Streamer Layout
Q-Fin, Nautilus and eBird devices allow In a typical 3D seismic acquisition layout,
streamer steering in addition to depth control, there are 8–16 streamers, two paravanes, 8–16
while DigiFIN provides only steering and the tail and front buoys, and two identical gun
3D spreads that use DigiFINs also need regular arrays generally consisting of three strings.
depth leveler birds to maintain the streamer The distance between the streamers is not more
depth. Nautilus is a 3D steering device specifi- than 100 m to achieve the required crossline
cally designed and developed to use with the sampling. An example 3D tow layout with eight
Sercel’s Sentinel streamers. Nautilus units do streamers and two gun arrays is shown in
not require an internal battery pack to operate, Fig. 2.42. In this example, the vessel tows
since the power is supplied by the wiring within 8 streamers, each with 8000 m overall length.
the streamer, which means fewer workboat The separation between the streamers and gun
operations to replace the internal batteries of arrays is 100 and 50 m, respectively, which pro-
the steering devices. vides 25 m crossline bin size. Minimum offset is
Constant streamer separation during the 300 m and the total crossline coverage is 700 m.
deployment and retrieval of the streamers, In order to accurately calculate the relative and
recording and line changes, even in the rough actual geographical positions of each in-sea
sea states and strong currents, is achieved by component, some of the surficial equipment
deploying these devices at regular intervals such as gun floats, front buoys and tail buoys
along the streamers. They also provide improve- is equipped with rGPS transponders.
ment in crossline sampling, a better subsurface In 3D seismic acquisition, it is crucial to main-
coverage, and more uniform offset and azimuth tain the geometry of the in-sea equipment in the
distribution in the crossline direction, reducing predetermined offset positions relative to each
the infill acquisition. This technology also allows other. It is also quite important to precisely
us to apply different streamer layouts, such as determine the positions of each source array as