Page 66 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 57

2.1.5 Streamer Depth Controllers of streamer depth control units on the market
(Fig. 2.15A and B):
In practice, the streamer is towed at a depth
range between 3 and 10 m to prevent the noise • DigiBird (ION Geophysical)
interference from the sea surface. During the • Navigator bird (OYO Geospace)
data acquisition, depth of the streamer can be
Birds have no electrical connection with the
controlled by the movements of the wings of
streamer internal components and the power
specific external devices called birds attached required for bird units is provided by their own
to the streamer cable. Depth leveler birds are internal battery pack. The communication with
mounted at regular intervals along the streamer the birds is maintained via electromagnetic com-
cable commonly with a maximum separation of munication coils located within each active and
300 m. Only streamer depth control units are stretch section of the streamer (Fig. 2.15C), where
required for 2D seismics. In 3D surveys, how- the birds are attached as close as possible using
ever, streamer positioning or steering equip- specific collar pairs. All communications between
ment used to maintain the depth and lateral
the birds and bird control units in the instrument
position of the streamers are also required
room are handled electromagnetically via com-
(Section 2.3.2). Currently, there are two types
municationcoilswithinthestreamerandthebirds.

FIG. 2.15 (A) ION 5011 CompassBird, (B) OYO Navigator bird and retriever, (C) electromagnetic communication coil
within the fluid-filled streamer, (D) ION 5010 DigiBird attached to the streamer. (E) Compass birds are used to monitor
the azimuth angle (α) of the streamer during the survey to calculate the possible cable feathering (ϕ).