Page 98 - Acquisition and Processing of Marine Seismic Data

2.3 3D MARINE SEISMIC ACQUISITION 89

FIG. 2.45 Different external devices attached to the streamers for acoustic networking. (A) Sips 2 XSRS (Sonardyne), (B)
CMX (ION Geophysical), (C) acoustic bird (ION Geophysical), and (D) velocimeter model 7000 (ION Geophysical).

sections of the spreads, and can both transmit and outer streamers as well as front and tail sec-
and receive acoustic pulses for range determina- tions may expose different sound velocities. The
tion. Fig. 2.45 shows the most common acoustic velocity information is obtained in real-time
positioning devices on the market today. SIPS 2 during the survey by velocimeters or speed sen-
XSRS (Sonardyne) and DigiRANGE-II CMX sors mounted on the front and tail sections of
(ION Geophysical) are the most widely used one or two selected streamers in the spread.
positioning units. Each XSRS has four receive Fig. 2.45D shows the model 7000 velocimeter
channels and one transmit channel, and as an of ION Geophysical. Sips 2 XSRS transceivers
example configuration, a typical layout of 6 and Nautilus devices also offer an option to
streamers of 8 km long requires 42 units to include sound velocity sensors.
derive the acoustic ranging solution. In addition All acoustic positioning devices attached to
to the aforementioned systems, Western Geco the streamers, vessel’s hull, the gun strings,
has developed a digital acoustic positioning sys- and the front and tail buoys form acoustic nodes.
tem known as IRMA (Intrinsic Ranging by Mod- These nodes compose an acoustic network
ulated Acoustics) to work with their streamer among the streamers and other in-sea equip-
steering system Q-Fin for a fully braced acoustic ment, which enable us to measure the ranges.
solution. In addition, the Nautilus system devel- The acousticnetworkof a3Dspreadmay bequite
oped by Sercel also has its own acoustic posi- complex consisting of several hundred nodes.
tioning module to operate with the Sercel’s Fig. 2.46 illustrates an example 3D acoustic net-
Sentinel streamer. ION Geophysical produces work and the ranges between the nodes for a four
a device called the acoustic bird which combines streamer case. The network consists of three sub-
a conventional depth leveler bird and a CMX networks of front-, mid- and tail-net. In addition
device to operate with ION’s DigiFIN lateral to the acoustic ranging units on the streamer,
streamer steering device (Fig. 2.45C). each gun string has also its own acoustic tran-
Since all the communications between the sponder to incorporate the ranging network.
acoustic positioning devices are performed by The main purpose is to find out the positions of
high-frequency sound pulses, it is necessary to each individual node within the network by
know the sound velocity in the water column incorporating the specific nodes whose positions
to precisely convert the arrival times of the are already known by some external measure-
acoustic pulses to distance ranges. Since the seis- ments, such as rGPS observations. These specific
mic spread has very large dimensions, the inner nodes are termed fixed nodes.