Page 113 - Acquisition and Processing of Marine Seismic Data

104 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.58 Photos of different ocean bottom cable sensors. (A) SeaRay (Sercel), (B) 4C sensor module of GeoRes (OYO), (C)
shallow, and (D) deep water OBC sensors of GeoRes (OYO).

gun array generates the seismic signal on the sea that does not require a separate recording vessel,
surface. The OBC survey can also be applied in which significantly reduces the operating cost.
shallow water areas, lakes and even in rivers. The GeoRes system employs OMNI series geo-
There are several different OBC systems avail- phones, which are also insensitive to tilt angle.
able worldwide today (Fig. 2.58): Its 4C sensors include an internal inclinometer
that measures the vertical orientation of all three
• Q-Seabed (Western Geco)
orthogonal axes of the geophones. The cable
• SeaRay (Sercel)
length of GeoRes system is typically 3.6 km with
• VectorSeis Ocean (ION Geophysical)
a custom sensor spacing of 25–200 m. Fig. 2.59
• GeoRes (OYO)
compares the seismic sections from a conven-
The Q-Seabed system employs a maximum of tional streamer and OBC P wave data from Val-
four cables in water depths up to 1500 m. Each hall Field in the North Sea.
cable has a maximum active length of 30 km, Some production fields have been equipped
with 25-m sensor intervals. The Q-Seabed sys- with permanent installations of cables with fiber
tem also uses an acoustic ranging system termed optic data transmission technology for time-
IRMA, similar to that used for the Q-marine 3D lapse monitoring of the reservoir for hydrocar-
towed streamer acquisition of Western Geco. bon recovery and injection processes in order
The IRMA system in connection with the tran- to optimize the production at periodic intervals,
sponders on the hull of the vessel provides pre- such as a couple of months. Such permanent
cise cable positioning. SeaRay is Sercel’s seismic installations are for time-lapse or 4D
omnidirectional 3C ocean bottom cable system seismic surveys and are termed Life of Field
capable of recording seismic data up to 500-m Seismic (LoFS), which significantly reduces the
water depths. It has a maximum of 37-km cable repeating survey costs as compared to conven-
length capability with 3C sensors, which are tional time-lapse seismics. Although the source
insensitive to tilting when coupled on the sea and receiver positions cannot be repeated pre-
floor. VectorSeis Ocean is ION’s redeployable cisely during each consecutive survey for con-
OBC acquisition system, which utilizes three ventional 4D surveys, permanently installed
identical microelectromechanical systems cables provide the highest repeatability, since
(MEMS) along with a hydrophone to record the receivers are in fixed positions at the sea
4C data. It can be operated at water depths up floor, and hence provide superior image quality
to 2000 m. It has a buoy-based recording system to evaluate the small changes in the reservoir