The use of fibre optic technology offers the potential for a low cost, highly reliable system with data quality that is an attractive alternative to repeated surveys using surface seismic or retrievable systems.
Repeatability from survey to survey will aid the operating oil companies in modelling the subsurface and the effect of production, thus maximising the oil recovery rate from the reservoirs. The cost and complexity of repeated surveys are significantly reduced as a single source vessel is required for the operation. Frequent surveys will become more cost-effective which will further improve the reservoir modelling.
The marine geophysical company Wavefield Inseis ASA is now introducing a 4C system for permanent installation on the sea floor – the Optowave OBC System.
The Optowave system is based on fibre optic technology developed by Optoplan AS in Trondheim, a company that has pioneered optical fibre sensing within oil and gas applications over the past 20 years.
Optoplan was a subsidiary of Weatherford International Ltd from 2002 but in 2007, Wavefield Inseis entered into an agreement with Weatherford to commercialise the Optowave OBC system. This agreement involved a 35percent partial-asset purchase of Optoplan with an option for Wavefield Inseis to purchase the remainder of Optoplan upon completion of the commercialisation period. This option was exercised in October this year and now Optoplan is a 100percent subsidiary of Wavefield Inseis.
The Optowave system is comprised of 4C seismic stations on the sea floor, including three accelerometers and a hydrophone, lead-in cables and an instrumentation system placed on the surface, on platforms or production vessels. An advanced multiplexing technique allows thousands of sensors to be interrogated by the instrumentation through a subsea optical fibre lead-in cable. The subsea components of the system are completely passive with no electronic components.

The Optowave seismic array provides
high-precision seismic measurements, much lower power consumption, improved system reliability and lower unit cost compared to other technologies. Fig.1 shows the system as envisaged, installed at the seafloor and connected back to the surface vessel.
This ocean bottom seismic system has been developed in a co-operation between Statoil and Optoplan. The motivation behind this co-operation was to combine the knowledge and experience of Statoil, as a leader within time-lapse seismic (4D) and multi-component ocean bottom seismic (4COBS), with the knowledge and experience of Optoplan in the field of optical sensing within the oil and gas industry.
Two significant field tests of the technology were conducted during 2006. The first test was carried out in Trondheim harbour in Norway during January 2006 where fibre-optic
4C cables were deployed at 40m water depth and buried at a depth of one metre into the seafloor sediments. The test also included the installation of an electrical MEMS cable for comparison. The test verified excellent performance of the optical system and repeated surveys have shown the technology to be very robust and with very good vector fidelity. Fig.2 shows comparable traces of the hydrophone, in-line, cross-line and vertical components from both the fibre optic and electrical stations. A high degree of similarity between the two different systems proves that the fibre optic sensors are able to provide the data quality required in permanent seismic applications.
The second field test was carried out November 2006 at Tjeldbergodden (Fig.3), at the west coast of Norway, where Statoil operates Europe’s largest methanol plant. Here a
fibre-optic cable was trenched into the seafloor at a water depth of approximately 300m. The main motivation of this test was to qualify installation procedures required for upcoming offshore installations. The installation went smoothly and following seismic surveys verified the excellent performance of the fibre optic system.
The Optowave OBC system signals the next phase in Wavefield Inseis’ evolution as a full service seismic company. The use of a permanent multi-component OBC system is an alternative to other marine seismic acquisition techniques for acquiring 4D data.l

Enter 16 or at www.engineerlive.com/ihss

Rick Donoghue is with Wavefield Inseis ASA, Lysaker, Norway.
www.wavefield-inseis.com