The solution–Fibre Optic Seismic
Fibre optic sensors have recently begun making a significant impact in the E&P field. This has mainly been in the area of pressure and temperature sensing, normally using either Bragg grating sensors or distributed techniques such as distributed fibre optic temperature-sensing technique. These approaches have proved very successful for these applications, but do not offer the performance required for seismic applications. More recently, downwell seismic sensors have also been successfully demonstrated by Weatherford using higher sensitivity interferometric techniques, although on quite a small scale (18 channels). None of the technologies so far demonstrated meet all the requirements for LOFS, which include high sensitivity, high channel count, high vector fidelity and low cost.
QinetiQ has been developing a solution that is directly aimed at the requirements for permanent seabed systems. The technical approach adopted, based on interferometric sensing, addresses all the requirements outlined above and represents a major step forward in the development of full-scale fibre optic seismic systems.
This approach has been derived from QinetiQ’s 20 years of experience in the field of high-sensitivity fibre optic sensing systems primarily for the defence market. This included the world’s first deployment of a seabed fibre optic hydrophone array in 1986, which demonstrated 11 years of successful underwater operation. In 2002, QinetiQ deployed a 96-channel seabed fibre optic hydrophone array, which was the largest such system at that time.
To convert this technology to the requirements of the seismic world, QinetiQ has been working with Input/Output, Inc., one of the world’s leading suppliers of seismic acquisition equipment.
The QinetiQ solution uses accelerometers and hydrophones based on fibre coils that are typically 40 metres in length. After detailed consideration of various systems architectures, QinetiQ has adopted a powerful system architecture based on the combination of time and wavelength multiplexing. This is believed to give the best combination of cost, performance and multiplexing ability. The benefits of the QinetiQ solution include the following:
- the ability to passively combine up to 500 sensors onto a single fibre pair and interrogate sensors over distances of > 20km, without any underwater electronics or electrical power;
- reduced size and weight of cables and sensor packages;
- increased reliability and reduced susceptibility to leak-induced failure; and
- redistribution of cost/complexity from the underwater array to a platform-based interrogator unit.
QinetiQ’s Solution
The QinetiQ solution comprises a central interrogation unit containing all of the optics and electronics and the underwater array. The array comprises passive fibre optic cable linking together the 4-C stations. QinetiQ has now developed all the sub-components of this system. The emphasis has been on developing a small diameter, ultralightweight seabed system that is electrically completely passive and contains a minimum number of components
Sensors
The accelerometers are cylindrical with a diameter of 30mm and a flat frequency response up to 500Hz. They are based on a 40-metre fibre coil and have a cross-axis rejection of < – 30 dB.
The hydrophones are also based on a 40-metre coil, in this case wound around a plastic tube with a diameter of 35mm and a length of ~ 100mm. The hydrophone has a flat response to greater than 1kHz, has currently been tested to a water depth of > 300 metres but is designed to operate down to 3,000 metres.
Figure 1: Fully Fibre Optic 4-C Package
4-C package
A lightweight, miniature 4-C package has been designed to accommodate the accelerometers (with the hydrophone attached to the outside). This package, shown in Figure 1, has an external diameter of 70mm and a length of around 240mm, and takes up less than a quarter the volume of most existing packages.
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Reservoir Engineering
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