PGS has been developing fiber-optic sensor and telemetry technology since 1997. A major application will be to permanently install reservoir monitoring (Figure 1). Such a system will ideally operate without physical intervention over a period as long as 20 years, unaffected

Figure 1. Schematic illustration of a permanent seafloor monitoring installation. The PGS solution uses all-optical sensor stations. (Images courtesy of PGS)
by corrosion, electrical leakage or sensor degeneration. Optical seismic technology presents the perfect match to such requirements. An optical system is entirely passive (no electronics) at the “wet end,” using Michelsen micro-interferometers to convert strain from the seismic wavefield into a phase change of the carrier light signal.

Dense wavelength division multiplexing (DWDM) is utilized in the PGS telemetry scheme to optically power the seismic sensors. The net benefit is that a very small number of carrier fibers are used to take a unique light signal to several thousand channels. At each sensor the seismic wavefield applies a strain to an interferometer, thus causing a phase shift in the modulated light signal. The degree of phase shift in units of radians is linearly proportional to the amount of strain. Both the PGS hydrophone and three-axis optical accelerometers are qualified to operate in 9,800 ft (3,000 m) depths without degradation in performance. The optical system has a dynamic range in excess of 140 dB and with a very low noise floor.

Extensive field-testing over several years demonstrates that the optical seismic data is identical to conventional seismic data and can in fact be recorded with better dynamic
Figure 2. Working 4-C optical sensor station, as showcased by PGS at SEG.
range and sensitivity given appropriate system architecture. PGS showcased its latest four-component optical seafloor sensor at the 2006 meeting of the Society of Exploration Geophysicists (SEG) in New Orleans (Figure 2). This particular system was built to record frequencies as high as 5,000 Hz. Commercialization plans for PGS are to begin delivering industrial systems in 2007.

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