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) |
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. |
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