Patent ID: 7755971

Claim:
A seismic sensing system for sensing undersea seismic vibrations including: an undersea optical fibre cable on a seabed comprising an optical fibre providing an optical communications link; at least one modulator arrangement comprising: an undersea vibration sensor in contact with the sea-bed so as to sense seismic vibrations and arranged to produce an electrical sensor signal in response to detecting a seismic vibration from which the presence of a seismic event can be detected; and an undersea transmitter unit electronically coupled to the vibration sensor and arranged to transmit acoustic waves through sea-water, the transmitter including: an electronic circuit for generating an acoustic signal in response to receiving the electrical sensor signal, and a transducer element arranged to transmit said acoustic signal as an acoustic wave through the sea-water to couple to said optical fibre cable, wherein, the transmitter unit is arranged to be retained in a raised position relative to the seabed by a flotation arrangement so as to transmit said acoustic wave in an angled downward direction through the seawater towards said optical fibre cable; and wherein said acoustic wave modulates one or both of a pair of retarded copies of an optical signal applied to said optical link, one copy being retarded in a forward direction and the other copy being retarded in a reverse direction by a time-delay relative to said optical signal, from which modulation the presence of a seismic vibration can be inferred, wherein the time delay is related to the frequency response of the optical fibre cable to phase modulation caused by said acoustic signal coupling to said optical fibre cable.