Array retroreflector apparatus for remote seismic sensing

Abstract

A remote sensing system is provided which operates as a bipolarized, differential mode, LDI (laser differential interferometry) system to detect an electrical signal produced at a remote location. The system is adapted to reduce the effects of turbulence induced frequency fluctuations by using an array retroreflector apparatus which converts a single sensing beam into a single return beam having two, overlapping, transversely polarized sensing signals; thus, turbulence induced noise will be common mode to both return signals and cancel at the receiver. In preferred embodiments, the system is used to remotely detect an amplified array signal from a plurality of seismic detectors (e.g., geophones or hydrophones) connected to form an array at the remote location. The target is coupled to common motions at the selected location and the amplified array signal is coupled to an optical component within the target. The target converts the sensing beam into a return beam by separating from the sensing beam two transversely polarized sensing signals, by Doppler shifting the sensing signals to have frequency components that represent common motions coupled to the array retroreflector and to contain between the signals a difference signal which represents the array signal. The target also combines the two sensing signals into a single return beam having two transversely polarized return sensing signals. The return beam is detected. The return sensing signals are separated from the return beam and combined by electronic and/or by optical heterodyning techniques to cancel any common mode signals on the sensing and return beam, thereby leaving a frequency modulated difference signal which represents the frequency modulated array signal. The frequency modulated difference signal may then be demodulated to produce a time varying signal which represents the seismic signal of the seismic motions detected by the seismic detectors.