Apparatus for remote seismic sensing of array signals using side-by-side

Abstract

A remote seismic sensing system is provided which operates as a bipolarized, differential mode, LDI (laser differential interferometry) system to detect electrical signals produced at a remote location. In preferred embodiments, the system is used in seismic surveying to detect array signals. An amplified array signal (the electrical signal is coupled to an array retroreflector apparatus (target). The array signal is obtained from an array of seismic detectors, e.g., geophones or hydrophones. The amplified array signal increases the velocity of a Doppler shifting optical component on the target. The target converts a defused sensing beam into two polarized return sensing signals, both of the return sensing signals having been Doppler shifted by the target to contain frequency components that represent motions, e.g., ground motions and wind motions, that are common to both signals and to contain a difference signal which represents the array signal. The difference signal has a greater deviation from the optical carrier frequency due to the amplification of the signal than the Doppler shifts imposed by the common motions. The return sensing signals are detected by a receiver and combined by electronic and/or by optical heterodyning techniques to cancel any common mode signals on the return sensing signals, thereby leaving a frequency modulated difference signal which, ideally, represents only the frequency modulated array signal. However, the difference signal may also contain turbulent noise due to atmospheric turbulence that causes frequency fluctuations on the sensing beams and the return signals. However, these frequency fluctuations will be much less than the Doppler shifts caused by the amplified signal. The frequency modulated difference signal may then be demodulated to produce a time varying signal which represents the seismic trace of the seismic motions detected by the seismic detectors of the array.