Shell Oil Comapny

Albert J Berni

Vitold R Kruka

Edward R Cadena, Jr

Lawrence D Park, Jr

John F Parrish

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.

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

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.

Array retroreflector apparatus for remote seismic sensing

A remote sensing system is provided which operates as a bipolarized, differential mode, LDI (laser differential interferometry) system to detect motion at a remote location. The system is adapted to reduce the effects of turbulence induced frequency fluctuations by using a 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 detect seismic motions. The retroreflector apparatus is coupled to ground motions at a selected location where it separates the sensing beam into two polarized sensing signals, frequency modulates one of the polarized sensing signals to contain frequency components that represent selected ground motions, and combines the polarized sensing signals into the return beam. The return beam is detected. The return sensing signals are separated from the beam and combined by electronic and/or by optical heterodyning techniques to cancel common mode signals (including turbulence induced noise) on the sensing and return beam, thereby leaving a difference signal which represents the selected ground motions. In one preferred embodiment, the target is illuminated with a horizontal sensing beam; a polarizing beamsplitter splits the sensing beam into a first and a second polarized sensing signal. The beamsplitter Doppler shifts the first signal and also deflects the first signal to a vertical path where it reflects from an inertially isolated retroreflector. The second signal travels along a horizontal-inline path where it is reflected and Doppler shifted by a seismically coupled retroreflector. The Doppler shifted signals are combined in the beamsplitter to form the return signal. The first signal contains Doppler shifted frequency components which represent vertical and horizontal-inline ground motions. The second return sensing signal contains Doppler shifted frequency components which represent horizontal-inline ground motions. The common mode signals (the horizontal-inline frequency components and turbulence induced noise) are canceled in the receiver, leaving a difference signal which represents vertical motions.

Retroreflector apparatus for remote seismic sensing

Methods and apparatus are provided for remotely sensing motions of the earth from the Doppler shift of reflected electromagnetic waves. The apparatus is based on a heterodyne continuous wave or pulsed laser system. The airborne laser system described herein is intended to remotely sense seismic motion. Motion of the earth's surface, when coupled to a reflector, causes a continuously shifting frequency (Doppler) of the reflected sensing laser beam, the Doppler frequency being proportional to the particle velocity of the ground. The method uses electromagnetic waves reflected from reflectors, some of which are coupled to the earth's surface. Similarly, 'inertial' reflective surfaces (i.e., vibrationally isolated from any earth motions) are also located at the surface of the earth and employed by the apparatus and methods of the present invention. Both an inertial reflector and an earth-coupled reflector are positioned at each location for which seismic signals are desired.

Remote seismic sensing

Methods and apparatus are provided for sensing motions of the earth caused by seismic waves from the doppler shift of reflected electromagnetic waves. The apparatus is based on a heterodyne continuous wave laser system.

A marine seismic cable comprising a core, a body of flotation material positioned around the core, an acoustic energy transducer positioned proximate the core, and a body of acoustically transparent material positioned around the acoustic energy transducer.

Marine streamer cable

A method for eliminating ghosts, or reflections from the air/water interface of a body of water, from seismic signals detected at a predetermined depth is provided. The method employs both a pressure sensor and a motion sensor. A seismic signal and its corresponding ghost signal detected by a pressure sensor is filtered as a function of depth of the sensor to provide a preselected amplitude and zero phase shifted band-limited spike signal at an arrival time corresponding to the arrival time midway between the seismic and ghost arrival times. Similarly, a seismic and ghost signal from a motion sensor is filtered as a function of depth of the sensor to provide a band-limited spike having an arrival time midway between the seismic and ghost arrival times. The filtered motion and pressure spikes are added together in proportion to their respective signal-to-noise ratios. The proportionally added signal is a ghost free seismic signal having a maximum signal-to-noise ratio for each frequency component and may be employed in further seismic processing steps.

Marine seismic system

A marine seismic streamer cable having both pressure responsive and acceleration responsive transducers disposed therein. The accelerometer responds to acceleration along one axis regardless of its orientation about the other axis. The accelerometer is mounted in the streamer cable and responds to accelerations produced by a seismic source but is insulated from the noise and motion produced as the streamer cable is towed through the water.

Low noise mounting for accelerometer used in marine cable

A method of producing a seismic signal indicative of the substrata beneath a body of water comprising the steps of: supplying acoustic energy to the body of water; sensing at a predetermined depth the pressure wave reflected from the substrata beneath the body of water and the secondary pressure wave caused by a secondary reflection of the pressure wave from the air-water interface and generating a first signal indicative thereof; sensing at the predetermined depth the particle velocity of the water accompanying the pressure wave reflected from the substrata and the secondary pressure wave and generating a second signal indicative thereof; filtering the second signal such that portions of the second signal that have a frequency that is less than a first frequency are attenuated by the filtering to generate a third signal, the first frequency being less than a second frequency which is equal to the wave propagation velocity of the body of water divided by two times the predetermined depth; and combining the first and third signals to produce a signal indicative of the substrata beneath the body of water.

An accelerometer for measuring the vertical component of acceleration independent of the accelerometer orientation or the pressure. The accelerometer is particularly adapted for use in marine surveys in combination with a hydrophone to cancel the surface reflected wave.

Vertical component accelerometer

A hydrophone mounting in a cavity of the hydrophone cable and a method of mounting the hydrophone. The cavity which is provided in the hydrophone cable may be constructed so that it is free-flooding, that is, the construction allows the liquid in which the hydrophone cable is submerged to completely surround the hydrophone. The cavity serves to insulate the hydrophone from movements and perturbations to the cable so that the hydrophone response is not distorted by these sources of noise.

Free flooding hydrophone mounting

A seismic amplifier for use in marine seismic survey work involving the transmission of signals from remote signalling stations over cable pairs to signal processing equipment. These signalling stations are comprised of capacitive transducers. The seismic amplifier of the surveying system includes operational amplifiers electrically interconnected with a feedback circuit comprised of a balanced resistive-reactive network which is a lumped parameter equivalent circuit of the transmission line and capacitive transducer. The lumped parameter equivalent circuit used for feedback provides a flat, zero-phase shift overall response over the cable pair from the transducer to the output of a differential amplifier stage which is connected to the operational amplifier outputs to improve common mode rejection.

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Houston, TX, United States of America

Albert J Berni