Abstract: Methods, structures, devices and systems are disclosed for implementing optical seismometers that detect seismic information based on optical interferometry. In one aspect, a device includes a first retroreflector attached to a mass of a seismometer, a second retroreflector attached to a member of a frame of the seismometer, the frame structured to suspend the mass, and optical components attached to the member of the frame and configured with the first and second retroreflectors to form an interferometer, in which a change in position of the mass is identified by detecting by a change in an optical path of a light beam generated by a light energy source transmitted to the interferometer.

Abstract: Methods, structures, devices and systems are disclosed for implementing optical seismometers that detect seismic information based on optical interferometry. In one aspect, a device includes a first retroreflector attached to a mass of a seismometer, a second retroreflector attached to a member of a frame of the seismometer, the frame structured to suspend the mass, and optical components attached to the member of the frame and configured with the first and second retroreflectors to form an interferometer, in which a change in position of the mass is identified by detecting by a change in an optical path of a light beam generated by a light energy source transmitted to the interferometer.

Abstract: Techniques and devices for digitally resolving quadrature fringe signals from interferometers such as optical interferometers and interferometer-based sensing devices. In one implementation, two quadrature fringe signals from an interferometer which causes two signals in two signal paths to interfere with each other are sampled to obtain digital data samples from the two quadrature fringe signals. The digital data samples are used to perform a linear least square fitting to establish coefficients for an ellipse traced by the two quadrature fringe signals as a phase difference between the two signal paths changes. A pair of digital data samples are respectively obtained from the two quadrature signals at a given moment and are used to compute a corresponding phase difference between the two signal paths of the interferometer from established coefficients of the ellipse. The coefficient for the ellipse can be updated over time. This digital processing allows for real time processing.

Abstract: Techniques and systems are disclosed for performing a gravity survey near the seafloor. In one aspect, a system includes an autonomous underwater vehicle that includes a sensor system holding area. The system includes a gravity sensor system to fit inside the sensor system holding area of the autonomous underwater vehicle. The gravity sensor system includes a motorized gimbal to provide a leveled sensor platform. Also, the gravity sensor system includes a gravimeter sensor mounted onto the motorized gimbal to measure gravity data. Further, the payload includes a motion sensor mounted onto the motorized gimbal to measure motion data associated with movements of the autonomous underwater vehicle.

Abstract: Techniques and devices for digitally resolving quadrature fringe signals from interferometers such as optical interferometers and interferometer-based sensing devices. In one implementation, two quadrature fringe signals from an interferometer which causes two signals in two signal paths to interfere with each other are sampled to obtain digital data samples from the two quadrature fringe signals. The digital data samples are used to perform a linear least square fitting to establish coefficients for an ellipse traced by the two quadrature fringe signals as a phase difference between the two signal paths changes. A pair of digital data samples are respectively obtained from the two quadrature signals at a given moment and are used to compute a corresponding phase difference between the two signal paths of the interferometer from established coefficients of the ellipse. The coefficient for the ellipse can be updated over time. This digital processing allows for real time processing.

Abstract: The invention is a method for monitoring subsidence of the sea-bed (14) of a survey area (8) caused by compaction of an underground hydrocarbon reservoir (1), and comprises the following steps: Conducting at least two series (S1, . . . ,Si, . . . ,Sm) of time-indexed depth measurements (13a, . . . ,13n), with separation in time &Dgr; between the measurement series characteristic of a significantly detectable long-term change of seafloor elevation due to compaction to take place in the reservoir. Measurements are time-indexed and corrected for tidal depth variations. Depth measurements (13) are conducted on survey stations (2) arranged on benchmarks (6) which have settled in the locally consolidated seabed (14). To handle short-term depth variations several stationary time-indexed short-time local reference depth measurement series (19r) are conducted at short-term local reference stations (18r) at benchmarks (6) during each separate measurement series (Si).

Abstract: Infrasound signals in the band 0.02 to 4 Hz are sensed in the presence of ambient noise generated chiefly by wind as integrated pressure variations, which induce detectable changes in the optical path length, along optic fibers, typically extending 100 m. to 1000 m. and more, arrayed at arbitrary geometries. Two fibers connected as a Michelson, Mach-Zehnder or equivalent interferometer where (i) one fiber is coupled to atmosphere while (ii) the other is not for being hermetically sealed in a tube, permit common mode rejection of noise from (i) temperature changes and (ii) strain, including ground vibration. Because the optic fiber infrasound sensors are longer than the distance over which wind-induced pressure changes are coherent, the effects of wind noise on the sensing of infrasound is reduced, and signal-to-noise ratio is increased over a wide bandwidth.

Abstract: The invention is a method for monitoring subsidence of the sea-bed (14) of a survey area (8) caused by compaction of an underground hydrocarbon reservoir (1), and comprises the following steps: Conducting at least two series (S1, . . . ,Si, . . . ,Sm) of time-indexed depth measurements (13a,.., 13n), with separation in time &Dgr; between the measurement series characteristic of a significantly detectable long-term change of seafloor elevation due to compaction to take place in the reservoir. Measurements are time-indexed and corrected for tidal depth variations. Depth measurements (13) are conducted on survey stations (2) arranged on benchmarks (6) which have settled in the locally consolidated seabed (14). To handle short-term depth variations several stationary time-indexed short-time local reference depth measurement series (19r) are conducted at short-term local reference stations (18r) at benchmarks (6) during each separate measurement series (Si).

Abstract: A gravity meter has a portable housing for holding a corner cube retroreflector that can be dropped within the housing. The housing also holds a laser and an optical fiber having a first end in light communication with the laser. Additionally, the fiber has a second end which terminates at a ferrule. Light from the laser propagates through the optical fiber, and a portion of the light is reflected by the second end of the fiber back through the optical fiber to a beam splitter, while another portion of the light propagates through the second end of the fiber and is reflected by the falling corner cube back through the fiber to the beam splitter. The two reflected portions of the laser light interfere with each other to generate an interference fringe pattern which is extracted by the beam splitter.

Abstract: A gravity meter has a portable housing for holding a corner cube retroreflector that can be dropped within the housing. The housing also holds a laser and an optical fiber having a first end in light communication with the laser. Additionally, the fiber has a second end which terminates at a ferrule. Light from the laser propagates through the optical fiber, and a portion of the light is reflected by the second end of the fiber back through the optical fiber to a beam splitter, while another portion of the light propagates through the second end of the fiber and is reflected by the falling corner cube back through the fiber to the beam splitter. The two reflected portions of the laser light interfere with each other to generate an interference fringe pattern which is extracted by the beam splitter.