A system (100) for monitoring a field (20) under a body of water, wherein the system (100) comprises a reference station (112) and a plurality of permanent seafloor sensors (120, 121). Each permanent seafloor sensor (120, 121) is fixed relative to a seafloor (2) on or at the field (20). The seafloor sensor (120, 121) further has a nearby survey station (111) sufficiently distant to ensure that a movable sensor (122) visiting the nearby survey station (111) does not disturb measurements from the permanent seafloor sensor (120). The distance is sufficiently close to ensure that the offset (Δp, Δg) from a value provided by the permanent seafloor sensor (120) is constant or can be modelled, e.g. to account for changes in the pressure/depth relation due to changes in water density. Each seafloor sensor is associated with a unique drift function d(t) at least comprising a drift rate (a). Thus, each permanent seafloor (120, 121) sensor provide an output that is corrected for drift at any time between calibration surveys. The system may be used for permanent monitoring of a seafloor.

A system (100) for monitoring a field (20) under a body of water, wherein the system (100) comprises a reference station (112) and a plurality of permanent seafloor sensors (120, 121). Each permanent seafloor sensor (120, 121) is fixed relative to a seafloor (2) on or at the field (20). The seafloor sensor (120, 121) further has a nearby survey station (111) sufficiently distant to ensure that a movable sensor (122) visiting the nearby survey station (111) does not disturb measurements from the permanent seafloor sensor (120). The distance is sufficiently close to ensure that the offset (Δp, Δg) from a value provided by the permanent seafloor sensor (120) is constant or can be modelled, e.g. to account for changes in the pressure/depth relation due to changes in water density. Each seafloor sensor is associated with a unique drift function d(t) at least comprising a drift rate (a). Thus, each permanent seafloor (120, 121) sensor provide an output that is corrected for drift at any time between calibration surveys. The system may be used for permanent monitoring of a seafloor.

An apparatus and method for measuring a local acceleration of gravity includes releasing a ferrous rod having a regular alternating pattern of reflective and non-reflective portions on a surface thereof from an electromagnetic holder so that the rod falls with a substantially vertical acceleration and substantially no angular velocity about a center of mass of the rod. The falling rod is illuminated with a light emitting diode (LED) configured to emit infrared (IR) light, and IR light emitted by the LED and reflected by the falling rod is detected with a photodiode. A two-state signal is generated corresponding to an illumination state of the photodiode by the reflected IR light. Times of transitions between the two states in the generated signal are calculated to determine kinematic data, and the kinematic data is fitted to a predetermined curve to calculate a local acceleration of gravity.

An apparatus and method for measuring a local acceleration of gravity includes releasing a ferrous rod having a regular alternating pattern of reflective and non-reflective portions on a surface thereof from an electromagnetic holder so that the rod falls with a substantially vertical acceleration and substantially no angular velocity about a center of mass of the rod. The falling rod is illuminated with a light emitting diode (LED) configured to emit infrared (IR) light, and IR light emitted by the LED and reflected by the falling rod is detected with a photodiode. A two-state signal is generated corresponding to an illumination state of the photodiode by the reflected IR light. Times of transitions between the two states in the generated signal are calculated to determine kinematic data, and the kinematic data is fitted to a predetermined curve to calculate a local acceleration of gravity.

An apparatus and method for measuring a local acceleration of gravity includes releasing a ferrous rod having a regular alternating pattern of reflective and non-reflective portions on a surface thereof from an electromagnetic holder so that the rod falls with a substantially vertical acceleration and substantially no angular velocity about a center of mass of the rod. The falling rod is illuminated with a light emitting diode (LED) configured to emit infrared (IR) light, and IR light emitted by the LED and reflected by the falling rod is detected with a photodiode. A two-state signal is generated corresponding to an illumination state of the photodiode by the reflected IR light. Times of transitions between the two states in the generated signal are calculated to determine kinematic data, and the kinematic data is fitted to a predetermined curve to calculate a local acceleration of gravity.

An apparatus and method for measuring a local acceleration of gravity includes releasing a ferrous rod having a regular alternating pattern of reflective and non-reflective portions on a surface thereof from an electromagnetic holder so that the rod falls with a substantially vertical acceleration and substantially no angular velocity about a center of mass of the rod. The falling rod is illuminated with a light emitting diode (LED) configured to emit infrared (IR) light, and IR light emitted by the LED and reflected by the falling rod is detected with a photodiode. A two-state signal is generated corresponding to an illumination state of the photodiode by the reflected IR light. Times of transitions between the two states in the generated signal are calculated to determine kinematic data, and the kinematic data is fitted to a predetermined curve to calculate a local acceleration of gravity.

The present application discloses a dead-zone-free cold atom interferometer with a high frequency output. The interferometer includes: a three-dimensional magneto-optical trap, wherein a predetermined angle is formed between the first group of light sources and an atomic beam path, the first group of optical stops are arranged at edges of the first group of light sources and downstream of the atomic beam path, the first group of optical stops block laser light emitted from the first group of light sources, the second group of light sources are orthogonally arranged with respect to the first group of light sources, the second group of optical stops are arranged at edges of the second group of light sources and downstream of the atomic beam path, and the second group of optical stops block laser light emitted from the second group of light sources.

Systems and methods for determining azimuth of a wellbore while drilling. A method of drilling a wellbore that includes rotating a drill bit to extend the wellbore into a subterranean formation. The method may further include measuring magnetic toolface with a first magnetometer while rotating the drill bit. The method may further include obtaining a cross-axial magnetic field measurement. The method may further include determining azimuth at a point in the wellbore using at least the magnetic toolface and the cross-axial magnetic field measurement. The method may further include using the azimuth as feedback in the drilling the wellbore.

Systems and methods for determining azimuth of a wellbore while drilling. A method of drilling a wellbore that includes rotating a drill bit to extend the wellbore into a subterranean formation. The method may further include measuring magnetic toolface with a first magnetometer while rotating the drill bit. The method may further include obtaining a cross-axial magnetic field measurement. The method may further include determining azimuth at a point in the wellbore using at least the magnetic toolface and the cross-axial magnetic field measurement. The method may further include using the azimuth as feedback in the drilling the wellbore.

A sensing element (10) for an intrinsic gravity gradiometer (IGG) for use in sensing variation in a gravity field at a location. The sensing element (10) is flexible, elongate and has unfixed opposed ends (12, 14) when part of the gravity gradiometer. The sensing element can be a metallic ribbon, and can be mounted by a number e.g. 3 or 5, pivot points or axes 30-40 at each of the opposed sides along the sensing element, with the opposed ends of the sensing element free to move. The pivot points or axes can include pins, preferably cylindrical pins (48) or the sensing element may be etched within the side wall and remain joined to the remainder of the side wall by connections. The sensing element (10) can form part of one or more resonant cavities or wave guide (44, 52-66), such as a side or dividing wall (46) or part thereof. A dual phase bridge (61,612) arrangement can be provided. Electrical current (I) can be injected into the sensing element. Feed forward motion compensation (MC or FFMC) can be applied as part of the determination of the current. Applying electrical current into the opposed longitudinal sides (20, 22), such as right and left sides, of the sensing element, such as a ribbon, can be used for several types of compensation. Displacement of the sensing element can be detected by a resonant cavity, electromagnetic sensor or optical sensor.