Silixa Limited

Chevron U.s.a. Inc.

Fotech Solutions Limited

Siiixa Ltd

Silica Limited

Sergey Shatalin

Mahmoud Farhadiroushan

Tom Richard Parker

Daniel Finfer

Dmitry Strusevich

Veronique Mahue

Julian Dajczgewand

Sotiris Emil Kanellopoulos

Jack Maxwell

A long range optical fiber sensor such as a distributed acoustic sensor has a sensing fiber located remotely from the interrogator, with a length of transport fiber path connecting the two. Because no sensing is performed on the transport fiber then the pulse repetition rate from the interrogator can be high enough such that the pulse repetition rate and pulse power are optimised according to the sensing fiber length and hence sensing frequency response and sensitivity are also optimised according to the sensing fiber length.

Long range optical fiber sensing systems

The present invention provides novel apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fibre. The present invention can be used for point sensors as well as distributed sensors or the combination of both. In particular this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fibre while achieving fine spatial resolution. The present invention offers unique advantages in a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.

Method and apparatus for optical sensing

An optical fiber distributed acoustic sensor system makes use of a specially designed optical fiber to improve overall sensitivity of the system by a factor in excess of 10. This is achieved by inserting into the fiber weak broadband reflectors periodically along the fiber. The reflectors reflect a small proportion of the light from the DAS incident thereon back along the fiber, typically in the region of 0.001% to 0.1%. To allow for temperate compensation to ensure that the same reflectivity is obtained if the temperature changes, the reflection bandwidth is relatively broadband. The reflectors are formed from a series of fiber Bragg gratings, each with a different center reflecting frequency, the reflecting frequencies and bandwidths of the gratings being selected to provide the broadband reflection. The reflectors are spaced at the desired spatial resolution of the optical fiber DAS.

An improved optical fiber distributed acoustic sensor system uses an optical fiber having reflector portions distributed along its length in at least a first portion. The reflector portions are positioned along the fiber separated by a distance that is equivalent to twice the distance an optical pulse travels along the fiber in a single sampling period of the data acquisition opto-electronics within the sensor system. No oversampling of the reflections of the optical pulses from the reflector portions is undertaken. The sampling points for data acquisition in the sensor system are aligned with the reflections that arrive at the sensor system from along the sensing fiber. Adaptive delay componentry adaptively aligns the reflected optical signals (or their electrical analogues) with the sampling points. Control over the sampling points can re-synchronise the sampling points with the returning reflections. Reflection equalisation componentry may reduce the dynamic range of the returning reflections.

An optical fiber distributed acoustic sensor system includes weak broadband reflectors inserted periodically along the fiber. The reflectors reflect only a small proportion of the light from the DAS incident thereon back along the fiber, typically in the region of 0.001% to 0.1%, but preferably around 0.01% reflectivity per reflector. In addition, to allow for temperate compensation to ensure that the same reflectivity is obtained if the temperature changes, the reflection bandwidth is relatively broadband. In some embodiments the reflectors are formed from a series of fiber Bragg gratings, each with a different center reflecting frequency, the reflecting frequencies and bandwidths of the gratings being selected to provide the broadband reflection. A chirped grating may also be used to provide the same effect. In preferred embodiments, the reflectors are spaced at half the gauge length i.e. the desired spatial resolution of the optical fiber DAS.

A prefabricated mat-like structure having lengths of fiber mounted thereon or therein in a predetermined deployment pattern that provides a high spatial density of fiber to give increased spatial sensing resolution is described. The prefabricated mat-like structures may be very easily deployed by being placed against and/or wrapped around an object to be monitored, typically being fastened in place by clamps or the like. In addition, easy removal from the object is also obtained, by simply unfastening the mat-like structure, which may then be redeployed elsewhere. The prefabricated mat-like structure having the fiber already mounted thereon or therein therefore provides a very convenient and easily installable and removable solution.

Flexible substrate fiber optic sensing mat for distributed acoustic sensing

Embodiments of the present invention provide a cable for optical fiber sensing applications formed from fiber wound around a cable core. A protective layer is then preferably placed over the top of the wound fiber, to protect the fiber, and to help keep it in place on the cable core. The cable core is preferably of a diameter to allow bend-insensitive fiber to be wound thereon with low bending losses. The effect of winding the fiber onto the cable core means that the longitudinal sensing resolution of the resulting cable is higher than simple straight fiber, when the cable is used with an optical fiber sensing system such as a DAS or DTS system. The achieved resolution for the resulting cable is a function of the fiber winding diameter and pitch, with a larger diameter and reduced winding pitch giving a higher longitudinal sensing resolution.

Optical fiber cable

The present invention provides novel apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fiber. The present invention can be used for point sensors as well as distributed sensors or the combination of both. In particular this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fiber while achieving fine spatial resolution. The present invention offers unique advantages in a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.

Apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fiber can be used for point sensors as well as distributed sensors or the combination of both. In particular, this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fiber while achieving fine spatial resolution. Advantages of this technique include a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.

Embodiments of the present invention address aliasing problems by providing a plurality of discrete acoustic sensors along a cable whereby acoustic signals may be measured in situations where the fiber optic cable has not been secured to a structure or area by a series of clamps. Acoustic sampling points are achieved by selectively enhancing the acoustic coupling between the outer layer and the at least one optical fiber arrangement, such that acoustic energy may be transmitted selectively from the outer layer to the at least one optical fiber arrangement. The resulting regions of acoustic coupling along the cable allow the optical fiber to detect acoustic signals. Regions between the outer layer and the at least one optical fiber arrangement that contain material which is acoustically insulating further this enhancement since acoustic waves are unable to travel through such mediums, or at least travel through such mediums at a reduced rate.

Non-isotropic acoustic cable

An interferometer apparatus for an optical fiber system and method of use is described. The interferometer comprises an optical coupler and optical fibers which define first and second optical paths. Light propagating in the first and second optical paths is reflected back to the optical coupler to generate an interference signal. First, second and third interference signal components are directed towards respective first, second and third photodetectors. The third photodetector is connected to the coupler via a non-reciprocal optical device and is configured to measure the intensity of the third interference signal component directed back towards the input fiber. Methods of use in applications to monitoring acoustic perturbations and a calibration method are described.

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Elstree, United Kingdom

Sergey Shatalin