Fibre optic cable with tuned transverse sensitivity

Abstract

This application relates to a fibre optic cable structure suitable for use as a sensing fibre optic for distributed acoustic sensing and having an improved sensitivity to transverse pressure waves. The application describes a fibre optic cable (300) having a longitudinal cable axis and comprising at least one optical fibre (301). The cable also comprises a compliant core material (303) mechanically coupled to the optical fibre(s), possible via a buffer (302) such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the optical fibre(s). At least one deformable strain transformer (304) is coupled to the compliant core material and configured such that a force acting on the strain transformer in a direction transverse to the cable axis results in a deformation of the strain transformer thereby applying a longitudinal force to the compliant core material.

Description

technical field [0001] The present application relates to fiber optic cables suitable for use in fiber optic sensing, especially for fiber optic distributed acoustic sensing, and in particular to fiber optic cables with tuned sensitivity to transverse strain and methods of manufacture thereof cable structure. Background technique [0002] Distributed Acoustic Sensing (DAS) is a known type of sensing in which an optical fiber is deployed as a sensing fiber and interrogated with electromagnetic radiation to provide insight into the acoustic activity (i.e. dynamic strain) sensing. By analyzing the backscattered radiation from within the fiber, the fiber can be effectively divided into discrete sensing sections which may (but need not be) contiguous. Within each discrete sensing section, mechanical perturbations of the fiber, such as dynamic strain due to incident acoustic waves, cause changes in the properties of the radiation backscattered from that section. This change can...

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Problems solved by technology

However, it will be appreciated that an effective change in the optical path length due to the incident pressure wave will also cause a change in the detected backscatter from the sensing fiber, and may mask the detection of any change due to changes in the magnetic field alone.

Likewise, distributed fiber optic sensing based on Rayleigh backscattering has been proposed to provide an indication of any dynamic temperature change along the length of the fiber, but again, this may be difficult to account for effects due to temperature changes and due to incident pressure The detected backscatter signal is discriminated between the effects of wave generation

Therefore, in such applications it may be desirable to use sensing fibers that are sensitized to the measurand in question but relatively insensitive to incident pressure waves

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