Fibre Optic Cable Strain Transformer for Transverse Sensing
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Solution Overview
Problem
Conventional fibre optic cables for distributed acoustic sensing (DAS) are predominantly sensitive to longitudinal strains, making them less effective for detecting transverse acoustic waves, which is a limitation in applications such as seismic surveying and surface seismic sensing where transverse signals are of interest.
Innovation Solution
A fibre optic cable design featuring a compliant core material and a deformable strain transformer that converts transverse forces into longitudinal strains, enhancing the sensitivity of the optical fibre to transverse acoustic waves by amplifying physical length changes and refractive index modulations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional fibre optic cable structure is used for DAS sensing, then the sensing of longitudinal strains is effective, but the sensitivity to transverse acoustic waves is insufficient
Solution Approach 1:
A compliant core material is introduced as an intermediary between the optical fibre and the external environment. This core material mechanically couples transverse forces to the optical fibre, enabling the fibre to detect transverse acoustic waves that would otherwise not effectively strain the sensing element
Solution Approach 2:
The cable structure parameters are modified by introducing a compliant core with specific mechanical properties and deformable strain transformers. These parameter changes enable the cable to convert transverse acoustic wave energy into longitudinal strain on the optical fibre, thereby improving transverse wave sensitivity
2Measurement precision
If the fibre optic cable is made highly sensitive to transverse strains, then detection of transverse acoustic waves is improved, but sensitivity to pressure variations may become excessive for certain applications
Solution Approach 1:
The cable structure is designed with localized strain transformers at specific positions along the optical fibre. These strain transformers are configured to convert transverse strains selectively while the overall cable structure can be tuned to filter out pressure variation effects, achieving directionally selective sensitivity
Solution Approach 2:
The compliant core material and strain transformers are designed with specific dynamic mechanical properties that allow them to respond differently to transverse acoustic waves versus pressure variations. The dynamic response characteristics enable selective enhancement of transverse wave detection while maintaining insensitivity to pressure changes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The fibre optic cable exhibits significantly improved sensitivity to transverse acoustic waves, with the strain transformer configuration allowing for either enhanced detection or reduced sensitivity to pressure variations, depending on the application, thereby improving the effectiveness of DAS systems in various sensing scenarios.
Implementation Method 1
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
Implementation Method 2
Light transmitted into an optical fibre will be Rayleigh scattered from the various inherent, i.e. intrinsic, scattering sites within an optical fibre. A mechanical vibration or dynamic strain acting on the fibre, such as caused by an incident acoustic wave, will effectively alter the distribution of scattering sites resulting in a detectable change in the properties of the Rayleigh backscattered light
Implementation Method 3
a compliant core material mechanically coupled to the at least one optical fibre such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the at least one optical fibre
Data Source
AI summary
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.