Multimode Fiber Distributed Acoustic Sensing via Coherent Rayleigh Backscattering
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Solution Overview
Problem
Current fiber-optic sensing technologies, particularly Distributed Acoustic Sensing (DAS), are limited by their reliance on single mode optical fibers, which restricts their application in existing infrastructure where multimode fibers are already installed for Distributed Temperature Sensing (DTS).
Innovation Solution
The use of multimode fibers for DAS measurements is enabled through coherent Rayleigh backscattering, where narrowband laser pulses are launched into multimode sensing fibers, and the backscattered light is detected using interferometric techniques, with strategically positioned detectors to optimize signal reception and mitigate signal fading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If single mode optical fiber (SMF) is used for DAS measurements, then measurement precision is improved, but adaptability deteriorates because existing wells have multimode fibers installed
Solution Approach 1:
The patent changes the operational parameters of multimode fiber by using specific wavelength ranges (C-band: 1530-1565nm, L-band: 1565-1625nm) and controlling coherence properties of the light source. This allows MMF to function for DAS measurements while maintaining measurement precision through parameter optimization rather than requiring SMF
Solution Approach 2:
The patent makes multimode fiber universal by enabling it to perform both its traditional DTS function and the previously SMF-only DAS function. The same MMF infrastructure can now support multiple sensing modalities, eliminating the need for separate fiber types for different measurement types
2Adaptability or versatility
If multimode fiber (MMF) is used for DAS measurements, then adaptability is improved, but signal quality deteriorates due to signal fading
Solution Approach 1:
The patent implements feedback through coherent detection techniques that measure the phase and amplitude of backscattered light. By using reference arms and interferometric detection, the system compensates for signal variations and fading effects, maintaining reliable measurements despite using MMF
Solution Approach 2:
The patent employs composite sensing approaches by combining multiple detection techniques (Rayleigh backscattering, Brillouin scattering, Raman scattering) and processing methods. This composite approach compensates for the weaknesses of individual methods when used with MMF, improving overall signal reliability
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
This approach allows for DAS measurements in existing wells with MMF installed for DTS, and future installations can utilize MMF alone, enhancing the versatility and efficiency of fiber-optic sensing systems by eliminating signal fading and expanding application scope.
Implementation Method 1
the light pulses undergo coherent Rayleigh backscattering and are reflected back, toward the optical source
Implementation Method 2
When the coherent Rayleigh backscattered pulse is detected back at the surface, interferometric techniques can be used to determine very small changes in the length or temperature of the fiber
Data Source
AI summary
A method and system for distributed acoustic sensing using multimode optical fibers.