Grating Position Dithering for Distributed Acoustic Sensing
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Solution Overview
Problem
Destructive interference in optical fibers used for acoustic downhole measurements results in weak optical signals due to uniform spacing between gratings, which hampers the effective measurement of acoustic signals in wellbores.
Innovation Solution
The optical system employs gratings in the optical fiber with offset distances selected to reduce destructive interference, where each grating is associated with a nominal site and separated by a specific offset distance, which can be determined using a dithering function, to enhance signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If gratings are uniformly spaced in the optical fiber, then the manufacturing process is simple, but destructive interference occurs resulting in weak optical signal
Solution Approach 1:
The patent applies asymmetry by intentionally introducing non-uniform spacing between gratings through offset distances from nominal sites. Each grating is positioned at a nominal site uniformly spaced along the optical fiber, but with a specific offset distance that breaks the uniformity. This asymmetric positioning eliminates destructive interference patterns while maintaining manufacturing feasibility through systematic offset application.
Solution Approach 2:
The patent implements local quality by applying different offset distances to different gratings based on their specific positions. Each grating receives a tailored offset distance selected to reduce destructive interference at that particular location. This localized adjustment optimizes the optical signal strength at each grating position while maintaining overall system coherence.
2Illumination intensity
If offset distances are introduced to reduce destructive interference, then optical signal strength improves, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies parameter changes by systematically varying the offset distance parameter for each grating. Instead of requiring extremely precise fixed positions, the system uses controlled variations in offset distances from nominal sites. This approach transforms the manufacturing challenge from achieving absolute precision to managing relative deviations, which is more feasible while still achieving the desired interference reduction.
Solution Approach 2:
The patent implements preliminary action by pre-calculating and pre-defining the offset distances for each grating before manufacturing. The offset distances are selected in advance to reduce destructive interference, allowing manufacturers to follow a predetermined pattern rather than requiring complex real-time adjustments during the manufacturing process.
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 configuration increases the signal-to-noise ratio by minimizing destructive interference, resulting in a stronger optical signal for accurate acoustic signal measurement in wellbores.
Implementation Method 1
An optical interrogator transmits a light into the optical fiber and records a reflection of the light from the scatterers
Implementation Method 2
destructive interference can occur due to uniform spacing between gratings, thereby resulting in a weak optical signal being received at the optical interrogator
Data Source
AI summary
An optical system employs a method for measuring an acoustic signal in a wellbore. The optical system includes an optical interrogator and an optical fiber. The optical fiber has a plurality of nominal sites uniformly spaced apart along a longitudinal axis of the optical fiber. A plurality of gratings are formed in the optical fiber. Each of the plurality of gratings is associated with a nominal site and is separated from its associated nominal site by an offset distance. The offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. The optical interrogator transmits a light pulse into the optical fiber to measure the acoustic signal via a reflection of the light pulse from at least one of the plurality of gratings.