Optical DAS Phase-Differential Processing for Long-Range Strain Detection
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Solution Overview
Problem
Existing distributed acoustic sensing (DAS) systems face limitations in operational range and spatial resolution due to signal-to-noise ratio degradation and overscale issues when extending the length of the optical path, particularly in coherent detection methods, leading to inaccurate strain measurements.
Innovation Solution
A signal processing method that involves digitally processing the scattered signal with a local oscillator to generate a first carrier signal modulated by phase difference, then converting it to a second carrier signal modulated by a spatial differential of the phase difference, allowing for accurate strain estimation over longer distances and higher resolutions by applying a rectangular-to-polar coordinate transform to the second carrier signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the length of the optical path is extended to increase operational range, then the detection range is improved, but the signal-to-noise ratio deteriorates leading to measurement inaccuracies
Solution Approach 1:
The patent transforms the measurement parameter from cumulative phase to spatial differential of phase by digitally processing the carrier signal. This parameter transformation allows the system to maintain measurement precision over extended optical paths by eliminating the overscale issue that plagues traditional coherent detection methods when used over long distances.
2Measurement precision
If the spatial resolution is improved by using shorter gauge lengths, then the measurement precision is improved, but the signal-to-noise ratio deteriorates due to reduced scattered signal accumulation
Solution Approach 1:
By changing the measurement parameter to spatial differential of phase and applying digital signal processing techniques, the system achieves high spatial resolution (1m or less) while maintaining reliable signal-to-noise ratio even for short gauge lengths where traditional methods would fail due to insufficient signal accumulation.
3Measurement precision
If coherent detection is used to improve signal-to-noise ratio and detection sensitivity, then the measurement precision is improved, but overscale issues occur when extending the optical path length
Solution Approach 1:
The patent replaces the traditional mechanical/optical phase unwrapping approach with digital signal processing. By digitally generating a second carrier signal from the first carrier signal and measuring its spatial differential, the system eliminates overscale issues while maintaining coherent detection sensitivity, effectively substituting digital processing for complex optical phase management.
4Productivity
If the pulse repetition frequency is increased to improve sampling rate, then the productivity is improved, but the operational range is limited by the fiber length
Solution Approach 1:
The patent enables higher pulse repetition frequencies to be used effectively by transforming the measurement parameter to spatial differential of phase. This allows the system to sample at higher rates (improving productivity) while extending the operational fiber length beyond traditional limits, as the digital processing approach does not suffer from the cumulative phase errors that constrain traditional systems.
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
Enables DAS systems to achieve operational ranges of over 60km with 10m spatial resolution, or extend to 80km or improve resolution to 1m or less, by enhancing signal-to-noise ratio and avoiding overscale issues through digital processing techniques.
Implementation Method 1
a scattered signal that was scattered at a scattering location along an optical path is received and interfered with a local oscillator signal to generate a first carrier signal that is modulated by a phase difference
Implementation Method 2
a pulse or pulses of laser light are launched into a length of optical fiber and the light that is scattered within the fiber is analysed
Data Source
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AI summary
The invention provides a signal processing method for a distributed acoustic sensing system (DAS), where two or more scattered signals that were scattered at scattering locations along an optical path are received at a detector stage. Each of the two or more scattered signals is interfered with a local oscillator signal to generate a first carrier signal that is modulated by a phase difference between the local oscillator signal and that scattered signal. The first carrier signal is then represented as a phasor, and a reference phasor based on a time average is determined. The phasor is rotated by an angle corresponding to a difference between the reference phasor and a common reference phasor determined for the two or more scattered signals. After rotation, the phasors corresponding to the two or more scattered signals are summed to generate a second complex carrier signal.