Distributed Acoustic Sensing Signal Processing With Pulse Compression
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Solution Overview
Problem
Existing distributed acoustic sensing (DAS) systems face limitations in operational range and signal-to-noise ratio due to the use of single pulse systems and shot noise in coherent detection, leading to reduced spatial resolution and inaccurate measurements of large acoustic strains or incorrect fiber scatter bias conditions.
Innovation Solution
The use of spread spectrum pulses in DAS systems, where scattered signals are interfered with a local oscillator to generate a modulated carrier signal, followed by pulse compression to enhance signal-to-noise ratio and operational range without reducing spatial resolution, by employing methods like pulse compression and digital interference simulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the duration of each pulse is increased to increase energy per measurement, then the signal-to-noise ratio is improved, but the spatial resolution of the system is decreased
Solution Approach 1:
The patent uses multiple pulses instead of a single continuous pulse. By transmitting a sequence of periodic pulses and coherently combining their scattered signals, the system accumulates energy over multiple pulse cycles, improving signal-to-noise ratio without requiring each individual pulse to be excessively long, thereby preserving spatial resolution.
Solution Approach 2:
The patent applies pulse compression processing to the received scattered signals before final measurement. By pre-processing the signals with compression algorithms that exploit the known pulse waveform characteristics, the system achieves effective energy concentration in the temporal domain, improving measurement precision without extending the actual pulse duration that would degrade spatial resolution.
2Ease of operation
If a single pulse system is used to interrogate the fiber, then the system is simpler to operate, but the pulse repetition frequency is limited by the fiber length
Solution Approach 1:
The patent employs periodic pulse transmission with optimized pulse repetition frequency. By using multiple shorter pulses transmitted in sequence with appropriate timing, the system achieves higher effective sampling rates and pulse repetition frequencies compared to single long pulses, while maintaining operational simplicity through automated pulse sequences.
Solution Approach 2:
The patent divides the interrogation process into multiple discrete pulse transmissions rather than using a single continuous pulse. Each pulse in the sequence independently interrogates the fiber, and the results are combined. This segmentation allows the system to operate at higher pulse repetition frequencies limited only by the pulse width and processing time, not by the full fiber length transit time.
3Length of stationary object
If only the amplitude of scattered light is measured, then the system can detect acoustic energy for long range installations, but the measurement significantly distorts for large acoustic strains
Solution Approach 1:
The patent uses coherent detection with multiple periodic pulses, measuring both amplitude and phase of the scattered light. By coherently combining the complex signals from multiple pulses and applying pulse compression, the system achieves high signal-to-noise ratio for long-range detection while maintaining measurement linearity and accuracy for large acoustic strains through proper phase unwrapping and differential processing.
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 improves the signal-to-noise ratio and operational range of DAS systems, maintaining spatial resolution and enabling accurate measurement of acoustic environments with enhanced sensitivity and precision.
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
These discontinuities lead to scattering of laser light passing through the optical fiber, particularly by Rayleigh scattering
Data Source
AI summary
Disclosed are signal processing methods for an optical detection system, and corresponding systems. An example is a signal processing method for a distributed acoustic sensing system which utilizes spread spectrum pulses transmitted along an optical path, where 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 between the local oscillator and scattered signals. The first carrier signal is then processed to generate a second carrier signal that is modulated by a spatial differential of the phase difference. Pulse compression is then performed on the second carrier signal. The spatial differential of the phase difference is directly related to the strain (or acoustic environment) of the optical path at the scattering location, and so enables the strain at the scattering location to be estimated.