Distributed Acoustic Sensing Using Multiple Pulse Widths
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Solution Overview
Problem
Fiber optic sensing systems face issues with Rayleigh fading, leading to signal-to-noise ratio drops and errors in distributed acoustic sensing due to destructive interference, which existing methods address by lengthening measurement periods or using additional hardware for multi-wavelength measurements, both of which are unacceptable for many applications.
Innovation Solution
Employing multiple pulse widths in distributed acoustic sensing systems to reduce Rayleigh fading effects by alternating pulse widths and selecting data with better quality, thereby minimizing interference and maintaining signal quality without increasing hardware complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If measurement period is lengthened to address Rayleigh fading, then signal quality improves, but output delay becomes unacceptable
Solution Approach 1:
The system dynamically adjusts pulse width based on detected fading conditions. When fading is detected in a channel, the system increases the pulse width for subsequent measurements in that channel, allowing the signal to average out the fading effects without requiring excessively long measurement periods. This dynamic adaptation resolves the contradiction by maintaining signal quality while reducing output delay.
Solution Approach 2:
The invention changes the pulse width parameter in response to detected fading conditions. By varying the pulse width (a key system parameter) based on real-time signal quality assessment, the system can compensate for Rayleigh fading effects without extending the overall measurement period, thus maintaining both signal quality and acceptable output delay.
2Reliability
If multiple wavelengths are used to address Rayleigh fading, then signal quality improves, but hardware complexity increases
Solution Approach 1:
Instead of changing wavelength (which would require additional hardware), the system changes the pulse width parameter. By varying pulse duration, the system exploits the frequency-dependent nature of Rayleigh fading to obtain diverse measurements that can be combined to overcome fading effects, achieving multi-wavelength benefits without multi-wavelength hardware.
Solution Approach 2:
The invention substitutes optical complexity (multiple wavelengths requiring additional lasers or filters) with temporal complexity (multiple pulse widths achievable with a single laser). This replacement of spatial/optical diversity with temporal diversity reduces hardware complexity while maintaining the ability to mitigate Rayleigh fading.
3Reliability
If pulse width is increased to improve signal energy, then signal quality improves, but spatial resolution deteriorates
Solution Approach 1:
The system dynamically adjusts pulse width based on detected fading conditions rather than using a fixed pulse width. When fading is detected, pulse width is increased to improve signal energy; when fading is absent, pulse width is reduced to maintain spatial resolution. This dynamic adjustment resolves the contradiction by adapting to real-time conditions.
Solution Approach 2:
The measurement process is segmented into multiple phases with different pulse widths. The system performs initial measurements with shorter pulses for high spatial resolution, then applies fading compensation with longer pulses only where and when needed. This segmentation allows the system to achieve both high resolution and good signal quality in different parts of the measurement 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 approach effectively reduces noise and improves signal quality by selecting fade-resistant phase measurements, enhancing the overall performance of fiber optic sensing systems in hostile environments like hydrocarbon wells.
Implementation Method 1
All optical fibers have a distribution of impurities that each scatter a small fraction of passing coherent light pulses. The scattered portions of coherent light can interfere constructively or destructively with each other.
Implementation Method 2
The scattered portions of coherent light can interfere constructively or destructively with each other. As the distribution of impurities is perturbed, the degree of interference can be volatile and seemingly random.
Data Source
AI summary
A distributed acoustic sensing method that includes sending a sequence of optical pulses along an optical fiber, of at least two different widths, demodulating backscattered light from the optical fiber to obtain interferometric phase measurements as a function of position, combining the interferometric phase measurements to obtain a set of fade-resistant phase measurements, and storing or displaying the set of fade-resistant phase measurements. A distributed acoustic sensing system that includes a transmitter that sends a sequence of optical pulses along an optical fiber, of at least two different widths, a receiver that demodulates backscattered light from the optical fiber to obtain interferometric phase measurements as a function of position and combines interferometric phase measurements to obtain a set of fade-resistant phase measurements, and a storage or display device.