Phase Based Sensing for Seismic Overloading
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Solution Overview
Problem
Fibre optic sensors used in seismic surveying face distortion and demodulation failure due to signals above a certain amplitude threshold, known as overloading or overscaling, which is particularly problematic with direct arrivals of high-amplitude pulses.
Innovation Solution
A method involving a single pulse signal interrogation, where a delayed version of the pulse is combined with an undelayed version to determine the rate of change of phase, allowing for shorter time differences between pulses and increased dynamic signal measurement capabilities, while reducing noise and improving signal-to-noise ratio (SNR) by correcting for signal wrapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase based sensing is used to measure vibration signals, then measurement precision is improved, but signals above a certain amplitude threshold cause phase distortion and demodulation failure
Solution Approach 1:
The patent changes the parameter being measured from phase directly to the derivative of phase with respect to time. This parameter transformation allows the system to measure high amplitude signals without the phase wrapping and distortion problems that plague direct phase measurement. The derivative signal maintains sensitivity to small vibrations while being immune to the overloading effects that cause demodulation failure.
Solution Approach 2:
The patent introduces an intermediary mathematical operation (differentiation) between the phase signal and the final measurement. By computing the derivative of the phase with respect to time, the system creates an intermediate representation that eliminates the harmful effects of large phase excursions while preserving the essential vibration information. This intermediary transformation resolves the contradiction between maintaining precision and avoiding demodulation failure.
2Reliability
If the derivative of phase is measured instead of phase directly, then reduced sensitivity is achieved which prevents overloading, but the amplitude of the derivative signal is much smaller than the original signal
Solution Approach 1:
The patent employs periodic square wave modulation at a frequency higher than the maximum signal frequency. This periodic action serves dual purposes: it amplifies the derivative signal by a factor related to the modulation frequency, and it maintains the immune-to-overloading property. The high frequency square wave oscillation effectively scales up the small derivative signal while preserving the reliability benefits of derivative measurement.
Solution Approach 2:
The patent transforms the static phase measurement into a dynamic derivative measurement with respect to time. This dynamic approach inherently scales the signal amplitude according to the rate of change of phase, which for high frequency vibrations provides natural amplification. The dynamic nature of the derivative measurement allows the system to capture fast vibrations with appropriate amplitude scaling.
3Adaptability or versatility
If pulse pairs with different separations are used to generate derivative signals, then multiple sensitivity levels are achieved, but the time difference between pulse pairs must be sufficiently large
Solution Approach 1:
The patent uses periodic square wave modulation to generate the derivative signal instead of relying on widely separated pulse pairs. The periodic high frequency oscillation effectively creates multiple cycles within a short time window, providing the necessary time resolution without requiring large time separations between measurement pulses. This approach maintains adaptability while reducing the time loss associated with pulse separation.
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 accurate measurement of large dynamic signals without distortion, improving sensitivity and SNR by integrating the derivative signal and correcting for signal wrapping, allowing for a wider range of signal amplitudes to be recorded without overloading.
Implementation Method 1
a sensed parameter causes a strain to be imposed on the fibre. Typically the fibre is arranged in a coil, although other arrangements are possible. Such strain causes a change in phase of optical signal propagation in that fibre
Implementation Method 2
which change can be detected by interferometric techniques
Data Source
Figure 1
Figure 2~3c
Figure 4~5
AI summary
Interrogation of a phase based transducer is performed by temporally overlapping and interfering a single pulse output from the transducer to determine the rate of change with time of the measurand represented as a phase change. The rate of change, or derivative of the phase change typically has a much smaller amplitude than the signal itself, and the derivative measurement therefore has reduced sensitivity. In this way, large amplitude signals which might otherwise be subject to overscaling effects can be measured more effectively.