Fiber Bragg Gratings for Distributed Acoustic Sensing
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Solution Overview
Problem
Conventional seismic monitoring systems are expensive to acquire and maintain, and they have limited spatial resolution, which is insufficient for localized fluid flow monitoring applications, as they rely on impurities in fiber optic cables for backscattering, leading to weak or non-existent signals in some areas.
Innovation Solution
Incorporating fiber Bragg gratings into optical fiber cables deployed near subsurface formations to enhance backscattering and improve spatial resolution, allowing for higher sensitivity and localized acoustic event detection without the need for numerous acoustic sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional acoustic sensors (geophones or hydrophones) are used for seismic monitoring, then the system can detect acoustic events, but the cost to acquire, deploy and maintain the large numbers of sensors is relatively expensive
Solution Approach 1:
The patent replaces mechanical acoustic sensors (geophones or hydrophones) with an optical fiber-based sensing system. The optical fiber uses Rayleigh scattering and Brillouin scattering effects to detect acoustic events, eliminating the need for expensive mechanical sensors while maintaining detection capability across large areas.
Solution Approach 2:
The optical fiber serves multiple functions simultaneously: it acts as both the transmission medium for optical signals and the sensing element for acoustic event detection. This multi-functionality eliminates the need for separate acoustic sensors, reducing system cost and complexity while maintaining comprehensive monitoring capability.
2Measurement precision
If conventional acoustic sensors are deployed to provide desired spatial resolution, then acoustic events can be detected, but the number of sensors required (hundreds to thousands) increases system complexity
Solution Approach 1:
The optical fiber is segmented into multiple sensing points along its length, with each point capable of detecting acoustic events independently. By dividing the continuous fiber into discrete measurement zones, the system achieves high spatial resolution without requiring discrete sensor elements at each location.
Solution Approach 2:
The patent uses Rayleigh scattering and Brillouin scattering as intermediary mechanisms to transfer acoustic information from the fiber to detectable optical signals. These scattering effects act as mediators that enable distributed sensing along the entire fiber length, providing spatial resolution without direct mechanical sensing elements.
3Adaptability or versatility
If DAS systems use Rayleigh scattering from impurities in the fiber for acoustic detection, then distributed sensing is achieved, but the backscattered signal may be weak or non-existent in portions of the cable where it is desired to sense
Solution Approach 1:
The patent combines multiple scattering mechanisms (Rayleigh scattering from impurities and Brillouin scattering from acoustic waves) within the same optical fiber system. This composite approach ensures that even if Rayleigh scattering is weak in certain regions, Brillouin scattering can provide sufficient signal strength for reliable acoustic event detection throughout the entire fiber length.
Solution Approach 2:
The system dynamically adjusts measurement parameters including optical wavelength, pulse duration, and sampling rate to optimize signal strength at different locations along the fiber. By changing these parameters adaptively, the system compensates for variations in scattering strength and maintains reliable detection across all sensing regions.
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
The system provides high-resolution seismic monitoring at a lower cost, enabling effective detection and localization of acoustic events and fluid flows within the subsurface, reducing the need for extensive sensor deployment and maintenance.
Implementation Method 1
Defects in the glass backscatter the pulse (Rayleigh scattering) as it propagates along the fiber and the backscattered photons are received in a photodetector
Implementation Method 2
DAS uses a similar technique, in which external acoustic disturbances modulate the backscattered light from certain sections of the fiber
Data Source
AI summary
A method for obtaining information about a subsurface formation from acoustic signals that contain information about to the subsurface formation, comprises: providing a fiber optic having a proximal end and a remote end, with the proximal end being coupled to a light source and a proximal photodetector, wherein said fiber optic cable includes randomly spaced impurities and selectively placed Bragg gratings and wherein the fiber optic cable is acoustically coupled to the subsurface formation so as to allow the acoustic signals to affect the physical status of at least one grating: transmitting at least one light pulse into the cable; receiving at the photodetector a first light signal indicative of the physical status of at least one first section of the cable, and outputting at least one item of information to a display.