Fiber Bragg Grating Calibration for Distributed Fiber Optic Sensing
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Solution Overview
Problem
Current methods for determining sensor channel locations in distributed fiber-optic sensing, particularly in wellbores and pipelines, face inaccuracies due to approximations in fiber index of refraction and reliance on limited calibration points, leading to errors in gauge-length computation and location estimation.
Innovation Solution
The method involves using Fiber Bragg-Gratings (FBGs) uniformly spaced along fiber-optic cables, interrogated with a distributed fiber-optic sensing laser and a broadband FBG laser, to calibrate and constrain sensor channel locations through optical time domain reflectometry, allowing for additional calibration points and improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If interpolation between two known locations (wellhead and bottom of wellbore) is used to estimate sensor channel locations, then the system is simple to operate, but the measurement precision of location information deteriorates due to lack of accurate knowledge of multiple locations inside the wellbore
Solution Approach 1:
The fiber-optic cable is segmented into multiple sections by placing FBGs at uniformly spaced intervals along its length. Each FBG acts as an independent calibration point, dividing the single interpolation span into multiple smaller segments. This segmentation allows for more frequent calibration points throughout the wellbore, improving location measurement precision while maintaining operational simplicity through automated interrogation of the segmented FBG locations
Solution Approach 2:
FBGs serve as intermediary calibration elements placed between the wellhead and bottom of the wellbore. These intermediaries provide additional known reference points that enable more accurate interpolation and calibration of sensor channel locations without requiring direct physical access to interior wellbore locations, thus improving measurement precision while keeping the system easy to operate
2Measurement precision
If FBGs are used to provide additional calibration points, then the measurement precision of sensor channel locations is improved, but the device complexity increases due to multiple FBGs and dual-laser interrogation system
Solution Approach 1:
The FBGs perform multiple functions: they serve as both the sensing elements for environmental measurements and as calibration reference points for location determination. The same fiber-optic cable that carries the sensing function also provides the calibration infrastructure through the embedded FBGs. This multi-functionality reduces device complexity by eliminating the need for separate calibration systems while improving measurement precision
Solution Approach 2:
The calibration system and sensing system are merged into a single integrated fiber-optic cable assembly. The FBGs are embedded within the same cable that contains the distributed sensing fibers, and both functions are interrogated using the same optical infrastructure and processing system. This merging reduces overall device complexity by consolidating multiple functions into a unified system while maintaining high measurement precision through the additional calibration points
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 enhances the accuracy of sensor channel location determination by introducing additional calibration points and reducing errors associated with fiber index approximations, thereby improving the precision of environmental property measurements within wellbores and pipelines.
Implementation Method 1
Each FBG reflects a wavelength of laser light that is proportional to the grating size
Implementation Method 2
Using an optical time domain reflectometer (OTDR) at the FBG wavelength, the distance to the particular FBG is computed in the optical domain
Data Source
AI summary
Methods for determining sensor channel location in distributed sensing of fiber-optic cables are disclosed. In one method, three or more Fiber Bragg-Gratings (FBGs) connected in series by a standard telecommunication fiber and interrogated using an input distributed fiber-optic sensing (DFOS) laser, where the input DFOS laser has a single wavelength. The input DFOS laser operates on a single wavelength that is different than the respective wavelengths of each of the three or more FBGs. The three or more FBGs are interrogated using an input broadband FBG laser. Each FBG reflects a wavelength of laser light that is proportional to the grating size, using an optical time domain reflectometer (OTDR) at the FBG wavelength, the distance to the particular FBG in the optical domain is computed and compared to the physical measurement of the FBG location. The sensor channel locations of the DFOS system are calibrated and constrained using this method.