The “Development of High Sensitivity Engineered Optical Fiber for Distributed Acoustic Sensing” project (HS-DAS) aims to build on the success of distributed acoustic sensing (DAS) technology for monitoring CO2 sequestration by increasing the sensitivity of DAS sensing cable. DAS is an emerging technology that uses commercial telecommunications optical fiber to sense acoustic waves. This is achieved by the use of an interrogation system that propagates laser light down an optical fiber, and then detects light backscattered by inhomogeneities of the optical fiber, a technique referred to as OTDR, optical time domain reflectometry. Careful processing of the backscattered light is able to recover information on the amplitude and phase of the acoustic energy impinging on the optical fiber. DAS has found applications in perimeter security, pipeline leak detection and more recently seismology.
Most commercial DAS fiber cables use standard telecommunications optical fibers as the acoustic sensor because telecom fibers are reliable, readily available, and inexpensive. But while the extremely low-loss performance of telecom fiber makes it ideal for long distance communications, its low-loss profile means only a small portion of the emitted energy returns to the interrogator – about one-millionth of the emitted light – which limits the ability of telecom fibers to sense seismic signals. Additionally, DAS cables using telecom fibers lack sensitivity to broadside acoustic waves, limiting the performance of DAS cables for waves arriving at high incidence angles to the fiber cable. Recent advances in commercially engineered fibers show an improvement in acoustic sensitivity as compared to standard telecom fibers, but engineered fibers are more expensive to fabricate and usually require a specialized DAS interrogator unit to operate, significantly increasing the cost of continuous CCS monitoring.
Our goal for this HS-DAS project is to develop, fabricate and field test a new custom fiber with improved acoustic sensitivity and enhanced broadside sensitivity that can be used with standard commercial DAS interrogators for cost-effective continuous seismic monitoring at CCS sites. As a joint effort between Lawrence Livermore National Lab and Berkeley National Lab, the project is split into three tasks: Project Management is led by LBNL in Task 1; LLNL led and completed Tasks 2-4; the design and fabrication of the enhanced fiber cable is led by LLNL in Task 5, and the field testing and data analysis are led by LBNL in Task 6.
Task 1.0: Project Management and Planning
Task 2.0: Investigate Writing Index Steps in Optical Fiber with UV LEDs
Task 3.0: Investigate Writing Index Steps in Optical Fiber with visible and UV lasers
Task 4.0: Procurement specification for an optical system for fabricating engineered DAS fibers
Task 5.0: Fabricate a custom fiber having in-line aperiodic index structures
Task 6.0: Field testing of an engineered fiber with aperiodic index structures