Levee Monitoring with Distributed Strain Sensing, Black Hawk, LA

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The goal of this project is to determine the feasibility of using distributed fiber optic sensing to detect and monitor seepage and/or boil formation in levees. This project proposes to use two types of distributed fiber optic sensing: distributed strain sensing (DSS) and distributed acoustic sensing (DAS). Both are established techniques for measuring strain and seismic signals using optical fiber. The optical cable, which typically contains several strands of fiber, can be tens of kilometers in length, thus providing the opportunity to monitor large areas. For both systems, the spatial resolution is on the order of meters. The emphasis in this Statement of Interest is in DSS. In this approach, an interrogator is connected to the fiber optic cable. The interrogator is a “black box” that contains a laser that injects laser pulses into the optical fiber. The interrogator receives light backscattered from imperfections in the fiber. Via a mechanism referred to as Brillouin scattering, the frequency of light backscattered from points in the fiber is shifted an amount proportional to the strain. These return signals are used to determine the strain at points all along the length of the fiber. A distributed fiber optic test bed will be designed and installed at the Black Hawk, LA site (Figure 3). For the DSS installation, vertical sections of fiber are proposed so that the fiber optic cable can extend through the clay blanket and into the sand substratum, as shown in Figure 4. The purpose of this configuration is to measure vertical strain. The idea is that during a flood event, seepage will cause the pressure to increase within the sand substratum, pushing up on the clay top stratum, or blanket, thereby inducing strain in the vertically oriented fiber. There are two areas of potential collaboration in applying DSS to the detection of seepage. One isto provide input in the design of the test bed. The second is in assisting to analyze and interpret data acquired with the DSS interrogator, to include developing algorithms to record, monitor, and display the signals over time. To interpret DSS signals and associate them to detected events, the signals will be recorded, analyzed, and compared with data acquired from corroborating sensors (e.g., piezometers and resistivity probes) to aid in their interpretation. Algorithms will need to be developed to interpret these signals and associate them to detected events. The objectives of the proposed effort include: Objective 1: To provide input in the design of the test bed to be installed at the Black Hawk site. The collaborators will provide the DSS interrogator to be installed at the site to record measurements over the length of the project. Objective 2: Aid in the analysis and interpretation of the data acquired using the DSS interrogator. This includesthe development of algorithms to record data, process data for continuous monitoring and display results.

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