Fiber Optic Monitoring in Mines

ABSTRACT: Fiber-optic cable can be deployed in underground mines over kilometers in distance. The sensing interrogator and data acquisition can be operated remotely using lead-in fiber. Fiber-optic sensors can be either discrete or distributed. Fiber-Bragg Grating (FBG) sensors were grouted into the walls and roof of a small alcove at a depth of 4100 ft (1250 m) to measure deformation while jacking the roof (Gage et al., 2014). The modulus as a function of depth showed the profile of the Excavation Damage Zone (EDZ). Although FBGs can be embedded into long runs of optical fiber to create a quasi-distributed fiber-optic sensor, Brillouin and Rayleigh optical scattering can turn optical fiber itself into Distributed Strain Sensing (DSS) and Distributed Acoustic Sensing (DAS) arrays, respectively. DAS has the greatest potential for mine monitoring. A 250- meter-long DAS array in the Lafarge-Conco mine in northern Illinois demonstrated the potential for using seismic tomography for monitoring velocity changes in pillars of a limestone mine (Zeng et al., 2022). At SURF we deployed 3050 meters of cable along the mine's ramp system between depths of 4100-ft (1250 m) and 4850-ft (1478 m). The total fiber path of 5440 meters included internet fiber to connect the DAS interrogator on the 4100-ft level to the cable on the ramp. Data were collected over a two-week period in May 2022 and the month of February 2023. We recorded numerous excavation blasts and mine activities, including cage movement, air doors opening and closing, and foot traffic, as well as regional and teleseismic earthquakes (Cunningham et al., 2023; Fratta et al., 2023). The DAS array distinguished the details of blast timing pattern, amplitude and duration. The importance of cable coupling to the mine floor was studied by comparing recordings on a 230-meter section of cable that was loosely coupled on the surface in 2022 but grouted into a groove a few centimeters deep in 2023.