Distributed Fiber Optic Sensing in Cold Regions

Distributed fiber optic sensing (DFOS) technology transforms a fiber optic cable into a temporally and spatially dense sensor. When connected to a DFOS interrogator, kilometers of fiber optic cable become capable of continuous spatial monitoring of in situ variables such as vibrational strain (via distributed acoustic sensing, DAS) and temperature (via distributed temperature sensing, DTS). The USACE, ERDC, CRREL is studying the application and performance of DFOS as a geotechnical engineering monitoring tool in cold regions. A field-testing program at a soil DAS test bed monitored the seasonal effect of below-freezing temperatures. Seismic source impacts were performed along the DAS array, and response to impact was quantified using signal-to-noise ratio (SNR) as a performance metric. The array was observed to perform similarly before and after a blizzard. Results from ongoing cold regions DAS research studies of fiber optic cable installations in pond ice and at CRREL's permafrost tunnel in Fairbanks, Alaska, support findings that DAS arrays perform well in cold regions. The ice-installed cable was continuously monitored during a breakthrough failure event, and results indicated that the DAS array responded to crack formation prior to the breakthrough. A DTS array was installed to monitor underseepage and the efficacy of a cutoff wall in a cold region earth embankment dam due to changing permafrost conditions. The DTS installation demonstrated significant temperature variability in an area of known structural instability, while structurally sound portions of the dam exhibited more consistent temperatures. These DFOS studies indicate that technologies such as DAS and DTS are useful cold regions geotechnical monitoring tools to help localize and remotely monitor in situ changes along kilometers of fiber optic array.