Permafrost Monitoring with Brillouin Based Fiber Optic Distributed Strain and Temperature Sensing: Findings from a Controlled Thaw Experiment

Permanently frozen ground or permafrost covers vast areas of the earth's polar regions. Global warming has led to increased rates and areas of permafrost thaw; this process can lead to damage of infrastructure due to the soil subsidence as well as promote the release of greenhouse gases into atmosphere. Novel techniques are needed to monitor the state of the permafrost over long distances, especially near infrastructure. In this study, we present the results of monitoring mechanical and thermal changes due to permafrost thaw with a fiber optics sensing technique based on Brillion optical time domain analysis (BOTDA), also known as distributed strain sensing (DSS). Using this technique, one can measure axial strain and temperature distribution along the fiber over tens of kilometers with one-meter spatial resolution.

In 2016, Lawrence Berkley National Laboratory in collaboration with Cold Regions Research Engineering Laboratory conducted a unique artificial permafrost warming experiment in Fairbanks, Alaska. 121 heaters warmed the subsurface at a depth of 3 m, perturbing a soil volume of 10.5 x 12.7 x 1.5 m³ for three months. Permafrost thawed in the heated area, leading to a measured surface subsidence of 10 cm. Commercial fiber optic cable was buried in a shallow trench directly above the heater array. 2700 m of the cable were used as a sensor for DSS measurements. We clearly observe the subsidence process in the DSS data. In the center of the subsidence zone we see the negative shift in Brillion frequency, which corresponds to compression along the fiber, while on the perimeter of the subsidence zone we observe the positive shift, which indicates tension. The measured response is similar to prior DSS observations in sinkhole studies. Besides the strain measurements, we also estimated the soil temperature using the BOTDA data from the unstrained parts of the fiber (outside the subsidence zone). The soil temperature inverted from BOTDA data is in good comparison with the temperature measured by thermocouples installed in the experiment area. We believe that our study Is the first experiment which demonstrates successful usage of DSS as an early detection system for permafrost thaw processes. We believe that this technique can be widely applied for monitoring critical infrastructure in permafrost regions.