Tailings Dam Performance Monitoring by Combining Coda Wave Interferometry with Distributed Acoustic Sensing

Advances in distributed fiber optic sensing technologies are enabling new methods to monitor changes in tailings dam performance. Distributed acoustic sensing (DAS), a distributed fiber optic sensing technology relying on Rayleigh light backscattering, can provide continuous spatial and temporal coverage along the length of a fiber optic cable extending tens of kilometers. In 2019, nearly six kilometers of fiber optic cable were installed at ∼1 m depth along an active upstream tailings dam in northern Canada. DAS seismic data were acquired at 400 Hz over a four-month period, from April to August 2021. We applied coda wave interferometry to a 120 m cable segment to obtain relative changes in seismic velocities (dv/v). Such coda waves are typically dominated by Rayleigh surface waves and dv/v can be used as a proxy for shear wave velocity changes. The dv/v estimates decrease by up to ∼1.9% over an initial two-month period of spring thaw and rainfall. Subsequently, dv/v recover by ∼1%, and generally show an inverse correlation with tailings pond levels up until the end of data acquisition. This correlation is supported by a known power-law relationship between shear wave velocity and effective stress. Rayleigh surface wave sensitivity kernels incorporating nearby seismic cone penetration testing data are used to estimate the approximate depths of dv/v sensitivity at ∼10 m. Despite active construction causing noise contamination, we obtain stable cross-correlation waveforms with as little as one hour of data per day. Overall, our results demonstrate how DAS can be used to augment geotechnical monitoring networks by providing in situ estimates of dv/v to inform changes in tailings dam performance over time.