Ocean Flow Monitoring Offshore the Oregon Coast with Distributed Acoustic Sensing

A wide range of water movements happen in oceans, including ocean surface gravity waves, tides, internal gravity waves and currents. Ocean Bottom Distributed Acoustic Sensing (OBDAS) experiments probe offshore fiber-optic cables to measure ground deformations, and therefore provide us with an unprecedented way to monitor water flows. In this study, we analyze four months of DAS data recorded using a telecommunication cable deployed offshore the coast of Oregon, USA. The data were recorded along the first 70 km of the fiber between August and December 2021 with a Febus interrogator. The gauge length was set to 40 m, the channel spacing to 20 m, and the sampling rate was 100 Hz. We first use a cross-correlation approach of the continuous DAS data to track the spatial and temporal evolution of ocean gravity waves. Cross-correlation functions (CCFs) are computed between the FK filtered 20-min data windows recorded by stations spaced by a few hundreds of meters to separate the shoreward and oceanward propagation of ocean gravity waves. We then compute dv/v measurements with regards to a reference waveform, which is the average of all the CCFs for each pair of stations. We observe clear temporal changes that correlate well with weather and buoy measurements. In addition, the CCF approach provides us with a proxy to monitor tides near the shore. Finally, we present a way to detect internal gravity waves in OBDAS data. This study shows that OBDAS can be used to monitor physical phenomena happening inside the water column and could also be a powerful tool for oceanographers.