Kilometer-resolution monitoring of Earth’s tidal response on the deep seafloor using fiber sensing

   Tidal deformation of the Earth’s surface results from the addition of two distinct processes. The first, known as the Solid Earth Tide (SET), corresponds to the deformation of the solid Earth caused by the gravitational attraction of the Moon and the Sun. The second, Ocean Tide Loading (OTL), arises from the redistribution of oceanic mass associated with tides, which imposes a variable load on the seafloor and surrounding crust, thereby inducing additional time-dependent deformation. Monitoring this response is crucial in geodesy for estimating the elastic and mechanical properties of the shallow Earth’s crust, for correcting geodetic measurements, and for constraining ocean tide models. On the other hand, tidal triggering of earthquakes suggests that Earth’s tidal forces influence seismic activity, particularly in the oceanic crust, highlighting the need to measure Earth tides in the deep ocean.
   
   However, the seafloor tidal response in the deep ocean remains sparse and poorly constrained due to the logistical challenges associated with continuous deployment of sensors in such an extreme environment. Here, we demonstrate that Distributed Acoustic Sensing (DAS) is able to monitor Earth tides in the deep sea. While DAS is challenged by high instrumental noise and environmental thermal fluctuations at low frequencies (< 0.01 mHz), we achieve a sensitivity on the order of picostrain per second from a submarine cable in the Mediterranean Sea by leveraging a signal processing approach for low-frequency noise suppression and the thermal stability of the deep Mediterranean Sea. While standard noise removal, an essential step in data pre-processing attenuates part of the Earth tide signals, it ultimately improves the continuous monitoring of Earth tides over distances of tens of kilometers, with kilometer-scale spatial resolution. The results from our measurements align closely with theoretical predictions. These findings validate the efficacy of Distributed Acoustic Sensing at extracting sub-nanostrain signals at periods exceeding several hours and demonstrate that DAS can serve as a new tool for seafloor geodesy applications.