Downhole Distributed Acoustic Sensing Provides Insights Into the Structure of Short‐Period Ocean‐Generated Seismic Wavefield.

Ocean‐generated seismic waves are omnipresent in passive seismic records around the world and present both a challenge for earthquake observations and an input signal for interferometric methods for characterization of the Earth's interior. Understanding of these waves requires the knowledge of the depth dependence of the oceanic noise at the transition into the continent. To this end, we examine 80 days of continuous acquisition with distributed acoustic sensor (DAS) system deployed in two deep boreholes near the south‐eastern coast of Australia. The iDASv3™ system deployed in a deep borehole at the CO2CRC Otway Project site provides sufficiently high sensitivity and low instrumentation noise for frequencies from 100 mHz to 20 Hz. Analysis of the seismograms and correlation with wave climate allows decomposing the DAS response into microseisms generated by swell from remote storms (∼ $\sim $0.15 Hz) and local winds (between 0.2 and 2 Hz), and strong body wave energy from large surf breaks at the coast (from 2 to 20 Hz). The depth dependence of the microseisms provides useful insights into the energy partition between the Rayleigh wave modes and may augment conventional kinematic analysis of the sparse surface seismological arrays. Overall, ocean‐generated signals at each channel along the borehole are strongly related to the wave climate, so that—with sufficient amount of training data—the passive seismic records on several downhole DAS sensors has a potential for high‐precision monitoring of formations surrounding the borehole as well as remote storms in the ocean. Plain Language Summary: This study attempts to decipher the complex ambient seismic wavefield in coastal regions. The microseisms have been under scrutiny for a long time: first, as a noise in earthquake monitoring systems and, later, as a target signal used for characterization of the upper Earth crust. The study relies on a unique data set acquired in a deep borehole using the most advanced distributed acoustic sensing (DAS) system deployed at the coast of Victoria (Australia). Compared with the conventional seismological networks, this new measurement technology adds a new dimension to the data–depth variation in the broad frequency range of the parameters of the seismic waves. Using just one borehole, we identified and quantitatively characterized the multitude of ambient seismic sources at the Victorian coast: remote storms, local winds, and surf breaks. Our measurements show a reasonable agreement with the established nonlinear models that predicts the statistical characteristics of seismic wavefield ocean waves. We found that the link between the amplitudes on DAS along the borehole and wave climate is so stable that with sufficient amount of training data, the passive seismic records may be used for high‐precision monitoring of both—formations surrounding the borehole and remote storms in the ocean. Key Points: A DAS system in a single borehole to identify and quantitatively characterize the multitude of ambient seismic signals at the Australian coastSeismic measurements are in a good agreement with theoretical predictions from the ocean waves spectraThe link between the DAS response and wave climate enables monitoring both the formations surrounding the borehole and remote storms in the ocean [ABSTRACT FROM AUTHOR]