Utilizing existing telecommunication cables for Distributed Acoustic Sensing (DAS) experiments has eased the collection of seismological data in previously difficult-to-access areas such as the ocean bottom. To assess the potential of submarine DAS for monitoring seismic activity, we conducted an experiment from mid-October to mid-December 2021 using a 45 km long dark fiber extending from the Greek island of Santorini along the ocean bottom to the neighboring island of Ios. This region is of great geophysical and public interest because of its historical and recent seismic and volcanic activity, especially along the Kolumbo volcanic chain. Besides recording anthropogenic noise and around 1,000 seismic events, we observe the primary and secondary microseisms in the submarine section, the latter inducing Scholte waves in a sediment layer where the cable is well-coupled. By using the spectral element wave propagation solver Salvus, we compute synthetic strains for earthquakes with varying degrees of model complexity. Despite including topography, a water layer, and a heterogeneous velocity model, we are unable to reproduce the lack of coherence in our observed earthquake waveforms. Backpropagation simulations for four observed earthquakes indicate that clear convergence of the wavefield, and thus the ability to constrain a source region, is only possible when all model complexities are considered. We conclude that, despite the promising emergence of DAS, monitoring capabilities are limited by often unfavorable cable geometries, cable coupling, and the complexity of the medium. Interrogating multiple cables simultaneously or jointly analyzing DAS and seismometer data could help improve future monitoring experiments.

中文翻译：

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海底光纤地震监测面临的挑战


利用现有的电信电缆进行分布式声学传感 （DAS） 实验，简化了以前难以进入的区域（如海底）的地震数据收集。为了评估潜艇 DAS 监测地震活动的潜力，我们在 2021 年 10 月中旬至 12 月中旬使用一根 45 公里长的暗光纤进行了一项实验，该光纤从希腊圣托里尼岛沿海底延伸到邻近的伊奥斯岛。由于其历史和最近的地震和火山活动，尤其是 Kolumbo 火山链沿线，该地区具有重大的地球物理和公共利益。除了记录人为噪声和大约 1,000 次地震事件外，我们还在海底部分观察了初级和次级微震，后者在电缆耦合良好的沉积层中诱发了 Scholte 波。通过使用谱元波传播求解器 Salvus，我们计算了具有不同模型复杂度的地震的合成应变。尽管包括地形、水层和非均匀速度模型，但我们无法重现观察到的地震波形中缺乏连贯性的情况。对 4 次观测到的地震的反向传播模拟表明，只有在考虑所有模型复杂性的情况下，波场的清晰收敛以及约束源区域的能力才有可能。我们得出的结论是，尽管 DAS 的出现前景广阔，但监测能力受到通常不利的电缆几何形状、电缆耦合和介质复杂性的限制。同时询问多条电缆或联合分析 DAS 和地震仪数据可能有助于改进未来的监测实验。