Distributed acoustic sensing along a shallow water energy cable

Distributed acoustic sensing (DAS) provides a means of measuring dynamic changes in strain along a fiber-optic cable and has many potential applications for monitoring infrastructure, earthquake early warning, and hazard assessment. Previous work has focused on submarine telecommunications cables, which contain only fiber-optic cables. Here, we focus on the use of energy cables, which transmit electricity from offshore generators powered by tides or wind and contain fiber-optic cables for communications with the generators. Specifically, we focus on the European Marine Energy Center in Orkney, Eday, U.K., a tidal power station. Energy cables fluctuate in temperature due to energy transmission, and there is strong wave action and tidal flows, which all generate noise for DAS. We show that noise levels vary along the cable during a time with no energy transmission, but many phenomena reported on telecommunication cables are still observable, including ocean waves and nearby small vessels. The character of the small vessel signals in frequency band energy plots varies along the cable length, in some areas exhibiting multiple frequency band energy peaks. This variation is diagnostic of the burial state of the cable. Knowing the burial state of energy cables is important for understanding the mechanical protection of the system for minimizing thermal interactions with the surrounding environments and ecosystems.