Quantitative Seabed Surface Wave Characterization Using Distributed Acoustic Sensing

Summary Bottom trawl fishing is a potential threat to subsea cables. If the trawl is towed across a shallow buried cable, the cable can be damaged by impact or by dragging. Distributed Acoustic Sensing (DAS) is an emerging technology capable to measure the strain in optical fibers from acoustic waves. We have analyzed data from a North Sea submarine optical fiber communication cable and characterize the propagating waves excited by a bottom trawl. A model describing seismic modes in a waveguide comprising shallow water over elastic sediment is derived and seabed parameters are found that allow to fit the theoretical modal dispersion curves to the field data. This shows that the seismic modes can be detected and resolved by DAS recordings. The result substantiates many interesting DAS applications, including geophysical investigations and surveys, protecting communication cables from trawling and other types of subsea activity, subsurface velocity profiling and monitoring, and classification of acoustic sources e.g. discrimination between sources acting on the sea floor and those located in the water column.