Using Distributed Acoustic Sensing with an Optical Fibre Cable in Galway Bay for Ocean Noise Monitoring

The oceans are a large part of our planet’s environment with highly diverse acoustic and seismic noise fields. These noise fields are made up of a plethora of natural (marine wildlife, ocean microseisms, earthquakes) and anthropogenic (ship traffic, seafloor construction) sources. It is very costly and time consuming to deploy and maintain offshore seismic and acoustic sensors to study this wide array of sources. Distributed acoustic sensing (DAS) applied on submarine fibre optic cables offers an unprecedented spatial resolution within the ocean environment for detailed analysis of seismic and acoustic noise at a relatively low cost.

Within this research project a 10 day long DAS dataset was acquired in June 2023 to characterise the submarine noise field of Galway Bay. The acquisition was performed using a Febus Optics interrogator on an optical fibre (5.56km length) connected to the Galway SmartBay offshore laboratory, located off the coast of Spiddal, Co. Galway, Ireland. In order to understand the cable sensitivity to a variety of different signals, both seismic and acoustic, present in the bay during the experiment, we compare the DAS data with data from other instruments such as seismometers (Irish National Seismic Network), hydrophones and wave buoys (both Galway SmartBay and Marine Institute Ireland).

The strain wavefield recorded with the DAS is being studied by isolating channels and treating them as stand alone sensors, and also utilising the densely spaced nature of DAS data to perform Frequency-Wavenumber (FK) analysis. Within the data there is a dominant signal between 0.1-0.3Hz, which is caused by the OSGW. Beamforming has been used to resolve the direction of propagation of these OSGW, which has been compared to the SmartBay wave buoy data. There is also a packet of ambient high frequency, 3.5-5.5Hz, Scholte waves present at intervals throughout the acquisition. Using FK analysis, a frequency vs phase velocity plot has been generated and shows a clear dispersion curve in the velocity range 300-1000m/s. There is also the presence of a, as of yet, undetermined signal in the 0.5-1.5Hz band. The presence of both of these waves (Scholte and 0.5-1.5Hz signal) aligns temporally with increases in the significant and maximum wave height recorded by the SmartBay wave buoy. The possibility of the increase in ambient seismic noise as a result of rougher sea conditions is being investigated further as such. Finally, during the acquisition there were several acoustic events picked up by the SmartBay hydrophone, these included ships passing overhead as well as North Atlantic minke whale calls. Unfortunately these signals were not observed on the DAS data despite the use of several techniques attempting to isolate them spatially and temporally (e.g. velocity filtering).