Detecting whale vocalizations up to 2.5 kHz using a seafloor distributed acoustic sensing array with incorporated loopsa)

Distributed acoustic sensing (DAS) is an increasingly popular tool for marine mammal monitoring. Until recently, most monitoring efforts targeted lower frequency sounds made by large baleen whales and were not optimized for the mid-frequency signals made by many other marine mammals. This is partially due to the directivity of the DAS system and the spatial averaging effect of the gauge length, which acts as a low-pass filter, markedly decreasing signal amplitude above 300-500 Hz, depending on the angle of incidence. Adding loops in the cable can mitigate this by integrating the strain around the circumference of the loop to improve sensitivity at higher frequencies. This study deployed a custom fiber optic cable with incorporated loops in Haro Strait (Salish Sea, WA) and recorded signals up to the Nyquist frequency of 2.5 kHz in looped sections, compared to approximately 500 Hz in straight sections. The study successfully recorded mid-frequency (0.5-2.5 kHz) vocalizations from Southern Resident killer whales, Bigg's killer whales, and humpback whales, and stacking channels within loops improved the signal-to-noise ratio by approximately 8 dB on average. These results show that a looped DAS array can supplement existing hydrophone networks or serve as a standalone tool when a large hydrophone array is not feasible.