Water column sound speed estimated from ambient sound measurements recorded on a vertical array of hydrophones

In the ice-free Arctic waters near the island of Jan Mayen (Nordic Seas), wind and wave-generated sound is used to estimate the sound speed between vertically separated hydrophones. The experimental data were recorded on a 52-element evenly spaced vertical line array moored on Jan Mayen Ridge, an undersea volcanic ridge with a depth of approximately 420 m at the array location. The coherence between vertically separated hydrophone pairs, sampled at a frequency of 8096 Hz and averaged over the 24-min recording period, provides sufficient resolution in the cross-correlation shape to determine features related to the acoustic travel time between hydrophones. The impact of hydrophone-pair spacing on sound speed estimates is explored, from the smallest separation distance of 7.25 m, determined by array design, to distances exceeding 100 m in high-wind conditions. The sound speed profile was estimated every 4 h over the yearlong experiment, and the fluctuations compared well with measurements made from thermistors mounted along the array. The method is robust, enabling accurate estimates (RMSE < 1 m/s) in a wide variety of environmental conditions over the course of the experiment, including low wind conditions, flow noise, and array tilt.