Normal-incidence bottom and sub-bottom reflection in the Canada basin of the Arctic Ocean

The normal-incidence reflection coefficient of a patch of seabed in the Arctic Ocean was analyzed using the signal from a source colocated with a vertical line array. A mooring in the Canada Basin of the Arctic Ocean in approximately 4000 m water depth, contained a sound source 54.7 m below the water surface and a vertical receiving array below it. The source transmitted a pulsed sine wave at nominally 35 Hz once every 3 days for over a year. Bottom and sub-bottom reflected signals were clearly detected. The baseband demodulated signals were recorded and used in the analysis. The results showed that the bottom reflection was very stable at an average reflection loss of 13.5 dB, with an annual sinusoidal variation of 0.15 dB. Three sub-bottom returns were detected. The first is likely a bottom-simulating reflector with significant phase reversal, probably due to free gas beneath a hydrate layer. Fluctuations in the first and second sub-bottom returns are likely due to the changing bubble population beneath the hydrate layer. The third sub-bottom return was very strong and highly variable in both arrival time and amplitude, indicating a very rough sediment-basement interface.