Underwater measurements of a sonic boom

During a sea trial on the Scotian Shelf, acoustic signals from an unexpected sonic boom were recorded on several hydrophones of a vertical array. The array spanned the lower 50 m of the water column above a sand bank at 76 m water depth. Based on the event time and location, the source of the sonic boom was deduced to be a Concorde supersonic airliner in transit between Paris and New York, travelling at about Mach 2 (roughly 600 m/s). The airborne sonic boom was heard on the deck of the research vessel during a routine collection of ocean ambient noise samples; the water-borne sonic boom was recovered later during playback of the recording tape for that sample. The horizontal speed of a sonic boom waveform-which matches the aircraft speed-is lower than the speed of sound in the water if the aircraft speed is below about Mach 4.4; the associated water-borne waveform is expected to decay as an evanescent wave below the sea surface. This decay of the amplitude of the waveform is observed along the array. The very calm weather resulted in low ambient noise and low self-noise at the hydrophones, resulting in good signal-to-noise ratio on the upper hydrophones; however, the decreased signal amplitude is more difficult to detect towards the lower part of the water column. The period of the observed waveform is of the order 0.16 s, corresponding to a peak frequency of about 6 Hz. The shape of the measured waveform differs slightly from the theoretical N-shape waveform predicted with Sawyers theory [J. Acoust, Soc. Am., 44, 523-4 (1968)]; this is possibly due to propagation effects caused by seismo-acoustic interaction of the infrasonic waves with the sediments underlying the water mass.