EXTRACTION OF THE SEABED REFLECTIVITY FUNCTION USING ICE CRACKING NOISE AS A SIGNAL SOURCE

A technique is described for measuring the reflection coefficient of t he Arctic seabed with a single vertical array of hydrophones. Naturall y occurring ice cracking noises were used as the acoustic sources. Thi s method circumvents the difficulties and expense of introducing artif icial sound sources through the thick Arctic pack ice. The measurement s were made in April 1988, with a 22-element array suspended from the ice in 420 m of water. The range of the source is first determined usi ng the direct arrival and multiple reflections from the seabed and und erice surface. Then the source directivity is parametrized using the d irect arrival path only, and extrapolated to reflected path angles. A plot of the reflection coefficient versus grazing angle clearly indica tes the value of the critical angle. The sound speed of the sea bottom corresponding to this critical angle agrees well with that measured f rom a bottom grab sample taken during the field trip. Finally, an inte resting phenomenon was an anomalous increase of the received pressure level around a grazing angle of 60'. This is associated with a leaky p late wave radiating at the ice-water interface.