Light aircraft sound for underwater acoustics experiments

Abstract

A propeller-driven light aircraft produces sound in the form of a sequence of harmonics from the propeller and from the reciprocating engine. The lowest frequency, from a two-bladed propeller, is around 80 Hz with detectable harmonics extending up to about 1 kHz. When flying over the shallow ocean at low level, some of the aircraft sound penetrates the air-sea interface to undergo multiple reflections between the seabed and the sea surface as it propagates through the channel. On approach to a sub-surface receiver station, the frequency of a given harmonic is Doppler upshifted, whereas on departure it is downshifted. Since these Doppler shifts depend not only on the speed of the aircraft but also on the geo-acoustic properties of the seabed, they provide the basis of a rapid and efficient inversion technique for surveying the seabed. To develop the technique, shallow-water experiments have been performed using various types of light aircraft as the sound source and a single hydrophone as the receiver. It was found that, with the aid of the grain-shearing theory of wave propagation in unconsolidated sediments, it is possible to recover the sediment geo-acoustic parameters from the Doppler-shifted aircraft harmonics. [Research supported by ONR.]