Sound from a light aircraft for underwater acoustic inversions

Experiments are being conducted in shallow (30 m) water off La Jolla, CA, to investigate the potential usefulness of the sound from a single-engine, propeller-driven, light aircraft for performing underwater acoustic inversions. The sound signature of the aircraft contains harmonics between 50 Hz and 1 kHz, which return the low-frequency geoacoustic properties of the seabed. A microphone approximately 1 m above the surface monitors the sound in air, a seven-element vertical array detects the acoustic arrivals underwater and a single, buried hydrophone receives the signals in the sediment. Aircraft overflights have been made at altitudes between 33 m and 330 m, yielding the altitude-dependence of the peak levels received underwater. Using the vertical array, the reflection coefficient of the seabed is being measured as a function of grazing angle. From the reflection coefficient, the critical angle of the sea floor and hence the sound speed in the sediment are inferred. The sound speed in the sediment should also be available directly from the Doppler shift on the buried hydrophone. These techniques and the available data sets will be discussed in the presentation. [Work supported by ONR.]