Geo-Acoustic Doppler Spectroscopy: A Novel Acoustic Technique For Surveying The Seabed

An acoustic inversion technique, known as Geo‐Acoustic Doppler Spectroscopy, has recently been developed for estimating the geo‐acoustic parameters of the seabed in shallow water. The technique is unusual in that it utilizes a low‐flying, propeller‐driven light aircraft as an acoustic source. Both the engine and propeller produce sound and, since they are rotating sources, the acoustic signature of each takes the form of a sequence of narrow‐band harmonics. Although the coupling of the harmonics across the air‐sea interface is inefficient, due to the large impedance mismatch between air and water, sufficient energy penetrates the sea surface to provide a useable underwater signal at sensors either in the water column or buried in the sediment. The received signals, which are significantly Doppler shifted due to the motion of the aircraft, will have experienced a number of reflections from the seabed and thus they contain information about the sediment. A geo‐acoustic inversion of the Doppler‐shifted modes associated with each harmonic yields an estimate of the sound speed in the sediment; and, once the sound speed has been determined, the known correlations between it and the remaining geo‐acoustic parameters allow all of the latter to be computed. This inversion technique has been applied to aircraft data collected in the shallow water north of Scripps pier, returning values of the sound speed, shear speed, porosity, density and grain size that are consistent with the known properties of the sandy sediment in the channel.