Abstract:

Recent experiments were conducted in the Mediterranean Sea with the intention of developing a better understanding of the physics of acoustic penetration into and scattering from the seabed. Two shallow water sites were studied, one near Viareggio, Italy and the other off the coast of Sardinia. One site consisted of a fine mixture of silt and sand, with a large amount of trapped gas, while the other included a layer of marine vegetation that produced a bright acoustic return ahead of the bottom echo. Echo soundings were collected with a broad beam impulsive sound source operating over a band between 1 and 15 kHz. The data acquisition system was configured to record the acoustic pulse before and after encountering the seabed, from which a calibrated measure of the seabed's reflective character was obtained. The measured reflection coefficient was compared with model predictions, based on a poroelastic theory for the seabed. In general the measured reflection coefficients exceeded the predictions significantly. A likely hypothesis is that trapped gas bubbles act to enhance the reflection. A description of various signal processing techniques that were employed will be included, along with a discussion of relationships between the processed acoustic data and known structural features. [Work supported by ONR.]