Search

Shallow‐water acoustic and geoacoustic measurements were made in the southern Adriatic Sea in May–June 1995 in combination with detailed oceanographic, geological, and geophysical surveys. Seismic subbotom profiling, narrow‐beam echosounding, and bottom coring results were used to select two shallow water sites with downward refracting conditions for acoustic experiments: (1) a flat seafloor at 90‐m depth with about 1 m of rigid, nongaseous clay over a sequence of laminated clays and (2) a seafloor sloping from 49‐ to 37‐m depth and consisting of soft gaseous mud over laminated clay. Broadband sparker and 180‐g TNT explosive sources were used for propagation and reverberation to a vertical array receiver in addition to cw sources for propagation. Initial results for the deeper water site show frequency‐independent propagation that is consistent with modeled results using geoacoustic parameters extrapolated from coring results. A modification of the Dix method to estimate sediment sound speed versus depth ...

During the Acoustic Reverberation Special Research Program (ARSRP) acoustic experiment of July 1993, an experiment was performed to identify seafloor scattering mechanisms. Explosive SUS sources were deployed on the western flank of the Mid‐Atlantic Ridge to provide omnidirectional broadband sound for long‐range reverberation measurements. In addition, pulsed cw tones in the frequency band 200 to 280 Hz from a vertical source array were used. The signals scattered by the seafloor were received by two 64‐wavelength subapertures of a towed array, processed to form beam‐time series, and then displayed as reverberation images on map projections. In order to obtain good signal‐to‐noise ratios, this analysis was performed in frequency bands of 4 Hz (time resolution 0.25 s) and then averaged to form beam‐time series of 20‐Hz bandwidth at harmonics of the bubble‐pulse frequency. Initial results show that the cw measurements are consistent with the broadband results and that scattering strengths of specific seafloor features are generally constant over the frequency band 100 to 1000 Hz. [Work supported by ONR.]

Signals from an explosive source backscattered from the seafloor and received at long range by hydrophones of a towed array are processed to estimate the directional distribution of energy for a given time increment. As assembly of these data shows the time and amplitude of scattering features, and after conversion to distance, the geographic location of the return. A frequency-domain beam-forming procedure is used in which beam levels are averaged over a given band of a broad-band source. The processing is applied to experimental data obtained in the southern Tyrrhenian Sea. The major backscattering occurred at the Baconi Seamounts and the coastal margin of Sardinia.

Backscattering from the seafloor from an explosive sound source is received by an array and processed to form images of backscattering features of the seafloor and to estimate scattering strength. The images are helpful in determining parameters used for calculating scattering strengths of specific physiographic features and understanding the results. Images of some features are improved by processing with a non-linear technique. Reverberation measurements in the Tyrrhenian Sea show that scattering from the coastal margin occurs in two distinct zones. Scattering strength increases slowly with frequency over the 700 Hz band.