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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.]

A narrow‐beam scanning sonar was used to measure the relative backscattering strengths at 48 kHz of the undersurface of arctic sea ice. The graphic records displaying the range and relative scattering levels were assembled into a sonar map that displays the location and shape of under‐ice features. The data indicate that there are two distinct types of backscattering: (1) very high level backscattering from well‐defined under‐ice ridges and (2) very low level backscattering from between the ridges. The higher scattering the ridges is probably due to the increase in roughness and the tilting of the average plane of the scattering surface. Comparison of the sonar map and the aerial photograph shows that most surface features have subsurface expressions and that their relationship can be complex.

Under-ice sonar surveys were carried out in pack-ice fields near Fletcher’s Ice Island and at two sites north of Pt. Barrow, Alaska, U.S.A. A narrow-beam scanning sonar was used to measure the location and relative back-scattering of features on the under surface of Arctic sea ice. The 48 kHz sonar had a 1.5° by 51 ° beam width. Graphic records displaying the range and relative scattering levels were assembled into sonar maps which display location and shape of under-ice features. Two distinct types of back-scattering were found: (1) very high-level back-scattering from well defined under-ice ridges and (2) very low back-scattering from areas between ridges. Higher scattering at ridges was probably due to an increase of roughness and tilting of the average plane of the scattering surface. To measure depths of features, the sonar transducer was adjusted to give a wide horizontal beam and a narrow vertical beam. Polar scans were taken at several depths of the transducer to determine depths of ridges. The tops and bottoms of features were compared and the average ratio of peak elevation to keel depth was about 1:7. Fuller accounts of some of this work have been published elsewhere (Berkson and others, 1973; Clay and Leong, 1974; Kan and others, 1974}.

Sea surface scattering experiments were conducted in which sound from underwater explosives was reflected from the sea surface at grazing angles 5° to 40° and was received by vertically separated sensors of a hydrophone array. The sensor separation varied from l0 to 250 m. The surface‐reflected arrivals were isolated and processed to obtain estimates of spatial coherence and reflection‐loss spectra for frequencies 20 to 250 Hz. The coherence estimates were obtained as a function of frequency, grazing angle, sensor separation, and bearing for different sea surfaces of low sea state. Simultaneous with the acoustic experiment, ocean wave height time series were measured by three oceanographic data buoys operating in the general region of the experiments. Processing of these data yielded estimates of the ocean wave roughness spectra. Sea surface roughness estimates from the acoustic data were consistent with the measured sea surface roughness. [Work supported by NORDA and Naval Electronic System Command.]

Coherence of bottom‐reflected sound in the Tagus Abyssal Plain was determined as a function of bearing at frequencies 50–500 Hz for a fixed grazing angle of 15°. SUS charges were dropped at 244 m depth in a circular pattern of 28 km radius around a receiver at 2500 m depth in water 5100 m deep. The bottom reflected signals were isolated and processed to remove the decorrelating effects of varying bubble pulse periods. Computed was a coherence function ρ(ω,φ) = |Smn(jω)|4/Sm(ω)Sn(ω), where S is a sample estimate of the power spectral density, j is −1, ω is the radian frequency, and m, n are the indices of adjacent shots at an average bearing φ. For a given frequency, the measured coherence function vs bearing exhibits a degree of symmetry in which features occur in pairs 180° apart. This observation suggests a directionally dependent, coherence‐degrading effect operating at the bottom, in the water column, or both, and significant over an aperture of 5 km. [Work supported by NORDA Ocean Programs Office.]

Acoustic reflectivity experiments were conducted over two different ocean bottom types in the northeast Pacific. Explosive sources were dropped at ranges up to 50 km from a vertical hydrophone array that was suspended near midwater depth. The experimental geometry allowed time isolation of the bottom‐interacting arrival. The bottom‐interacting arrival received by the array was processed to obtain spatial coherence for frequencies in the 20‐ to 250‐Hz band. The behavior of spatial coherence vs frequency, grazing angle, and sensor separation is distinctly different for the two sites. At the Abyssal Plain site, coherence is nearly 1.0 at grazing angles 10° to 25°, decreases with increasing grazing angle until about 60°, and then increases after 60°. At the Abyssal Hills site, coherence is low (about 0.3 for 280‐m sensor separation) and variable for 10° to 60°, and increases after 60°. The results are consistent with the combined effects of scattering from a rough interface and changing sensor geometry with g...