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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.]

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.

An underice survey was carried out in a pack ice field near Fletcher's Ice Island (T-3) by means of a 48-kHz Kelvin Hughes transit sonar. The transducer was lowered through a hole, and discrete underice scans were made through 360°. Five sets of data have been collected and transformed into polar coordinate mosaics. The echo length, the shadow zone, the arrival time of echoes, and the sounding depth provide the basic information for interpretation. A complete underice map has been constructed to show the distribution of the underice features. The keel depths of the ridges have been estimated. A surface survey that included a cross-ridge survey was carried out. The top and bottom features have been compared. The average ratio of the peak top elevation to the keel depths is 1:7.6.

A wind-blown water surface can be characterized in terms of a traveling correlation function in which the group and phase velocities are given by the values at the frequency of maximum spectral energy density. The two-dimensional (space and time) surface displacement correlation function has been measured for low-divergence wind-blown waves in a laboratory tank. The measured phase and group velocities of the correlation function agreed with theoretical values for the peak frequency water-wave component when the measurements were transformed to coordinates that had the velocity of the surface (drift) current. This description, which may be useful in the ocean as well, has recently been used to predict the temporal correlation of sound scattered from a model sea of known three-dimensional surface correlation function (Clay and Medwin, 1970).

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.

Long-range images of the seafloor are obtained by using low-frequency sound signals backscattered by the ocean bottom and received by an acoustic array. The technique differs from the side-scan sonar mapping method in that the source is omnidirectional and directional receiving beams are formed from forward to backward end fire by processing signals from hydrophones of the array receiving sound scattered from large-scale topographic features at long range via low grazing angle propagation. For processing, the hydrophone signals are split into time increments. For each increment, a beamforming procedure is used to obtain the angular distribution of energy over a given frequency band. A display of these data shows the time and amplitude of the scattering features, and after conversion to distance, the geographic location of the returns. The spatial resolution is a function of the size of the in sonified area of the seafloor and the method of scanning. However, the ultimate resolution is further constrained by the sound speed structure, bathymetry, and experimental geometry. Processing the beamformed data by an iterative high-resolution method appeared to improve the angular resolution of the seafloor images. Determination of the location of seafloor features requires knowledge of the sound speed structure and any spurious images or artifacts due to shipping noise and right/left ambiguity.

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.

Coherence of bottom-interacting sound in the Tagus Abyssal Plain was determined as a function of bearing and frequency (20 to 2000 Hz) for grazing angles between 11° and 13°. An acoustic experiment was performed in which SUS charges were dropped in a circular pattern of 28 km radius around a deep hydrophone. The bottom-interacting arrival was isolated and processed to remove the decorrelating effects of varying bubble-pulse periods. A spatial coherence function was calculated between shot pairs corresponding to 5, 9, and 13 km separation. For frequency bands of 20 to 500 Hz and 1200 to 2000 Hz, the spatial coherence of the bottom-interacting arrival is high. For the frequency band of 500 to 1200 Hz, the spatial coherence is lower and more variable. The high coherence values are consistent with the Eckart theory for scattering from an interface having rms roughness less than 0.1 m. The sharply tuned nature of the low coherence values in a discrete frequency region suggests that the effect is due to interference from sediment multi-paths rather than scattering from bottom roughness. Bandpass filtered impulse responses show that the energy of the sediment-refracted arrival predominates at low frequencies and the reflected arrival predominates at high frequencies. The interference effects occur in the middle frequency region where the two types of arrivals have nearly equal amounts of energy, and may be expected to depend on sediment properties, bottom topography, measurement geometry, and oceanographic environment.

Short-term omnidirectional measurements of ambient noise below 1 kHz were made for 14 deep-water stations in the Southwest Atlantic off the coast of South America during January 1981. At the lower frequencies (10 to 150 Hz) the noise levels agree with the middle range of the prediction curves for normal shipping densities in the northern oceans as reported by Wenz (1962). No consistent major geographical or physiographic dependence of the ambient noise levels was found for these reported measurements. The levels reported here for this South Atlantic region are generally higher than those found for the South Pacific regions. This result was unexpected as the total shipping in the southern ocean basins is considerably less than that in the northern ocean basis. One possible explanation of why the noise levels are high is that the sources from a high density shipping lane along the east coast of South America may have been effectively coupled to the main propagation paths in the deep sound channel.

Long-range, low-frequency directional measurements of acoustic backscattering from the seafloor were made in the Tyrrhenian Sea using explosive sound sources. The signals were received by a horizontal towed array of hydrophones and were processed by a beamforming procedure to obtain the directional distribution of the scattered field as a function of time. These data are used to form images of scatterers and to estimate the backscattering strength of specific physiographic features. From the data obtained the scattering strengths were estimated to range from -25 dB to -35 dB and did not exhibit strong dependence on frequency.

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...

The floor of Lake Superior northwest of the Keweenaw Peninsula has three zones of small-scale relief based on echogram character. The zones are roughly depth dependent. Zone A, located between the shore to about a 54-m depth, is generally smooth on the echogram and consists mainly of sand and boulder gravel deposits. Zone B, between about 54 and 165 m, has microroughness features with a 2- to 5-m relief and a 90- to 300-m spacing; the bottom consists of glacial till and lacustrine clay. Zone C, below depths of about 165 m, has narrow troughs with depths to 12 m and separation of 60 to 600 m; the bottom consists of lacustrine clay. The microrelief of Zone B consists of an intersecting network of grooves having widths of 5 to 75 m and lengths of as much as 1,950 m. Regular parallel features 15 to 30 m apart are also found in scattered areas of Zone B and the deepest parts of Zone A. Sand and boulder gravel deposits of Zone A may be beach and dune material of lower glacial-lake stages, and the border with Zone B may mark the lowest shoreline during the sequence of glacial lakes in the Superior basin. The grooves of Zone B were probably formed by scouring by icebergs during an earlier lake stage. Relief in Zone C probably was formed by a lacustrine process.