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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 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.

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.

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}.

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.