The scattering strength of the sea surface was measured for a range of wind velocities, grazing angles, and frequencies, in octave bands in the frequency range from 400 to 6400 cps. An empirical equation was obtained relating the scattering strength of the sea surface to the above variables, for grazing angles below 40°. At low grazing angles, scattering of sound from a subsurface layer of isotropic scatterers, probably of biological origin, frequently masked the reverberation due to scattering from surface roughness. For a given wind speed, the scattering strengths measured in this study at grazing angles below 20° were appreciably less than those obtained by other observers at higher frequencies. At higher grazing angles, of the order of 40°, there was little systematic difference between the measurements made at high and low frequencies.

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Surface backscattering strengths were measured in octave bands, using explosive sound sources in an experiment similar to that reported by Chapman and Harris [J. Acoust. Soc. Am. 34, 1592–1597 (1962)]. In the earlier experiment, measurements were made in the frequency range from 400–6400 cps and at grazing angles up to 60°. In the present experiment, a wider band of frequencies was covered, 100–6400 cps, and the maximum grazing angle was increased to 80°. Where results could be compared, the scattering strengths in the two experiments were in general agreement, any differences being consistent with the observed differences in ocean swell.

Reverberation from deep scattering layers was measured at 37 sites in the western North Atlantic. Resonant scatterers, presumably the swimbladders of bathypelagic fish, were responsible for the bulk of the observed reverberation. Three scattering layers, each populated with a characteristic size of scatterer, were found to persist over distances of several hundred kilometers. During the day, these layers were at depths between 300 and 900 m. A migration towards the sea surface of scatterers from the shallowest layer at sunset appeared to be responsible for most of the low frequency reverberation observed at night. The majority of the scatterers from the deepest of the layers did not experience a daily migration in depth. Two of the layers studied showed pronounced decreases in depth with increasing latitude. No seasonal variations in scattering strengths were observed.