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.

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Cavitation caused by shock pulses reflected from the sea surface is studied. A breaking pressure of zero is assumed and a model based on spalled layers is proposed to determine the prominent characteristics. These characteristics include the extent of cavitation, and a return pulse with an initial spike followed by an approximately rectangular pulse that, for larger charges fired near the sea surface, is very much longer than the incident pulse. Experiments using explosives to produce exponential shock pulses of various peak pressures and characteristic lengths verify the prominent features of the model. The acoustic effects indicate a breaking tension at least a factor of 10 smaller than that indicated by visible effects.

The characteristics of shock pulses reflected at normal incidence from the sea surface are examined as a function of charge weight and charge depth. The reflected tension pulse possesses a number of prominent features such as an initial spike, which is considerably shorter than the incident shock pulse, followed by a plateau that becomes longer for shallower or larger charges. These differences between the shapes of the incident and reflected pulses are attributed to cavitation effects near the sea surface. The acoustic results indicate that cavitation takes place for charge depths considerably greater than the maximum depth at which visible effects are caused at the sea surface. The acoustic effects indicate a breaking tension at least a factor of 10 smaller than that indicated by the visible effects.

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