New Technique for the Investigation of Ocean-Bottom Acoustic Structure

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Abstract
A technique is described for investigating the acoustic structure of the ocean bottom by spectral analysis of signals radiated from a broad-band sound source at the surface and received at hydrophones on or near the bottom. Spectrograms made of the received signals show a periodic structure, due to the interference between direct and bottom-reflected sound. The spacing in frequency between maxima depends on the sound speed, arrival angle, and distance between the receiver and the plane of reflection. Spectrograms recorded in an area south of Bermuda exhibit a complex interference pattern apparently associated with reflections from several sedimentary layers. To obtain quantitative measurements of the characteristics of the layers, the output of a sound spectrograph, making successive sweeps through a given range of frequencies, was itself analyzed on a second spectrograph. In this spectrogram, which displays the so-called cepstrum, each maximum corresponds to a bottom-layer reflection. Examples are shown of both types of records, along with their interpretations. [Work supported by the Bureau of Ships, U. S. Navy.]

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