Abstract
An upward-looking echo-sounder system has been designed to measure the time varying waveheight of sea-surface waves. A narrow beam of pulsed acoustic energy was focused on the sea surface by utilizing the transitional region from the near to farfield characteristics of a circular piston transducer. The transducer, operating at 360 kHz, was pulsed at 10-msec intervals to sample the waveheight. By transmitting short pulse lengths, the wide-band characteristics of the transducer were used to discriminate against near-surface bubble reverberation. The system is linear and can presently measure surface wave frequencies below 4.5 kHz. Waveheights as small as 0.05 in. were measured in a model tank by comparing the echo-sounder results with results obtained from a micrometer. An experiment, conducted in the Thames River, compared the performance of the echo sounder to a 15-ft resistive wave staff. Comparisons of ensemble-averaged power spectral densities obtained from the wave staff and the transducer, positioned 10 ft below the surface, were excellent for wave frequencies below 4.0 Hz.
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