A simple low‐frequency acoustic technique for remote measurement of the temporal ocean wave spectrum

A theory is presented that describes a method for using low‐frequency sound (≲150 Hz) to measure the waveheight variance and the nondirectional temporal waveheight spectrum of a random rough surface such as the sea surface. The technique requires a vertically pointing broadbeam acoustic source and a colocated broadbeam receiver which records the amplitude and phase fluctuations of the backscattered field. When the rms surface waveheight is much smaller than the acoustic wavelength, the temporal spectrum of the amplitude and phase fluctuations can be directly related to the nondirectional temporal surface waveheight spectrum. The theory predicts that the temporal waveheight spectrum out to frequencies of roughly 0.5 Hz can be retrieved.