Open Ocean Surface Wave Measurement Using Doppler Sonar

In October-November 1983, Doppler sonars mounted on the research platform Flip were used to scatter 75-kHz sound from the underside of the sea surface at low angle, as well as from the interior of the mixed layer. Surface gravity waves were seen in velocity estimates from the surface scattering sonar, even though the wave conditions were unusually calm. Valid measurements of sea surface motion were obtained over the range interval 600 to 1400 m from Flip. A second sonar which transmitted horizontally and scattered from the interior of the mixed layer also sensed surface waves, although with amplitude reduced in proportion to the decay of the motions with depth. Wave number-frequency spectra of the observed motions are consistent with linear theory. The existence (emergence) of this technology enables the synthesis of kilometer-long surface wave arrays in the open sea, without the cost and logistical support usually associated with large-aperture arrays. In addition, the Doppler acoustic approach can provide information on lower-frequency surface currents and on the spatial variations in these lowfrequency currents, such as Langmuir cells, which might affect wave propagation.