Marine Physical Lab.-0238, Scripps Inst. of Oceanogr., 9500 Gilman Dr., La Jolla, CA 92093-0238
The presence of bubbles in the near surface layer of the ocean causes significant reduction of the sound speed (A. B. Wood, 1957). The result is an upward-refracting sound-speed profile which acts as an acoustic waveguide. An experiment was performed in Saanich Inlet off the south east coast of Vancouver Island to investigate the spectral structure of sound which had propagated through a bubble layer. A motorboat, driven in a circle, created a bubbly wake which advected through the experimental region. Acoustic measurements were obtained at depths of 1.5 and 3.5 m at a range of 10 m from a broadband source. The spectral structure of the data depends on depth, but in both cases, shows a regular positioning of the peaks. The Green's function solution for an isovelocity sound-speed profile (Lloyd's mirror) and an upward-refractng inverse-square profile were fitted to acoustic spectra. The bubble-free records are well fitted by the Lloyd's mirror theory, and good inverse-square fits were found for acoustic ``snapshots'' of the bubbly field at 2 and 8 min after field generation. It is possible to invert for the sound-speed profile in the near-surface bubble layer by fitting the spectral structure predicted by the inverse-square theory to that of sound that has propagated through the bubbly field.