Acoustic Doppler evidence for a near-surface bubble layer

Abstract

Acoustic reverberation data were collected with a horizontally directed pulse-to-pulse incoherent Doppler sonar operating at 75 kHz and deployed near the surface. The spectral moment estimation method was used to estimate velocity profiles. Clear evidence of interaction with the surface is presented by both the reverberation intensity and velocity signatures. However, an interesting anomaly is revealed in the joint behavior of surface velocity and surface reverberation intensity. Specifically, the surface velocity event does not coincide in range with the corresponding intensity feature. An interpretation of this behavior is offered based on the sonar beam geometry and the hypothesized presence of a thin bubble layer entrained just below the surface. Realistic computer simulations are used to provide support for this hypothesis and to estimate the backscattering coefficient of the volume reverberation produced by the bubble layer. This backscattering coefficient, combined with the theoretical target strength of a resonant bubble, leads to an estimate of bubble concentration. [Work supported by ONR.]