Measuring currents with acoustic propagation

Abstract

Water currents refract and otherwise modify acoustic signals providing a basis for their remote detection. In recent years this concept has been explored with high-frequency propagation in the coastal environment, but the concepts have a broader application to ocean measurement. The component of flow resolved along the acoustic path may be detected through changes in effective sound speed. Phase coherent reciprocal transmission allows separation of current-induced effects from scalar contributions due to temperature or salinity, leading to measurements of great accuracy and precision. For turbulent flows this approach has been extended into the inertial subrange, permitting measurement of path averaged dissipation. In contrast, coherence of the refractive variability permits measurement of the component of flow orthogonal to the acoustic path through detection of scintillation drift with an array. Additionally, current advection of the pulse may be detected as a change in horizontal arrival angle. Resolution of both current speed and refractive variability as a function of path position can be achieved by combining multielement source and receiver arrays in a spatial aperture filter. These concepts are now being applied in the Bosphorus to the measurement of exchange with the Black Sea using a two-level reciprocal scintillation array.