Acoustic monitoring of flow through the Strait of Gibraltar

Abstract

The Strait of Gibraltar Acoustic Monitoring Experiment was conducted during April–May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, transport measurements in the Strait of Gibraltar, as well as to explore the acoustic scattering caused by the internal wave bores generated in the Strait. Three different approaches to monitoring the flow were explored: (i) high-frequency (2-kHz) reciprocal transmissions; (ii) high-frequency (2-kHz) horizontal arrival angle measurements of ray bending due to currents; and (iii) one-way transmissions from a low-frequency (250-Hz) source to a vertical receiving array on the opposite side of the Strait. Extensive independent measurements of the temperature, salinity, and velocity fields were also made. The deep-turning rays are stable and yield differential travel times that give reasonable values for the current components along the acoustic paths. Horizontal arrival angles (phase differences) are strongly correlated with rough estimates of the current perpendicular to the acoustic path made using current meter data, suggesting that this approach is feasible at ranges up to at least 15 km. Finally, the acoustic signals show the effects of the nonlinear internal waves present in the Strait. [Work supported by ONR.]