Acoustic scattering and remote sensing in the Strait of Gibraltar: The role of internal solitary waves

The Strait of Gibraltar is a challenging environment to observe with conventional instruments. Internal undular bores of 100 m in amplitude propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water, eventually evolving into a packet of internal solitary waves. The interface is also strongly modulated by internal tides of comparable amplitude. High-frequency underwater acoustic transmissions across the Strait are used to examine acoustic scattering caused by this unique internal wave field and the feasibility of acoustically remote sensing several physical processes.; Although the observed acoustic scattering is quite complicated, it is surprisingly robust, making it a good candidate for modeling. Chapter 2 describes a model of the complex hydraulics in the Strait, which when used in the forward problem, successfully reproduces many key features of the observed acoustic arrival pattern. Acoustic scattering from internal solitary waves can be explained qualitatively; as internal waves cross the acoustic path, they cause sharp soundspeed gradients which intermittently refract acoustic rays away from normal sound channels. Internal tides vertically shift soundspeed profiles for additional travel time variability.; Chapter 3 provides demonstrations of a few techniques used to extract physical oceanography information from the acoustic record, and in doing so, has offered some new insight into the processes at work in the Strait. Internal bores crossing the acoustic path can be recognized by their scattering effects in the acoustic record. The time between internal bore crossings is influenced more by the tidal phase of the bore release at the Camarinal Sill than by variability in the bore's propagation time down the Strait. When internal bores were present, the acoustic arrival patterns could be classified as one of three types with different internal bore and internal tide amplitudes. The arrival types alternate during spring to neap tide transitions, suggesting that internal bore amplitude is not linearly related to tidal height. The sensitivity of acoustic observables to several physical parameters is investigated using the forward model of Chapter 2, and inverse techniques provide estimates of several physical parameters from spring tidal cycles.