Single-phone Source Tracking in a Variable Environment

Internal tides commonly occur along ocean coasts. They are internal waves driven by the usual tidal force and generated by scattering at a sharp bottom feature such as the shelf break. The internal tides are dramatic features with crests typically 10-30 km apart and wave heights of 20 m. On the ocean surface they cause only a gentle ripple about 10 cm high but they affect the shine of the surface. As a result, astronauts often see them as the tides propagate away from the shelf break. In June 1996, a shallow-water tomography experiment (INTIMATE 96) was conducted off the coast of Portugal to observe these internal tides and learn about their acoustic effects. A source was towed around a vertical hydrophone array to produce acoustic sections along several slices. The experiment also provided an ideal opportunity for testing model-based source tracking. Acoustic sections taken parallel to the Portuguese coast allowed us to understand the propagation physics in a range-independent area. With this we have been able to develop a matched-field algorithm suitable for use in the far more complicated downslope direction which, being perpendicular to the crests of the internal tides, also experiences strong ocean-temperature variations. We will discuss both the approach and the source tracking.