Shallow-water tracking in the Sea of Nazare

In the summer of 1996, an experiment was conducted off the coast of Portugal to study the effects of internal tides on sound propagation. This experiment-called INTIMATE '96 (Internal Tide Investigation by Means of Acoustic Tomography Experiment)-has provided a great deal of insight about the variability of pulse transmission over space and time. In contrast to a common view of shallow water propagation as complicated and unpredictable, the authors find a steady pattern of echoes. The echo-pattern stretches and shrinks in a systematic way with the tides and allows the authors to infer the components of the first few oceanographic modes. They also used the echo-pattern to track the source over a period of several days. During this period the isotherms in the ocean wavered by 20 m as a result of the tides, providing a challenge for model based tracking. They discuss these acoustic results with emphasis on the source tracking.