The long-range ocean acoustic propagation experiment (LOAPEX) observable: Modal content as a function of arrival time. Part A: Relevance

Abstract

It is well known that for gradual enough changes in the sound speed structure, the mode number is nearly constant. Thus, changes in mode number are the most direct measure of scattering by smaller structures, such as internal waves. The mode number changes can be determined from data by separating the signal by arrival time. It is shown how the separation happens in an ideal case without scattering, allowing an optimum choice of the time window. The modal content in a small time window can also be estimated from the depth dependence of the intensity in the region of the turning depths of the modes. Each mode has a rapid falloff near its turning depth, and the extension beyond that depth measures the higher mode content. The deep vertical line array at LOAPEX measured this depth dependence. In addition, for certain times, all of the arrival is in the deep array, allowing unambiguous projections onto modes, especially if the source is above the sound axis. [Work supported by the Office of Naval Research.]