Arrival time fluctuation of normal modes caused by solitary internal waves

Abstract

Internal waves (IW) can significantly affect sound propagation in shallow water. When arrival times of normal modes are used for ocean tomography, IW-caused variations of the modal arrival time should be taken into account. The sensitivities of the normal mode arrival time to solitary IWs are analyzed by using the SWARM'95 environments. It is shown through theoretical analysis that acoustic modes with more energy distribution in the thermocline are more sensitive to the internal waves. The arrival times of these normal modes fluctuate greatly with thermocline depth variation. Moreover, the sensitive mode and insensitive mode can be the same mode, but with different frequencies. By contrast, the modes with smaller grazing angles and propagating mainly below the thermocline have stable arrival times. The results are meaningful for source depth location and frequency selection in shallow water tomography.