Passive ranging in strongly range-dependent environments: Effects of mode coupling on the waveguide invariant

Abstract

Prior work has shown that the value of the shallow water waveguide invariant changes in range-dependent environments due to non-uniform phase and group speed along the propagation path caused by either a range-dependent bathymetry or sound speed profile. Much of the work on these scenarios has dealt with weak range-dependence and the effects of mode coupling were neglected. In certain situations when mode coupling occurs, energy from higher order, surface-reflected-bottom-reflected modes may be lost to lower order, surface-refracted-bottom-reflected modes. These lower order modes, which do not interact with the surface, are associated with waveguide invariant values that differ greatly from the standard shallow water approximation where the waveguide invariant equals one. This talk will examine a case from the 2007 CALOPS experiment where the adiabatic approximation for modal propagation is no longer valid and mode coupling appears to be important. Acoustical data and analysis will be presented to demonstrate the effect that mode coupling has on the waveguide invariant in strongly range-dependent environments, and methods for incorporating coupled modes into waveguide invariant estimates will be discussed. [This research was supported by the Applied Research Laboratory, at the Pennsylvania State University through the Eric Walker Graduate Assistantship Program.]