ACOUSTIC PROPAGATION THROUGH A LOW MACH NUMBER, STRATIFIED FLOW

Propagation of sound in an oceanic waveguide in the presence of a low Mach number, stratified flow is investigated. Equations for the wave-n umber integration (WI) and normal-mode (Nh?) approaches are simultaneo usly derived in a consistent formulation. At low Mach numbers acoustic azimuthal coupling is negligible and the effect of current can be acc ounted for by a simple modification of existing WI and NM, codes. Nume rical results for low- and high-frequency sources ina simple waveguide show a high degree of agreement between modified versions of GASES an d KRAKEN, and confirm previous theoretical results. An approximate mod al closed-form solution based on the medium-at-rest mode set is then d erived assuming adiabatic propagation (no current-induced mode couplin g). Comparison with the modified versions of KRAKEN and GASES shows th is assumption breaks down if there is bottom penetration or if the cur rent profile exhibits somewhat sharp variations with respect to depth. An application to matched-field current tomography is eventually pres ented. Assuming accurate knowledge of the acoustic waveguide, a curren t velocity of 1.5 m/s can be measured at a range of 2 km with a resolu tion of 0.2 m/s using a multitone cw signal spanning 200-250 Hz. (C) 1 997 Acoustical Society of America.