Abstract:

Broadband parabolic-equation simulations have been performed with and without internal-wave sound-speed fluctuations. The simulations have a center frequency of 75 Hz, a bandwidth of 30 Hz, and a propagation range R of 1000, 2000, and 3000 km. For these cases it is found that acoustic propagation through internal waves is strongly nonadiabatic. In terms of modal travel times, low modes have a negative bias because they couple into higher, faster modes, while the higher modes show a positive bias, indicating preferential coupling into lower, slower modes. The lowest modes show the least travel-time spread and bias, and these quantities increase rapidly with increasing mode number. Empirically and approximately it is found that bias grows like R[sup 2] and spread grows like R[sup 3/2]. The modal power distributions over frequency are markedly different from the source distributions. Power is distributed roughly equally across the 30-Hz frequency band for each mode, with 5.6-dB scintillations consistent with an exponential probability distribution function for intensity. In spite of the dramatic spread and bias in the higher modes, it is found that a synthesis of these modes results in coherent wavefronts, whose characteristic timing fluctuations at 3000-km range are two orders of magnitude less than those of the corresponding modes.