Mode coupling by internal waves for multimegameter acoustic propagation in the ocean

We have performed broadband parabolic-equation simulations with and without sound-speed fluctuations induced by internal waves. The simulations have a center frequency of 75 Hz, a bandwidth of 30 Hz, and propagation range R of 1000, 2000, and 3000 km. In these cases it is found that long-range acoustic propagation through internal waves is strongly nonadiabatic. In terms of modal-travel times, low modes have a negative bias (they have a higher effective group speed than without internal waves) 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(2) and spread grows like R(3/2). This average slowing down and spreading of the higher modes causes a depth broadening of the reception finale. The modeled modal power distributions over frequency at multimegameter ranges are markedly different from the initial distributions at the source. 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 the 3000-km range are two orders of magnitude less than those of the corresponding modes. The modal spreads found in these simulations imply limits to the precision with which modal travel times can be measured using standard techniques. A nonstandard approach, using the geometric mean of independent realizations after transformation to the frequency domain, effectively averages the phase fluctuations and eliminates much of the spread. The strong modal coupling nonetheless suggests that nonadiabatic modal inversions will be necessary to do modal tomography at multimegameter ranges. (C) 1996 Acoustical Society of America.