Energy flow in interference fields

The purpose of this paper is to examine ocean acoustic field interference patterns and their associated energy flows both through numerical modeling and analysis of actual ocean acoustic data. The numerical modeling is performed using the environmental reconstruction obtained from data collected during the SWARM 95 experiment. The ocean acoustic data were collected during the 1990 NATIVE 1 experiment by a set of infrasonic (0.4 to 20 Hz) freely drifting vector sensors ("combined receivers"). The numerical modeling results indicate that, whereas under adiabatic mode propagation the interference pattern responds to introduction of water column fluctuations by simply shifting in range and frequency, effects of mode coupling result in more complicated behavior. Interference patterns can be destroyed in the presence of weak mode coupling, particularly where the dynamic range of the pattern (peak-to-trough ratio) is small. This dynamic range in the range/frequency plane is depth dependent. It also depends upon which property of the acoustic field is considered; the striation patterns in reactive intensity magnitude spectra can have much greater dynamic range, and therefore be more resistant to effects of mode coupling, than those in the acoustic pressure spectra at a given depth, and vice versa. Acoustic energy flows at 7.0 Hz measured by the acoustic vector sensing Swallow floats as a 120-m-deep source was towed out to a distance of nearly 19 km display interesting features; the active intensity predominantly is in the radial direction away from the source and represents net flux of energy down the waveguide, and the reactive intensity is mostly vertical since the field's spatial structure typically varies the greatest in that direction. However, near a pressure field minimum, the vertical net flux becomes significant and changes direction, and both components of the reactive intensity change direction. (Author)