Deterministic forward scatter from surface gravity waves.

Deterministic structures in sound reflected by gravity waves, such as focused arrivals and Doppler shifts, have implications for underwater acoustics and sonar, and the performance of underwater acoustic communications systems. A stationary phase analysis of the Helmholtz-Kirchhoff scattering integral yields the trajectory of focused arrivals and their relationship to the curvature of the surface wave field. Deterministic effects along paths up to 70 water depths long are observed in shallow water measurements of surface-scattered sound at the Martha's Vineyard Coastal Observatory. The arrival time and amplitude of surface-scattered pulses are reconciled with model calculations using measurements of surface waves made with an upward-looking sonar mounted mid-way along the propagation path. The root mean square difference between the modeled and observed pulse arrival amplitude and delay, respectively, normalized by the maximum range of amplitudes and delays, is found to be 0.2 or less for the observation periods analyzed. Cross-correlation coefficients for modeled and observed pulse arrival delays varied from 0.83 to 0.16 depending on surface conditions. Cross-correlation coefficients for normalized pulse energy for the same conditions were small and varied from 0.16 to 0.06. In contrast, the modeled and observed pulse arrival delay and amplitude statistics were in good agreement.

Reflection of underwater sound from surface waves.

A tank experiment has been conducted to measure reflection of underwater sound from surface waves. Reflection from a wave crest leads to focusing and caustics and results in rapid variation in the received waveform as the surface wave moves. Theoretical results from wavefront modeling show that interference of three surface reflected eigenrays for each wave crest produces complicated interference waveforms. There is good agreement between theory and experiment even on the shadow side of caustics where there are two surface reflected arrivals but only one eigenray.

Temporal patterns in ambient noise of biological origin from a shallow water temperate reef.

A systematic study of the ambient noise in the shallow coastal waters of north-eastern New Zealand shows large temporal variability in acoustic power levels between seasons, moon phase and the time of day. Ambient noise levels were highest during the new moon and the lowest during the full moon. Ambient noise levels were also significantly higher during summer and lower during winter. Bandpass filtering (700-2,000 Hz and 2-15 kHz), combined with snap counts and data from other studies show that the majority of the sound intensity increases could be attributed to two organisms: the sea urchin and the snapping shrimp. The increased intensity of biologically produced sound during dusk, new moon and summer could enhance the biological signature of a reef and transmit it further offshore. Ambient noise generated from the coast, especially reefs, has been implicated as playing a role in guiding pelagic post-larval fish and crustaceans to settlement habitats. Determining a causal link between temporal increases in ambient noise and higher rates of settlement of reef fish and crustaceans would provide support for the importance of ambient underwater sound in guiding the settlement of these organisms.

Shallow water sound propagation with surface waves.

The theory of wavefront modeling in underwater acoustics is extended to allow rapid range dependence of the boundaries such as occurs in shallow water with surface waves. The theory allows for multiple reflections at surface and bottom as well as focusing and defocusing due to reflection from surface waves. The phase and amplitude of the field are calculated directly and used to model pulse propagation in the time domain. Pulse waveforms are obtained directly for all wavefront arrivals including both insonified and shadow regions near caustics. Calculated waveforms agree well with a reference solution and data obtained in a near-shore shallow water experiment with surface waves over a sloping bottom.

Wavefronts and waveforms in deep-water sound propagation.

A new method of calculating waveforms in underwater sound propagation is presented. The method is based on a Hankel transform-generalized Wentzel-Kramers-Brillouin (WKB) solution of the wave equation. The resulting integral leads to a new form of ray theory which is valid at relatively low frequencies and allows evaluation of the acoustic field on both the illuminated and shadow sides of caustics and at cusps where two caustics meet to form a focus. The integral is evaluated by stationary phase methods for the appropriate number of stationary points. Rays of nearby launch angle which have a travel time difference less than a quarter period must be considered together. The description of all other ray arrivals corresponds to simple ray theory. The phase, amplitude, and travel time of broadband acoustic pulses are obtainable directly from a simple graph of ray travel time as a function of depth at a given range. The method can handle range dependence but is illustrated here in long-distance propagation in deep water where the ray paths do not pass close to surface or bottom. The method is fast and gives close agreement with normal-mode calculations. The field on the shadow side of a caustic is properly given in terms of rays with complex launch angles, but good approximations can be obtained without the need to find complex rays.