INTRODUCTIONSea surface scattering by wind-generated waves and bubbles is regarded to be the main nonplatform-related cause of the time variability of shallow acoustic communication channels.Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modelling of these dynamic sea-surface effects. It is known that, for frequencies in the range 1-4 kHz, the effect of bubbles on sea surface reflection loss is mainly due to refraction, which can be modelled with a modified sound-speed profile accounting for the bubble void fraction in the surface layer (Hall-Novarini model). The upward refraction induced by the bubble cloud then effectively acts as a catalyst for increasing the rough-surface scattering.In the present work, it is shown that, for frequencies in the range 4-8 kHz, bubble extinction also provides a significant contribution to the surface loss, including both the effects of bubble scattering and absorption. As this is the frequency band adopted in the European Defence Agency (EDA) project RACUN (Robust Acoustic Communication in Underwater Networks [1]), in which the reported research has been conducted, both bubble refraction and extinction effects should be modelled for acoustic channel simulations in RACUN. These model-based channel simulations will be performed by applying a Gaussian-beam ray-tracer (BELLHOP), together with a toolbox for generation of realistic rough sea surfaces based on both fully-developed ocean and short-fetch North Sea wave-height spectra and angular spreading functions (WAFO). SEA SURFACE MODELLING IntroductionThe objective of the present work is to improve channel modelling for underwater acoustic communication by incorporation of the effects of time-varying ambient conditions, especially windgenerated sea surface wave effects. These effects are probably the main cause of time-varying multipath and Doppler spread when both the transmitter and receiver are static. The sea surface dynamics can roughly be divided into two basic mechanisms:1. Periodic vertical motion of the sea surface; 2. Near-surface bubbles created by (breaking) waves.See Figure 1 for a schematic illustration of these effects.In order to keep things practical, we will at this point assume a separation of time scales for the sea surface dynamics and the underwater acoustic propagation. That is, we will treat the problem as "piece-wise frozen". For each "frozen" realization of the sea surface and the bubble distribution, acoustic computations are then performed without accounting for the instantaneous velocity of the sea surface and the bubbles. The main Doppler effects come in as a consequence of the variation of the path lengths between consecutive realizations [2]. This is called the range rate: dL k (t)/dt, with

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Abstract-During naval operations, sonar performance estimates often need to be computed in-situ with limited environmental information. This calls for the use of fast acoustic propagation models. Many naval operations are carried out in challenging and dynamic environments. This makes acoustic propagation and sonar performance behavior particularly complex and variable, and complicates prediction. Using data from a field experiment, we have investigated the accuracy with which acoustic propagation loss (PL) can be predicted, using only limited modeling capabilities. Environmental input parameters came from various sources that may be available in a typical naval operation.The outer continental shelf shallow-water experimental area featured internal tides, packets of nonlinear internal waves, and a meandering water mass front. For a moored source/receiver pair separated by 19.6 km, the acoustic propagation loss for 800 Hz pulses was computed using the peak amplitude. The variations in sound speed translated into considerable PL variability of order 15 dB. Acoustic loss modeling was carried out using a data-driven regional ocean model as well as measured sound speed profile data for comparison. The acoustic model used a two-dimensional parabolic approximation (vertical and radial outward wavenumbers only). The variance of modeled propagation loss was less than that measured. The effect of the internal tides and sub-tidal features was reasonably well modeled; these made use of measured sound speed data. The effects of nonlinear waves were not well modeled, consistent with their known three-dimensional effects but also with the lack of measurements to initialize and constrain them.

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