Chaos in ocean ambient ''noise''

Using a data analysis toolkit designed to explore data from nonlinear systems using time domain methods, it has been determined that the observed number of degrees of freedom in one pristine measurement of ocean ambient background pressure fluctuations or ''noise'' is nine. While the true number of degrees of freedom in the ocean is probably large, the observed measurements have nine degrees of freedom. There is a positive Lyapunov exponent, which identifies the system as being chaotic. The largest exponent is very large, indicating that the prediction horizon for these data is confined to a few samples. Determination of the degrees of freedom is important for the construction of physical models and nonlinear noise reduction filters, which may be based on characteristics of the observed few degrees of freedom from the background acoustic source. The magnitude of the largest Lyapunov exponent provides a measure of confidence for signal state prediction and mitigation. (C) 1996 Acoustical Society of America.