Environmental information content of ocean ambient noise.

In recent years, methods have been developed to estimate a variety of environmental parameters based on measurements of the ocean ambient noise. For example, noise has been used to estimate water depth using the passive fathometer technique and bottom loss estimated and used to invert for seabed parameters. There is also information in the noise about the water column sound speed, volume attenuation, and the sea-state. The Fisher information can be used to quantify the basic information available in the noise measurements and its inverse, the Cramér-Rao lower bound (CRLB), provides the lower limit on the variance of an unbiased estimator of a particular parameter. The CRLB can be used to study the feasibility of various measurement configurations and parameter sensitivities. In this paper, the CRLB is developed for ocean ambient noise and the environmental information contained in the measurements is determined. The CRLBs provide an estimate of the underlying information in the data, however, it is independent of the estimation methodology. This is useful to determine if a given estimation method is reaching the lower bound. Results illustrating the bounds as well as sensitivities and performance of estimators are demonstrated using both simulations and data.