Propagation and analysis issues in the prediction of long-range reverberation

Abstract

Data collected from the Office of Naval Research-special research program (ONR-SRP) bottom reverberation research cruises consist of various environmental measurements (sound-speed profiles, bottom properties, etc.), extensive bathymetric mapping of the 300 km×150 km natural laboratory, and high-quality acoustic reverberation data recorded in both monostatic and bistatic geometries. The analysis of the acoustic data and the effects of propagation are investigated. Accurate GPS tracking of the participating research vessels provided the precision necessary to attempt to correlate ‘‘spiky’’ reverberation events with bathymetric features. Understanding the limitations of our ability to resolve such features—due to imperfect signal processing, environmental variability, and complex multipath structures—is the main objective of this work. Using a simplified time-to-range conversion, the measured reverberation data can be displayed over the local bathymetry. This process shows good correlation between large-scale [O(km)] high-level returns and bathymetric ridge structures. Employing a PE propagation model in a quasi-cw manner, many of the large-scale reverberation features can be shown to correlate well with predicted high-level ensonification of the total field [low transmission loss (TL)]. Extending the model predictions to include time domain broadband pulse propagation, it can be shown for ranges greater than ∼20 km that multipaths and forward scatter produce complex ensonification patterns which are unresolvable in time and/or range. The effect of this on the ability to correlate finer scale returns and bathymetric features will be discussed. Preliminary analysis of data from a bottom mounted vertical array in the vicinity of 1/2 CZ exhibits strong evidence for the existence of these predicted multipaths and provides a means of confirming the relative intensities predicted by propagation models.