Mapping seabed variability: Rapid surveying of coastal regions

Abstract

Directional broadband reverberation can be exploited for rapidly probing seabed variability over large areas O (103–104)km2; the data need not be calibrated. Seabed variability thus mapped inherently combines spatial variability in geoacoustic properties that control reflection and those that control scattering. It is shown that in some cases, the variability can be separated. The method is applied to measurements in the Straits of Sicily where the data indicate the existence of two provinces where the statistics of the geoparameters are distinct. Three attributes of each province are quantified: the province boundaries, measures of spatial variability and/or uncertainty, and the location of discrete scattering features (that may lead to sonar clutter). One province is quite large and exhibits remarkably little spatial variability. The variability is probed on lateral scales ∼200×400m, yet the ratio of the seabed geoparameters varies less than 3 dB from 300−1600Hz over ∼1000km2. While this result seems contrary to prevailing notions of extreme seabed variability in shallow water, it indicates that certain kinds of geophysical (and concomitant geoacoustic) variability may be averaged in such a way that the seabed appears approximately homogeneous to long-range acoustic systems.