Acoustic monitoring of the bottom characteristics of a harbor

Abstract

Acoustic monitoring of the underwater environment typically found in a harbor requires knowledge of the spatial and temporal variability of the noise, the water properties, the currents, and the bottom characteristics. For safety of navigation, repeat bottom surveys must be performed at regular intervals to monitor changes in water depth along navigation channels due to accumulation or depletion of sediments, and to detect potential hazards on or near the bottom. Imaging side-looking sonars, particularly those with synthetic aperture capabilities, are most effective for hazard detection but their performance in waters 5-10 m deep can be hampered by multipath and by changes in water properties at the scale of the scene being imaged. Their inherent spatial filtering characteristics make multibeam echosounders effective tools for synchronous bathymetry and acoustic backscatter imagery of the water column and the bottom. In harbor surveys, such echosounders achieve decimetric spatial resolution by operating at several hundred kilohertz, but their measurements are sensitive to spatial and temporal changes in water properties at the scale of the survey swath, potentially reducing lateral coverage. The corresponding variations in acoustic absorption affect acoustic backscatter imagery and yield increased uncertainty in the outcome of a repeat harbor survey.