Impacts of Flow Variability on Fixed Side-Looking 100 kHz Sonar Performance in a Shallow Channel

Abstract

A 100 kHz side-looking sonar was operated nearly continuously from September 1996 to the end of May 1997 in Drogden Channel, near Copenhagen, Denmark. This busy shipping channel, 1-km-wide by 12-m-deep, connects the Baltic Sea with the North Sea through the Kattegat. The purpose of this operation was to maintain surveillance for migratory herring during the construction of a bridge and tunnel, however a variety of shipping traffic was also observed. Water temperature, salinity, and current profiles along with surface meteorology were simultaneously monitored at this site. Under normal, weakly stratified flow conditions, ships and fish schools were observed up to 400 m range, detectable against a background of low-grazing-angle seabed backscatter. Occasional saline intrusions were observed to create strong upward-refracting conditions that significantly restricted the available range for target detection. Example echograms and reverberation results from normal and upward-refracting conditions are shown. Rayg-tracing analysis is used to account for the observed backscattered reverberation.