SOFAR signal shaping : influence of topography and relative source depth

Abstract

The shaping of SOFAR signals by topographic interference and the depth of the source relative to the axis of the sound channel is investigated. Eight shot signals from five different sites near Cape Town were received at three widely spaced hydrophones at Ascension over a number of 4.4 Megametre lines. These signals show the effects of (1) sources at different depths relative to the axis of the sound channel, (2) blocking and partial blocking of direct rays by seamounts and narrow undersea ridges and (3) a sound speed profile displaced downwards by a filament of warm surface water. Comparisons with ray trace and normal mode models show that some of the received signals consist of a combination of slower, refracted rays and faster rays reflected off Walvis ridge. The reflected energy starts earlier and has a greater amplitude than would be expected from ray trace modelling. This energy will act as noise and make ATOC (Acoustic Thermometry of Ocean Climate) determinations more difficult.