HEARD ISLAND SIGNALS THROUGH THE AGULHAS RETROFLECTION REGION

Heard Island Feasibility Test sound transmissions were received at sea some 300 km southwest of the Cape of Good Hope on a four-hydrophone vertical array deployed from the vessel SAS UMZIMKULU. This area, 53-min travel time along the Atlantic path from Heard Island, lay immediately down path from strong regional variability in the acoustic properties of the SOFAR channel, associated with the retroflection of the Agulhas current. Many transmissions were not received at all whilst the four successful recordings exhibited a relatively high transmission loss. It appears that interaction between a warm water Agulhas ring and the local seamount bathymetry may have been responsible for the deterioration in the HIFT signal received off Cape Town and at other sites further along the Atlantic path. Nevertheless, the overall quality of these received signals with respect to phase stability and intensity was sufficient for the standard HIFT signal processing to be carried out.