THE TASMAN BLOCKAGE - AN ACOUSTIC SINK FOR THE HEARD ISLAND FEASIBILITY TEST

Low-frequency (57-Hz) signals from the Heard Island Feasibility Test, conducted in January 1991 were received on many hydrophones around the globe. The Atlantic and Indian Ocean paths were relatively clear and unimpeded, so received signal-to-noise ratios (after some averaging) were generally high. The Pacific paths were not as clear and considerable interaction with bathymetric features attenuated the signals received off the west coast of the U.S. A New Zealand-manned receiver in the Tasman Sea received the transmissions strongly, but a Japanese ship, placed strategically northeast of the south Fiji (Lau) Ridge did not detect any signals. There is good evidence from U.S. west coast receivers with beam forming capability, that the signals arrived from 20 deg further south (i.e., south of New Zealand, then through the Polynesian window) than if they had passed through the Tasman Sea window. Doppler-derived matching of transmitted and received azimuths further indicates that all U.S. west coast signals traveled south of New Zealand. A series of transverse ridges lying between northern New Zealand and New Caledonia and Fiji appears to have effectively blocked the transmissions from exiting the Tasman Sea. An adiabatic modal model of refracted geodesic raypaths sheds some light on the reasons for such effective blockage.