DISTRIBUTION OF REVERBERANT SUBBOTTOM LAYERS IN THE SOUTHWEST PACIFIC BASIN

Vertical reflection profiles made with a large volume (7628 cm3) sound source and airgun-sonobuoy reflection and refraction profiles in the southwest Pacific Ocean basin reveal significant accumulations of acoustically reverberant material that is difficult to penetrate with small (328-656 cm3) airguns. An ispoach map of the reverberant layers, called 'smooth acoustic basement' or the 'opaque layer' in earlier studies, is presented along with an isopach map of the overlying weakly stratified (and low velocity) sediment. The maps shows that misidentification of the reverberant material as igenous basement would result in average basement depth about 500m too shallow, and in three separate areas, 1500 to 2000m too shallow. The new data generally agree with depth versus age curves of basement in the oldest Pacific crust as determined by DSDP boreholes.-from Authors