Results and Methods Used to Determine the Acoustic Properties of the Southeast Asian Margins

Abstract

More than 850 new sonobuoy sound velocity solutions and some published solutions have been analysed statistically to develop 16 regional velocity functions from the Southeast Asian shelves and marginal basins. The velocity functions are least-squares regressions of the form V = V_O + Kt, where t is one-way vertical travel-time to layer mid-points. The value of K shows surprisingly little variability on the shelves and adjoining basins from the Bay of Bengal to the Japan Sea. The average value is 1.86 ± .30 km/s². Very large values of 3.2 km/s² from Western Australia (not included in the average) are unique, and may result from the special combination of very low sedimentation rates on quite old oceanic crust. Paired comparisons of interval velocities and refraction velocities from the same sonobuoys show that the two measurements are interchangeable if they are compared at equal depths at layer mid-points. More than 30 high-resolution velocity inversions from the Southeast Asian shelf data have been compiled to yield 5 average seafloor velocity gradients. Gradients predicted from statistically derived functions, based on refraction solutions from relatively thick layers, are not in satisfactory agreement with these averaged values. These differences can be reconciled by picking thinner layers or by using greater (and more realistic) seafloor sound velocities than those obtained as intercepts from the statistical regressions.