Application of 3-D acoustics to the CTBTO processing system

For low-frequency propagation ( 20 Hz), the ocean is ostensibly flat, with seafloor topography driving propagation. Refraction and diffraction around islands, ocean ridges and continents means that the usual Nx2D methodology for running acoustic propagation is no longer accurate. The United Nations Comprehensive Test Ban Treaty Organization has deployed a hydrophone network to detect and localize at-sea nuclear testing. The system for localization is based upon 2-D adiabatic mode propagation models computed in the early 1990s. In this paper we demonstrate the need for using 3-D acoustic propagation and a methodology for generating blockage maps and travel time tables for use in the automated association algorithm run at CTBTO. The effect of 3-D propagation on the observation of distant earthquakes is demonstrated.