Three-dimensional Modeling of the Hydroacoustic to Seismic T-phase Transition

Plans call for five T-phase stations to be installed as part of hydroacoustic segment of the International Monitoring System (IMS), for use in detecting nuclear explosions in the oceans. The ability to detect T-phase signals at the seismic T-phase stations relies on an understanding of the transition from ocean-borne acoustic energy to seismic energy. Observations of the seismic T-phase indicates that the hydroacoustic energy may convert to either compressional or shear body waves, or to highly-attenuative interface waves that can be observed near the ocean/land boundary. Previous 2-D modeling efforts have shown that T-phase amplitudes depend strongly on the velocity structure of the seafloor and land portion of the propagation path, as well as the depth of the source within the water column. However, Snell’s law indicates that 3-D effects, i.e. the angle of incidence to the coast, must also be considered in modeling the acoustic-to-seismic transmission. In this paper, we model upslope propagation of acoustic energy at a sloping wedge using a 3-D finite-difference time-stepping (3D-FDTD) method. We synthesize both vertical and horizontal velocity waveforms for sources at varying angles of incidence to the ocean/land boundary. We investigate the dependence of signal characteristics on both seismic velocities and source direction. Although the 3-D model simulations are both simple and small-scale, the following conclusions may be made based on this work. For high slopes at the ocean/land interface, T-phase amplitudes on land increase with increasing seafloor slope. This contradicts previous results computed using 2D modeling at lower slope values. T-phase amplitudes on land are strongly dependent on seafloor velocity, with lower amplitudes resulting from higher seafloor velocities. T-phase amplitudes on land drop off rapidly with increasing angle of incidence of the acoustic phase to the shoreline, then level off past the critical angle. For reflected waves recorded on hydrophones, the amplitude ratio of the reflected to the direct acoustic arrivals increases with both increasing angle of incidence at the shorelineand with increasing impedance mismatch between ocean and land.