Estimates of seismo-acoustic transfer functions relevant to CTBT IMS T-phase stations

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) is a world-wide network of stations and laboratories designed to detect nuclear explosions underground, in the oceans and in the atmosphere. The IMS incorporates four technologies: seismic, hydroacoustic and infrasound (collectively referred to as waveform technologies), and radionuclide (particulate and noble gas). The focus of this presentation is the hydroacoustic component of the IMS, which consists of 6 hydroacoustic stations employing hydrophones deployed in the oceans and 5 near-shore seismic stations, called T-phase stations, located on islands or continental coastal regions. The purpose of T-phase stations is to detect water-borne hydroacoustic pressure waves converted into seismic waves that propagate on the earth’s crust and are detected by land seismometers. However, the conversion process from in-water pressure to near-interface seismic waves is complex and strongly dependent on the properties of the local underwater and geological environment. To further understand this conversion process, state-of-the-art hybrid seismo-acoustic wave propagation models have been applied to simplified environments and to scenarios representative of the conditions encountered at IMS T-phase stations to compute broadband pressure time-series in the water and particle  velocity components on-land. Transfer functions from in-water pressure to on-land seismic particle velocity and vice versa were estimated both from modelling results and from real data acquired in locations where the hydrophones and (non-IMS) seismic stations were within 50-km distance. The presented results have been used to give a first assessment of the feasibility of characterizing the hydroacoustic phase of an in-water event by on-land seismic recordings at IMS T-phase stations, subject to limited a-priori environmental information and limiting factors, such as band-width and instrumental and/or environmental noise.