We are continuing the development of the Hydroacoustic Blockage Assessment Tool (HABAT) which is designed for use by analysts to predict which hydroacoustic monitoring stations can be used in discrimination analysis for any particular event. The research involves two approaches (1) model-based assessment of blockage, and (2) ground-truth data-based assessment of blockage. The tool presents the analyst with a map of the world, and plots raypath blockages from stations to sources. The analyst inputs source locations and blockage criteria, and the tool returns a list of blockage status from all source locations to all hydroacoustic stations. We are currently using the tool in an assessment of blockage criteria for simple direct-path arrivals. Hydroacoustic data, predominantly from earthquake sources, are read in and assessed for blockage at all available stations. Several measures are taken. First, can the event be observed at a station above background noise? Second, can we establish backazimuth from the station to the source. Third, how large is the decibel drop at one station relative to other stations. These observational results are then compared with model estimates to identify the best set of blockage criteria and used to create a set of blockage maps for each station. The model-based estimates are currently limited by the coarse bathymetry of existing databases and by the limitations inherent in the raytrace method. In collaboration with BBN Inc., the Hydroacoustic Coverage Assessment Model (HydroCAM) that generates the blockage files that serve as input to HABAT, is being extended to include high-resolution bathymetry databases in key areas that increase model-based blockage assessment reliability. An important aspect of this capability is to eventually include reflected T-phases where they reliably occur and to identify the associated reflectors. To assess how well any given hydroacoustic discriminant works in separating earthquake and in-water explosion populations it is necessary to have both a database of reference earthquake events and of reference in-water explosive events. Although reference earthquake events are readily available, explosive reference events are not. Consequently, building an in-water explosion reference database requires the compilation of events from many sources spanning a long period of time. We have developed a database of small implosive and explosive reference events from the 2003 Indian Ocean Cruise data. These events were recorded at some or all of the IMS Indian Ocean hydroacoustic stations: Diego Garcia, Cape Leeuwin, and Crozet Island. We have also reviewed many historical large in-water explosions and identified five that have adequate source information and can be positively associated to the hydrophone recordings. The five events are: Cannekin, Longshot, CHASE-3, CHASE-5, and IITRI-1. Of these, the first two are nuclear tests on land but near water. The latter three are in-water conventional explosive events with yields from ten to hundreds of tons TNT equivalent. The objective of this research is to enhance discrimination capabilities for events located in the world's oceans. Two research and development efforts are needed to achieve this: (1) improvement in discrimination algorithms and their joint statistical application to events, and (2) development of an automated and accurate blockage prediction capability that will identify all stations and phases (direct and reflected) from a given event that will have adequate signal to be used in a discrimination analysis. The strategy for improving blockage prediction in the world's oceans is to improve model-based prediction of blockage and to develop a ground-truth database of reference events to assess blockage. Currently, research is focused on the development of a blockage assessment software tool. The tool is envisioned to develop into a sophisticated and unifying package that optimally and automatically assesses both model and data based blockage predictions in all ocean basins, for all NDC stations, and accounting for reflected phases (Pulli et al., 2000). Currently, we have focused our efforts on the Diego Garcia, Cape Leeuwin and Crozet Island hydroacoustic stations in the Indian Ocean
