Acoustic performance of new International Monitoring System hydrophone stations

The International Monitoring System (IMS) is a global network of hydroacoustic, infrasonic, seismic and radionuclide sensors to detect, identify, and locate the signals generated by a nuclear explosion to verify compliance with the Comprehensive Nuclear-Test-Ban Treaty. The IMS hydroacoustic network, which monitors the major world oceans, consists of eleven stations located with an emphasis on the vast ocean areas of the Southern Hemisphere. Presently, two new stations based on hydrophones and one new station based on seismometers, called a T-phase station, are operational and transmitting real time data to the International Data Centre (IDC) in Vienna, Austria. Signals have been observed from man-made sources including seismic exploration using air guns and shipping. Water-borne (T-phase) and earth-borne (P-phase) signals originating from seismic events located undersea and near the continental margin have been detected at distances up to 9000 km. For the closer range undersea events the T-phase energy is significant across the 1 to 100 Hz bandpass of the system. At longer ranges propagation effects reduce the energy at the lower (<~3 Hz) and higher (>~30 Hz) frequencies producing a maximum signal to noise ratio in the 5 to 20 Hz band. The triplet hydrophone sensor geometry allows the determination of the azimuth of the T-phase signals from measurements of time of arrival differences. Natural events with known positions have been used to refine the triplet geometry allowing azimuth determination to within a few degrees