Implementation of a hydroacoustic monitoring system for the comprehensive nuclear-test-ban treaty

For the purpose of monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty, a global hydroacoustic network is being established in order to detect, locate and identify the signal generated by a nuclear explosion. The network is composed of 6 hydrophone and 5 T-phase stations. A hydrophone station is based on an ocean-deployed hydrophone sensor with a frequency range of 1 Hz to 100 Hz. The hydrophone sensor elements are at a fixed depth near the axis of the SOFAR channel, floated from an ocean-bottom anchor and cabled back to shore. A T-phase station is based on an island-deployed seismometer sensor. Each T-phase station is sited near the shore of a small island with steep bathymetry. It relies on the detection of a signal that has propagated predominantly through the ocean, but has been converted to seismic energy at the margin of the island. The implementation of the network is carried out in a phased-approach and starts with a site survey for each station. This survey is meant to assess the suitability of the site for hosting a station and to prepare for the installation work. It involves various tasks, primarily bathymetric measurements, seafloor characterization, noise measurements, evaluation of the environment constraints, studies of acoustic propagation, and assessment of the efficiency of the hydroacoustic/seismic conversion. There are currently no existing stations that meet the CTBT requirements and a complete and complex design phase has to be implemented, especially for hydrophone stations. Particular attention is given to the following topics: system noise, calibration and quality of the data measurements at low frequency, deployment and reliability of the equipment