Comprehensive Nuclear Test Ban Treaty monitoring: The hydroacoustic network and its applications.

In the 14 years since the Comprehensive Nuclear Test Ban Treaty was signed, a network of 262 International Monitoring System (IMS) stations (mostly seismic) has been constructed out of a planned total of 337 facilities. 11 are hydroacoustic stations, of which six use hydrophones and five use T‐phase seismometers. While the hydroacoustic network’s primary purpose is to detect, locate, and identify underwater explosions, a secondary benefit is civil and scientific applications of the data. Given the ocean’s remarkable transparency to low‐frequency sound, it is not surprising that the hydrophone stations detect many other events and activities, e.g., marine mammal vocalizations, iceberg calving, underwater volcanoes, and seismic exploration. A variety of scientific studies have already used CTBTO hydroacoustic data, and the new virtual Data Exploitation Center (vDEC) is designed to facilitate and encourage more studies. Tsunami warning centers in eight countries around the world now receive real‐time seismic and hydroacoustic data from the CTBTO. Future applications include ocean climate studies using acoustic thermometry, taking advantage of the permanency of the installed hydrophones and the repeatability of sources of opportunity, such as seismic exploration airguns. The challenge of sustaining the hydroacoustic network for decades must not be underestimated, however.