T-phase Stations for the International Monitoring System of the Comprehensive Nuclear-Test Ban Treaty: A Global Perspective

Abstract

The enforcement of the Comprehensive Nuclear-Test Ban Treaty (CTBT) requires the monitoring of acoustic disturbances in the world's oceans in order to guard against the possibility of a nuclear test being hidden either in the water column itself or on a remote island. The traditional means of acoustic monitoring in the water column builds on the experience of the SOSUS project ( e.g. , Mikhalevsky, 1999), and consists of deploying hydrophones within the SOFAR channel, the layer of low acoustic velocity extending between depths of approximately 600 to 1800 m in the world's oceans, and functioning as a waveguide for acoustic waves with frequencies greater than 2.5 Hz. While such stations offer superb recording capabilities, notably at high frequencies, they must be tethered to a nearby island or continental shore for the purpose of providing power and retrieving data, which makes for technologically complex, and hence expensive, installation and maintenance procedures ( e.g., Schrodt, 1999). On the other hand, the acoustic energy in T waves is readily converted into seismic energy upon hitting the shore of an island or continent (I will show that T waves were indeed first identified on seismic records); in addition, under exceptionally favorable circumstances, T phases have actually been felt by island populations at distances of several thousand kilometers from both natural sources (Talandier and Okal, 1979) and man-made ones (the underwater nuclear test WIGWAM on 14 May 1955 was felt as far as the Bonin Islands [Wadati, 1960]). The concept of a so-called “ T -phase station” is simply that of recording seismically the elastic energy resulting from the conversion of the acoustic wave at the continental or island shore; issues such as powering, data storage, and retrieval then become relatively straightforward, resulting in considerable savings during installation, operation, and maintenance. Thus, the …