Seismic Monitoring
"T-wave" refers to a relatively high-frequency waterborne seismic phase generated by submarine earthquakes. ("T" corresponds to tertiary, since these waterborne phases travel slower than solid-earth P- (primary) and S- (secondary) waves, and therefore arrive third on seismograph records.) Waterborne "T-waves" propagate more efficiently than solid-earth body waves due to the presence in most of the global ocean of an acoustic wave guide , commonly referred to as the SOFAR (SOund Fixing And Ranging) channel. Relative to solid-earth seismic waves that propagate spherically, acoustic waves within the oceanic SOFAR channel propagate cylindrically and can therefore travel great distances with little attenuation (Figure 1). The finite thickness of the wave guide results in inefficient propagation below 2 Hz, but at higher frequencies small seismic events can be detected at ranges of thousands of kilometers.

Direct, indirect, and theoretical methods have been applied to data from various sources to estimate the detection thresholds of fixed hydrophone systems. These thresholds, when combined with frequency-magnitude scaling relationships derived from the global networks, provide predictions of the mean number of events expected for the Juan de Fuca Ridge system. The results indicate that while the global networks are limited to a detection threshold of mb = 4.2 and 1 event/year; earlier work with PMR/MILS hydrophones and analog techniques was limited to Mb = 3.4 and 15 events/year; SOSUS hydrophones combined with digital signal processing techniques can detect a minimum Mb = 2.5 and 265 events/year, perhaps as low as 2.4 as indicated by events recorded from northern California; and beam forming of the SOSUS arrays can reduce the detection threshold to Mb = 1.8-2.1 and 1,000-2,000 events/year.

Marine Mammal Monitoring
The same systems used for seismic monitoring are capable of detecting vocalizations from large marine mammals at long ranges, in the open ocean, an environment where very little is known about their behavior, distribution, and habitat preferences. In the case of large blue whales, accurate locations can be derived for individual animals at ranges of several hundred kilometers by applying mathematical matched filters to the acoustic signals. Other species under study include fin whales, humpback whales, Minke whales, and potentially other large whales such as Sei's whales, Bryde's whales, and sperm whales.

Acoustic Thermometry
In support of the seismic program, a low-power acoustic beacon was installed on Axial Seamount in August, 1993. This known acoustic source projects twice per day to allow calibration of ocean sound speed models. The travel time is routinely derived for each SOSUS array at ranges of up to 3600 km. . The resulting time series provide a means of monitoring acoustic travel times within the oceanic sound channel over thousands of kilometers.