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.
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.
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.
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 resulting time series provide a means of monitoring acoustic
travel times within the oceanic sound channel over thousands of kilometers.