Definition and Introduction
T phases are defined as seismic recordings of signals having traveled an extended path as acoustic waves in the water body of the oceans. This is made possible by the “sound fixing and ranging” (SOFAR) channel, a layer of minimum sound velocity acting as a wave guide at average depths of 1000 m (Okal 2007). It allows the efficient propagation of extremely small signals over extremely long distances, in practice limited only by the finite size of the ocean basins. The existence of the SOFAR channel results from the dependence of the velocity of sound in water on temperature, pressure, and salinity. As a result, the detailed structure of the channel (including the value of the minimum velocity) varies both geographically (mainly with latitude) and seasonally, but at first order, the SOFAR channel can be regarded as a quasiuniversal feature of the world’s oceans.
At the shoreline of a continent or island, the acoustic wave is converted into a seismic wave which...
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