R. A. Stephen
Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
In marine seismology two types of transient sound sources are used. Controlled sources such as airguns or explosions are used in seismic reflection and refraction surveys to study geological structure below the seafloor. Naturally occurring earthquakes are also observed and they are used to study tectonic and regional and whole earth structure problems. In both cases three-component particle motion, either velocity or acceleration, is measured on or within the seafloor. However controlled source studies work in the VLF band (1.0--100 Hz) and earthquake studies generally run in the lower ULF band (0.001--5 Hz). A major advantage of transient sources versus harmonic sources is that multi-pathing can be distinguished by the time separation of phases. In fact, many schemes for inferring geological structure from transient seismic data are based solely on the arrival times of certain phases. Also time spread (coda or signal-generated noise) can be observed directly as reverberation after the coherent arrivals. The polarization of the separated phases can be used to study propagation effects. Examples of quantitative data of both controlled and earthquake sources will be shown and discussed.