We describe the design, development, and testing of an
autonomous oceanic device that should revolutionize earth structure
determination via global seismic tomography by detecting earthquakes
teleseismically, in the oceans. One prototype of the Mermaid, as our
instrument is called, for Mobile Earthquake Recording in Marine Areas by
Independent Divers, was constructed and tested in situ. It consists
of the combination of two readily available, relatively low-cost, but
state-of-the-art components, namely a Sounding Oceanographic Lagrangian
Observer, or Solo float, and an off-the-shelf hydrophone, interacting by
custom-built data logging hardware. The final design, which is yet to be
completed, must have, in addition, a depth sounder, a Global Position System
receiver, and onboard detection and discrimination software operating on a
low-power processing platform, as well as be endowed with satellite
communication capabilities for telemetered data transfer. In this paper, we
describe the necessary, and successful, seismological tests required to
justify and prepare for these improvements, and to make the vision of a
floating global array of seismic sensors a reality. We report on three pilot
experiments conducted at a depth of 700 m offshore La Jolla, during which
over 120 hours of data were gathered; on the development of efficient
wavelet-based signal processing algorithms; and on the analysis of the
actual pressure time series collected using them. Five signals from
earthquakes, of which one teleseismic, were successfully recorded and
identified, and this information is sufficient to allow quantitative
estimates of the likely success of our instrument in collecting data useful
for seismic tomography during dedicated campaigns, as will be planned for
the future.
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