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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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