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<h1 class="display-3 title">
<b>THE FIRST GENERATION</b>
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<p>
Born from ideas hatched by <b>Guust Nolet</b> in the 1990s, and
further developed by <b>Frederik J. Simons</b> at Princeton,
MERMAID-001 was built by <b>Jeff Babcock</b>, <b>John
Orcutt</b>, <b>Russ Davis</b>, and their groups at the Scripps
Institution of Oceanography in 2003, with funding from the US
National Science Foundation. A prototype in the traditional
sense of the word, it fulfilled the core requirements of
seismological functionality. A modified SOLO (<i>Sounding
Oceanographic Lagranian Observer</i>) float, the acoustic payload
was a High Tech HTI-90-U hydrophone. Data were sampled at 20 Hz,
recorded with 16-bit precision, and stored on a flash memory
card.<br><br>
All in all, MERMAID-001 gathered a mere 120 hours of acoustic
pressure data from a depth of around 700 m offshore La Jolla,
without telemetry. The results were nevertheless encouraging:
over the course of three recovered field tests in November 2003,
September 2004, and August 2007, several positive earthquake
identifications stood out from the noise. One of these was a
tremor offshore Colombia, of sufficient magnitude (moment
magnitude 6.0) and epicentral distance (46.5°, an equivalent
5170 km measured along the surface of the earth, which qualifies
it as <i>teleseismic</i>) to prove the potential of MERMAID
ultimately improving seismic coverage in the oceans. Several
smaller-magnitude events were, strictly
speaking, <i>bycatch</i>: not useful for deep-earth imaging, but
suitable for assessments of local and regional seismicity, and
to study crustal structure.<br><br>
Funding from the UK Natural Environment Research Council (NERC)
and the Nuffield Foundation, and from Princeton University,
proved essential to ensuring continued innovation.<br><br>
Central to the early development was the crafting of algorithms
that enable MERMAID to, completely independently and on an
ongoing basis, detect segments of incoming energy, and identify
any signals of seismological interest. Detection was carried out
by way of traditional STA/LTA analysis, whereby ratios of
short-term over long-term average amplitude are monitored for
threshold exceedance, as is commonly done for telemetered
seismometers worldwide. The discrete wavelet transform (DWT)
proved particularly adept at the task of discrimination and
identification, with a custom algorithm developed by <b>Frederik
J. Simons</b> at Princeton University, later at University
College London.<br><br>
Implemented in integer arithmetic with in-place calculation via
the fast lifting algorithm, the streaming time-domain samples
are reduced to an evolving bar code whose character quite
clearly reveals which records are due to distant earthquakes,
compared to arrivals from regional and local events whose
high-frequency energy has survived selective removal by
intrinsic attenuation and geometrical spreading in the solid
Earth. Another distinct acoustic fingerprint is from <i>T</i>
phases from very shallow sources, whose energy carries over long
distances, bouncing around in the Sound Fixing and Ranging
(SOFAR) low-velocity channel of the ocean itself. Most lately,
these have been used for seismic thermometry over long spatial
and temporal baselines.<br><br>
Simple pattern recognition (no machine-learning required!) in
the two-dimensional space of time versus wavelet scale removes
potential triggering due to non-seismological sources
(e.g. shipping noise, volcanic eruptions, glacial calving, rain,
biological signals—not to mention mysterious plane crashes). The
breakdown of time-domain acoustic pressure variations into
scale-dependent wavelet coefficients provides useful ways of
data denoising and compression, and has revealed its utility in
applications for seismometry on land, e.g., for earthquake
early-warning studies. Adaptations targeting rather than
avoiding such competing sources of energy are straightforward:
after all, one maid’s noise is another one’s signal. Indeed,
independently developed profiling floats with acoustic packages
tailor-made for cetacean (whales!) census research, and wind and
rainfall intensity, i.e., meteorology, are to be considered part
of the extended family of "Lagrangian" "quasi-Eulerian"
hydroacoustic marine devices.<br><br>
MERMAID-001 retired to the University of Rhode Island, where she
went on display on the Narragansett campus in the lab
of <b>Harold T. "Bud" Vincent</b>.<br><br>
Read on about
the <a href="https://geoweb.princeton.edu/people/simons/earthscopeoceans/secondgeneration.html">Second
Generation</a>.
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