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Tsunami - Reading in the Deep Sea
Tsunami - Reading in the Deep Sea
November 19,2007.
In
order to extend alert times and avoid false alarms, a new
seafloor pressure recording system has been designed to detect
tsunamis shortly after their development in the open ocean.
The project is directed by scientists of the working group
'Marine Observation Systems' at the Alfred Wegener Institute
for Polar and Marine Research, part of the Helmholtz Association.
Successful testing of the recording system off the Canary
Islands in November 2007 means that a new mile stone for the
development of the Indian Ocean Tsunami Early Warning System
(GITEWS) has been reached.
The GITEWS project is supervised by the German National Research
Centre for Geosciences (GFZ) in Postdam. Scientists of the
Alfred Wegener Institute, in collaboration with companies
Optimare and develogic, and with the Zentrum für Marine Umweltwissenschaften
(MARUM) and the University of Rhode Island, are developing
part of the simulation component and the so-called pressure-based
acoustically coupled tsunami detector (PACT) for real-time
detection of sea level rises in the deep ocean.
The German tsunami early warning system is unique in that
it processes a multitude of information as the basis for a
comprehensive and accurate evaluation of every particular
situation. Within just few minutes, measurements of the vibrations
and horizontal seafloor movements off the coast of Indonesia
provide a clear picture of the location and intensity of a
seaquake, which, at the warning centre, facilitate the appropriate
selection of a previously calculated tsunami propagation model.
However, not every
seafloor quake causes a tsunami. "There is only one way to
be clear about this and avoid nerve-wrecking and costly false
alarms: we must measure sea level directly", says PACT-project
leader Dr Olaf Boebel of the Alfred Wegener Institute.
For this purpose, sea level recordings must take place off
the coast, in the deep ocean. At water depths of thousands
of meters, a tsunami wave travels at several hundred km/hr,
but is only inches high, and approximately 60 miles long.
Not before it reaches the coast or shallower waters, does
a tsunami wave develop into a massive wall of water several
meters high.
Being able to detect the very slight sea level rise in the
deep ocean reliably and precisely requires the use of bottom
pressure sensors. These instruments are installed on the seafloor
where they measure any sea level changes in the water column
above. In this process, the weight of any additional water
leads to minute pressure increases at the seafloor which are,
nevertheless, reliably recorded by the PACT bottom units,
precision instruments built by Optimare in Bremerhaven.
Representing one of the greatest challenges of the PACT project,
this problem was addressed by the Stuttgart-based company
develogic through use of highly modern technology: Similar
to a fax machine, an acoustic modem uses a sequence of sounds
- the so-called telegram - to transmit information to a second
modem which is connected to a buoy near the surface, sending
the data via satellite to the warning centre.
Within the overarching GITEWS project, the primary PACT objective
consists of the new development of a reliable, compact and
highly energy efficient system which will record and analyze
seafloor pressure every 15 seconds, and which will transmit
the information to the surface modem if a tsunami event is
detected. After approximately two years of development work
on PACT, an important milestone was reached recently, when
in-situ tests of the system were completed successfully north
of the Canary Islands, using a trial fixture supplied by MARUM
(Zentrum für Marine Umweltwissenschaften in Bremen).
From depths below 10,170 feet and over periods of several
days, pressure data were transmitted repeatedly to the surface
modem. The most important result: none of the data telegrams
were lost, a crucial requirement for the reliable functioning
of the warning system.
Having been tested successfully, the new system will now be
integrated into the GFZ-developed surface buoy and the entire
early warning system. Further tests, scheduled for early next
year in the Mediterranean, will investigate the transmission
reliability under various weather conditions. "Surely, the
upcoming winter storms will give us the opportunity to discover
the limits of the system", suggests Boebel.
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