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Catamaran
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A catamaran is a sailing
ship made with two parallel adjoined hulls. Very stable thanks to its width -
and non-magnetic when made of fiberglass - a catamaran, with its very small
draft, is the perfect ship for magnetometric surveys conducted even in shallow
water areas. For the discovery of the "San Diego", Franck Goddio's team used his
20m long, 10m wide fiberglass-hull catamaran called "Kaimiloa".
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Anomaly
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The geophysical analysis techniques consist of
highlighting what we call "discontinuities" of the measured parameters, such as
the ambient magnetic field or the acoustic response from the bottom. These
discontinuities or contrasts are called "anomalies". In archaeological
applications, the interesting anomalies can be particularly small and are often
hidden between the complex and frequently occurring natural, geological
anomalies, which are of course detected at the same time by the sensors. The
discrimination between those anomalies - between these with possible
archaeological significance, and those without - once again relies on both
technology, and on the judgement of trained professionals. It comes down to the
sensitivity of the sensors, and the expertise with which they are implemented;
on the computer processing of the measurements, and on the analysis of the
results.
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Differential Global Positioning System
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The GPS
is an absolute geographical positioning system, providing for the entire surface
of the earth the position in latitude, longitude, and altitude of the mobile
receiver, through information sent by a constellation of satellites. To get an
accuracy of better than one metre, which is necessary in archaeological
research, a GPS ground station is installed to send real time corrections by
radio to the mobile. The positions given by the mobile after correction are then
called differential positions.
Franck Goddio's team has also developed
an original underwater DGPS to collect the position of the archaeological
remains during excavation.
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Computing Systems
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The survey
catamaran is equipped with acquisition and navigation computers, collecting,
recording, and graphically displaying the data from the geophysical sensors
(magnetometers, echo sounders, sidescan sonar, etc.) and from the positioning
sensors (GPS, acoustic positioning, pressure sensors, heading, winch control
parameters, etc.) An on-board geophysical data processing station is then used,
running specialized software to lead to the edition of the results map for
analysis.
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