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NMR Magnetometer
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The main sensors of the detection system are Nuclear Magnetic Resonance (NMR)
Magnetometers. These highly sensitive sensors were developed by the French
Atomic Energy Commission (CEA). Based on simultaneous proton and electron
magnetic resonance (the Abragam-Overhauser effect), the NMR magnetometers
measure more than one thousand times per second the absolute value of the
earth's magnetic field - with an accuracy of one fifty-millionth of its value!
Theory
The probe is a field/frequency transducer based on
the principle of nuclear magnetic resonance, amplified using dynamic electron
polarization.
The hydrogen atoms of standard solvents have a magnetic
moment which is proportional to their spin. In the earth's magnetic field, they
precess around this field at a frequency proportional to the modulus - this is
called the Larmor frequency, 1 to 3 Khz in the earth's field. The resonant
electromagnetic excitation creates by spin phase coherence a macroscopic
magnetization component precessing at the Larmor frequency. This component
induces a voltage in the detection coil. The measured frequency gives the value
of the field!
The nuclear magnetism is not directly detectable in the
earth's field. The dynamic electronic polarization amplifies by a factor of a
thousand the nuclear signal. The nuclear spins are coupled to the free electron
spins of a radical in solution. Two excitation frequencies of the electronic
resonance are possible: one gives a positive polarization, the other negative.
The frequencies depend on the solvent used.
The correct choice of a pair
of solvents, containing the same radical, will give - at the same frequency - a
positive polarization factor in one and a negative one in the other (double
effect).
The probe is composed of two hydrogenated solvents in
individual flasks containing a free radical in solution, a high frequency
dynamic polarization excitation circuit and a low frequency circuit, which
simultaneously excites the nuclear resonance, and measures the signal. It is
constructed of two symmetrical coils mounted in opposition.
MRM-2000 Frequency-meter
The very accurate measurement and the conversion of the frequency signal issued from the probe is
realized through a specialized frequency-meter allowing, with a sampling of 1000Hz (1 measurement
every millisecond), the delivery of the absolute value of the magnetic field with an accuracy of one
thousandth of a Gamma, which means an accuracy of a frequency measurement to the seventh
decimal place (107 Hz).
A high-integrated version of this frequency-meter, named MRM-2000 has been developed by the
engineering team of Franck Goodio in collaboration with the French Atomic Energy Commission.
Geomagnetism
The earth's magnetic field superposes very
complex temporal and spatial phenomena - the earth's crust geology, dynamo effect,
sun-earth interaction, ionospheric and telluric currents, etc. - based on
magnetic and electric properties. The earth's magnetic field can be estimated as
a bipolar field with a value between 20000 nano.Tesla (nT) on the equator and
60000 nT on the poles, added to a field of world-wide anomalies (of about 10000
nT), a field of geological local anomalies and temporary phenomena of a few
tenths of nT per day.
Application to Archaeology
In
archaeological remains, interesting localized magnetic anomalies, are
superimposed on these natural anomalies. The discrimination between these
anomalies is grounded in the high sensitivity of the sensors, and, when needed,
a local magnetic gradient measurement between two simultaneously towed
magnetometers. This gradiometer permits removal in real time of the temporal
variations of the earth's magnetic field and permits us to reject a great
proportion of the geological anomalies. This method, implemented with NMR
magnetometers, enables the detection of very weakly magnetic objects, even those
deeply buried in the sediments.
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Sidescan Sonar
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This seismic sensor provides an image of the acoustic contrasts on the
sea-bed, on a band 50 metres on each side of the boat. Highlighting rocks or
other prominent objects on the sea-bed, the sonar also gives an idea of their
height by a measurement on the real time pulldown screen of the projected
shadows. At the same time, the magnetometers will determine if a particular spot
seen on the sonar is magnetic or not. The computer processing of the sonar data
allows us to build a mosaic of the covered area by laying side by side the
geographically positioned bands.
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Echo Sounder
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A precise bathymetric or relief map of the area is obtained by the implementation
of echo sounders on the boat. These acoustic sensors deliver a non-stop accurate
measurement of the depth along the survey lines.
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Acoustic Positioning System
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The system, called "short base acoustic
positioning", is grounded on a regular time transmission of an acoustic signal
from a transmitter - called a "pinger," and mounted on the mobile unit to be
positioned - to an immersed boat-fixed receiver. Several pingers sending on
request from the transceiver can be used at the same time. The positioning of
each mobile unit is then calculated in terms of range and bearing seen from the
receiver. Taking into account the geographical position of the boat recorded by
the DGPS, and the known fixed position of the transceiver on the boat, the
geographic position of all pinger-equipped sensors can be determined in real
time.
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