NMR Magnetometer

The main devices of Franck Goddio’s 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!

The probe is a field/frequency transducer based on the principle of nuclear magnetic resonance, using amplified 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.

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). It delivers 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 (10 puissance-7 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.

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.

These natural anomalies are superimposed by archaeological remains, indicated via local magnetic anomalies. The discrepancies between these anomalies can be measured due to the high sensitivity of the sensors. If needed, a local magnetic gradient measurement between two simultaneously towed magnetometers can be applied. 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.