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May 5 2014

GEM’s Proton Precession Magnetometer

Proton Precession Magnetometers have long had a niche as inexpensive portable magnetometers, despite limitations such as relatively large power consumption and relatively low sensitivity. Typical applications include environmental and engineering surveys where targets are relatively near surface and do not require high sensitivities to detect and map, or production-oriented reconnaissance surveys for resource exploration.

Proton Precession Magnetometer

Operating Principles

A proton magnetometer uses hydrogen atoms to generate precession signals. Liquids, such as kerosene, are used because they offer very high densities of hydrogen and are not dangerous to handle.

A polarizing DC current is passed through a coil wound around a liquid sample (water, kerosene, or similar). This creates an auxiliary magnetic flux density of the order of 100 Gauss.

Protons in this field are polarized to a stronger net magnetization corresponding to the thermal equilibrium of the stronger magnetic flux density. When the auxiliary flux is terminated quickly, the “polarized” protons precess to re-align them to the normal flux density. The frequency of the precession, fo, relates directly to the magnetic flux density, B, (units of which are Teslas, T), according to the following equation:

fo= (γp / 2π) B                    γp / 2π = 42.5763751 MHz/T (3)

The precession signal is present from a fraction of a second to up to 2 seconds, and can be measured using a special counter. Signal quality can also be derived from the signal amplitude and its decay characteristics, which are averaged over the recording period.

Proton precession measurements are necessarily sequential. This means that there is an initial polarization, followed by a frequency measurement – after which, the cycle is repeated. This differs from continuous measurements where the nuclei are polarized and frequency measurements are made simultaneously.

In summary, proton precession uses a simple method of polarization, but it allows measurement of sensitivities to a fraction of an nT and up to 2-3 readings per second.

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