Ground Magnetometers

---

Quantum magnetometers are widely used geophysical instruments for mineral exploration, hydrocarbon exploration; archeology; environmental and engineering; unexploded ordnance detection (UXO / EOD); and monitoring (magnetic observatory, volcanology, earthquake hazard) applications.  

 A standard proton precession magnetometer uses hydrogen atoms to generate precession signals. Liquids such as kerosene and methanol 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 that is wound around the sample. In a magnetometer, such as the GSM-19T, this creates a high-intensity magnetic field of over 100 Gauss. Protons in this field are polarized to a stronger net magnetization corresponding to the thermal equilibrium of stronger magnetic flux density.  When the auxiliary flux is released, the “polarized” protons precess to re-align themselves with the “normal” magnetic flux density. The frequency of the precession relates directly to the magnetic field strength.

The Overhauser Effect is a nuclear method that takes advantage of a “quirk” of physics that affects the hydrogen atom. This effect occurs when a special liquid (containing electrons) is combined with hydrogen and then exposed to a radio frequency (RF) magnetic field (i.e. generated from a radio frequency source).  RF fields are ideal for this type of application because they are transparent to the Earth’s DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. does not contribute noise to the measuring system).  The unbound electrons in the special liquid (normally a mixture of free radicals) transfer their excited state (i.e. energy) to the hydrogen nuclei (protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like in a proton magnetometer – but with much less power and to greater extent.  The proportionality of the precession frequency and the magnetic flux density is linear and can be described through a simple equation.

Optically pumped magnetometers use gaseous alkali metals from the first column of the periodic table, such as cesium and potassium (or He 4 in metastable state). That means that the cell containing the metal must be continuously heated to approximately 45 degrees C. These magnetometers operate on virtually the same principle.  First, a glass cell containing the gaseous alkali metal is exposed (or pumped) by light of a very specific wavelength – an effect called light polarization. The frequency shift of light is specifically selected and circularly polarized for each element to shift electrons from level 2 to the excited state 3.  Electrons at level 3 are not stable, and these electrons spontaneously decay to both energy levels 1 and 2. Eventually, the level 1 is fully populated (i.e. level 2 is depleted). When this happens, the absorption of polarizing light stops and the vapour cell becomes more transparent.  This is when RF depolarization comes into play. RF power corresponding to the energy difference between levels 1 and 2 is applied to the cell to move electrons from level 1 back to level 2 (and the cell becomes opaque again). The frequency of the RF field required to populate level 2 varies with the ambient magnetic field and is called the Larmor frequency.  The effect of polarization and depolarization is that the light intensity becomes modulated by the RF frequency. By detecting light modulation and measuring the frequency, we can obtain a value of the magnetic field.