Site Characterization for Using Overhauser Magnetometer

Dr. Ivan Hrvoic, Mike Wilson and Francisco Lopez (GEM Systems, Inc., Markham, Ontario)

Magnetometers and gradiometers are being used increasingly in monitoring roles (i.e. to monitor atmospheric magnetic disturbances, volcanoes or earthquakes).

The Overhauser magnetometer, with its unique set of features, represents a pillar of modern magnetometry of the Earth’s magnetic field. Its sensitivity matches costlier and less convenient cesium magnetometers, for example. The Overhauser magnetometer also offers superior omnidirectional sensors; no dead zones; no heading errors; or warm-up time prior to surveys; wide temperature range of operation (from –40 to 50 degrees Celsius standard and –55 to 60 degrees Celsius optional); rugged and reliable design; and virtually no maintenance during its lifetime. Other advantages include high absolute accuracy, rapid speed of operation (up to 5 readings per second), and exceptionally low power consumption.

Overhauser magnetometers use proton precession signals to measure the magnetic field – but that’s where the similarity with the proton precession magnetometer ends.

Overhauser magnetometers were introduced by GEM Systems, Inc. following R&D in the 80’s and 90’s, and are the standard for magnetic observatories, long term magnetic field monitoring in volcanology, geophysical ground and vehicle borne exploration, and marine exploration.

Operating Principles

The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source).

RF magnetic fields 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. they do not contribute noise to the measuring system).

The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality, γ_ρ, is known to a high degree of accuracy.

Overhauser magnetometers achieve some 0.01nT/Hz^1/2 noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.

Site characterization

One of the primary goals in long term monitoring projects is to ensure that sensors used for measurements locate in magnetically very quiet zones. This can be ensure by a magnetometer or gradiometer ground survey. Data presented in this article generated in a site in Oaxaca, Mexico where specialists from GEM Systems and University of Mexico installed a SuperGradiometer for earthquake prediction applications.