Information Sheet PPM 1
Proton Precession Magnetometer
Department of Earth Sciences, The University of Hong Kong. //www.hku.hk/earthsci
Theory
The outer core of the Earth is a fluid, metallic layer located at about 2980 km-5120 km beneath the
Earth’s surface. Continuous advective motions of the conductive fluid in the outer core produce a
strong magnetic field. This external field permeates most materials, and in turn, causes the materials
to possess an induced magnetisation. The intensity induced magnetisation of a material depends on
the strength of the external field as well as the magnetic
susceptibility of the material, i.e., how easily can it
become magnetised. In magnetometer surveys,
geophysicists measue variations in earth’s magnetic field
in order to identify bodies of anomalous magnetic
susceptibilities. The technique is often used to locate
mineral deposits, archaeological sites, buried tanks and
ordnance, subsurface contacts between geological units,
or submerged objects such as submarines or shipwrecks.
Proton precession magnetometer
Measurement of Magnetic Field Intensity
A magnetometer is an instrument used to measure the strength of a magnetic field. The proton
precession magnetometer operates on the principle that protons in all atoms are spinning on an axis
aligned with the magnetic field. Ordinarily, protons tend to line up with the earth's magnetic field.
When subjected to an artificially-induced magnetic field, the protons will align themselves with the
new field. When this new field ceases, the protons return to their original alignment with the earth's
magnetic field. As they change their alignment, the spinning protons precess, or wobble, like a
gyroscope. The frequency at which the protons precess is directly proportional to the strength of the
earth's magnetic field. This is the Proton Gyromagnetic Ratio, equal to .042576 Hertz / nanoTesla. (The
magnetic field intensity of Hong Kong is about 44,000 nT.)
In a proton precession magnetometer, a container of fluid rich in hydrogen
atoms, usually a hydrocarbon, is surrounded by a coil of wire which can be
energized by a direct current to produce a strong magnetic field, forcing
all protons to align in a certain direction. The current is then shut off,
letting the protons to precess back to their original direction. The
precession induces a very weak signal into the same coil, which is
measured by a frequency counter to give a direct readout of the magnetic
field strength.
Corrections and Modeling
Several corrections are needed in order to extract sensible
information from magnetic field intensity data. In Hong Kong,
magnetic field surveys are often done in nighttime to minimize
noises and interferences from artificial sources such as mass
transit systems. Often two magnetometers are employed
simultaneously, one as a base station to record diurnal
variations and occasional intensity spikes, while the other to
collect magnetic data in the study area. Data modeling is often
necessary to determine the depth and geometry of the object
responsible for observed magnetic variations.