Geometrics G-877, Operation Manual

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1.2 MAGNETOMETER
THEORY
In order to understand the proton precession principle in a magnetometer, one needs to
understand what happens inside the sensor of the proton magnetometer. Sensors come in many
shapes and sizes, but for the proton precession magnetometer, sensors share the same operational
design: A non-magnetic container is filled with a liquid (typically water or hydrocarbon fluids
like kerosene, white mineral spirits) in which a coil of wire is immersed. Free protons in the
fluids have spin and small magnetic fields, and therefore the protons will align themselves with
the flux lines of the earth’s magnetic field. However, as soon as “polarizing” current is applied
to the coil the protons will align themselves with the electromagnetic field of the energized coil.
When the current is removed from the coil, the protons will “turn away” from their alignment
with the electromagnetic field of the coil to become realigned with the earth’s magnetic field.
As they “turn away”, the protons do not directly turn to align themselves with the earth’s field,
but because of their inherent spin, “precess” like a spinning top. As they precess, the protons
behave like tiny magnets inside the sensor coil , inducing a small AC signal in the coil. This
signal is amplified and its frequency counted to produce a highly accurate and repeatable
measurement of the total intensity of the magnetic field at the sensor. The frequency of the
measured signal is proportional to the absolute value of the ambient earth’s magnetic field.
Thus, a proton precession magnetometer is an instrument that measures the absolute value of the
earth’s magnetic field by measuring the proton precession frequency and converts that frequency
into the units that are used to quantitatively represent the earth’s magnetic field in gammas or
nanoTeslas (nT).
You may want to obtain more information on the operation, application of magnetometers and
data processing procedures for magnetic data. This information can be found in the Geometrics
publication, Applications Manual for Portable Magnetometers, by Sheldon Breiner available
from our web site
www.geometrics.com under Downloads or included in your equipment
documentation.
G–877 MARINE MAGNETOMETER OPERATION MANUAL
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