MAGNETIC METHOD - Field Geophysics - page 72

transmitted using direct current, producing fields of several tens of thousands
of nT that may extend for more than a kilometre on either side of the power
lines. Useful magnetic surveys are simply impossible in such areas.
3.3 Magnetic Instruments
Early magnetometers were compass needles mounted on horizontal axes,
and measured vertical fields. These torsion magnetometers were in common
use until about 1960, when they began to be replaced by fluxgate, proton
precession and alkali vapour magnetometers. Instruments of all three types
are still used, are available with built-in data loggers and can be set to record
automatically at fixed time intervals. All three can be used singly or, in
tandem, as gradiometers , although care must then be taken with precession
instruments to ensure that the polarising field from one sensor does not affect
the other.
3.3.1 Proton precession magnetometer
Proton precession magnetometers make use of the small magnetic moment
of the hydrogen nucleus (proton). The sensing element consists of a 'bottle'
containing a low-freezing-point hydrocarbon fluid about which is wound a
coil of copper wire. Although many fluids can be used, the manufacturer's
recommendation, usually for high-purity decane, should always be followed
if the bottle has to be topped-up. A polarising current of the order of 1 amp
or more is passed through the coil, creating a strong magnetic field, along
which the moments of the protons in the hydrogen atoms tend to align.
When the current is switched off, the protons realign to the direction of the
Earth's field. Quantum theory describes this reorientation as occurring as an
abrupt 'flip', with the emission of a quantum of electromagnetic energy. In
classical mechanics, the protons precess about the field direction, as a gyro-
scope precesses about the Earth's gravity field, at a frequency proportional
to the field strength, emitting electromagnetic waves as they do so. Both
theories link the wave frequency to the external field via two of the most
accurately known of all physical quantities, Planck's constant and the proton
magnetic moment. In principle the proton magnetometer is capable of almost
any desired accuracy, but in practice the need for short reading times and
reasonable polarisation currents sets the limit at about 0.1 nT. In the Earth's
field of about 50 000 nT, the precession frequency is about 2000 Hz, and
sophisticated phase-sensitive circuitry is needed to measure to this accuracy
in the half or one second that is all that modern geophysicists will tolerate.
Proton magnetometers may give erratic readings in strong field gradients
or because of interference from power lines and radio transmitters, or even
from eddy currents induced in nearby conductors by the termination of
the polarising current. Also, they can only measure total fields, which may