Geology Reference
In-Depth Information
of a second. The proton magnetometer is sensitive to
acute magnetic gradients which may cause protons in
different parts of the sensor to precess at different rates
with a consequent adverse effect on precession signal
strength.
Many modern proton magnetometers make use of
the
Overhauser Effect
.To the sensor fluid is added a liquid
containing some free electrons in 'unpaired' orbits.
The protons are then polarized indirectly using radio-
frequency energy near 60 MHz. The power consump-
tion of such instruments is only some 25% of classical
proton magnetometers, so that the instruments are
lighter and more compact. The signal generated by the
fluid is about 100 times stronger, so there is much lower
noise; gradient tolerance is some three times better;
sampling rates are faster.
7.6.5 Magnetic gradiometers
The sensing elements of fluxgate, proton and optically
pumped magnetometers can be used in pairs to measure
either horizontal or vertical magnetic field gradients.
Magnetic gradiometers
are differential magnetometers in
which the spacing between the sensors is fixed and small
with respect to the distance of the causative body whose
magnetic field gradient is to be measured. Magnetic gra-
dients can be measured, albeit less conveniently, with a
magnetometer by taking two successive measurements
at close vertical or horizontal spacings. Magnetic gra-
diometers are employed in surveys of shallow magnetic
features as the gradient anomalies tend to resolve
complex anomalies into their individual components,
which can be used in the determination of the location,
shape and depth of the causative bodies. The method
has the further advantages that regional and temporal
variations in the geomagnetic field are automatically
removed. Marine and airborne versions of magne-
tometers and gradiometers are discussed by Wold
and Cooper (1989) and Hood and Teskey (1989),
respectively.
7.6.4 Optically pumped magnetometer
Optically pumped
or
alkali vapour magnetometers
have a
significantly higher precision than other types. They
comprise a glass cell containing an evaporated alkali
metal such as caesium, rubidium or potassium which is
energized by light of a particular wavelength. In these
alkali atoms there exist valence electrons partitioned
into two energy levels 1 and 2.The wavelength of the en-
ergizing light is selected to excite electrons from level 2
to the higher level 3, a process termed polarization. Elec-
trons at level 3 are unstable and spontaneously decay back
to levels 1 and 2. As this process is repeated, level 1 be-
comes fully populated at the expense of level 2 becoming
underpopulated.This process is known as optical pump-
ing and leads to the stage in which the cell stops absorb-
ing light and turns from opaque to transparent. The
energy difference between levels 1 and 2 is proportional
to the strength of the ambient magnetic field. Depolar-
ization then takes place by the application of radio-
frequency power.The wavelength corresponding to the
energy difference between levels 1 and 2 depolarizes the
cell and is a measure of the magnetic field strength. A
photodetector is used to balance the cell between trans-
parent and opaque states. The depolarization is extre-
mely rapid so that readings are effectively instantaneous.
The sensitivity of optically pumped magnetometers can
be as high as ±0.01 nT.This precision is not required for
surveys involving total field measurements, where the
level of background 'noise' is of the order of 1 nT. The
usual application is in the magnetic gradiometers de-
scribed below, which rely on measuring the small differ-
ence in signal from sensors only a small distance apart.
7.7 Ground magnetic surveys
Ground magnetic surveys are usually performed over
relatively small areas on a previously defined target.
Consequently, station spacing is commonly of the
order of 10-100 m, although smaller spacings may be
employed where magnetic gradients are high. Readings
should not be taken in the vicinity of metallic objects
such as railway lines, cars, roads, fencing, houses, etc,
which might perturb the local magnetic field. For simi-
lar reasons, operators of magnetometers should not carry
metallic objects.
Base station readings are not necessary for monitoring
instrumental drift as fluxgate and proton magnetometers
do not drift, but are important in monitoring diurnal
variations (see Section 7.9).
Since modern magnetic instruments require no
precise levelling, a magnetic survey on land invariably
proceeds much more rapidly than a gravity survey.
7.8 Aeromagnetic and marine surveys
The vast majority of magnetic surveys are carried out in
the air, with the sensor towed in a housing known as
