Geology Reference
In-Depth Information
cause problems in interpreting large anomalies where the direction as well
as magnitude of the field changes rapidly from place to place. However,
these are only minor drawbacks, and 1 and 0.1 nT proton magnetometers
are probably still the commonest instruments in ground surveys. They have
the advantage of producing drift-free absolute measurements, but corrections
must still be made for diurnal variations. Since the protons align themselves,
the sensor need not be precisely orientated and can be mounted on a pole and
carried well away from both the observer and from small magnetic sources
at ground level (see Figure 1.8). To obtain adequate signals it is necessary
only to ensure that the polarising field is at a high angle to the measured field;
i.e. it should be horizontal in high latitudes and vertical in low latitudes. The
manufacturer's instructions need to be read carefully before mounting the
sensor, since the polarising field is not necessarily directed along the axis of
the sensor housing.
3.3.2 High-sensitivity (alkali vapour) magnetometers
Proton magnetometers can be made more sensitive using the Overhauser
effect, in which a VHF radio wave acts on paramagnetic material added to
the bottle fluid. This increases the precession signal by several orders of
magnitude, considerably improving the signal/noise ratio. However, high
sensitivity is now more commonly achieved using electron magnetic mo-
ments, which are about 2000 times larger than proton moments. Effectively
'free' electrons are required, and these occur in the outer electron 'shells'
of alkali metal (usually caesium) vapours. The principle is similar to that
of the proton magnetometer, in that transitions between energy states are
observed, but the much higher energy differences imply much higher fre-
quencies, which can be measured with much smaller percentage errors. The
actual measurement processes are quite complicated, involving the rais-
ing of electrons to high energy states by laser beams ('optical pumping')
and then determining the frequency of the HF radio signal that triggers a
transition to a lower state. This is all, however, invisible to the user. The
effects of electrical noise and high field gradients are less serious than
with proton precession instruments, and measuring times are very short.
Readings can be routinely obtained every tenth of a second (approximately
every 5-10 cm at walking speeds), which can be important in archaeology,
where very high rates of coverage are required, and can be achieved using a
non-magnetic trolley with a trigger actuated by the rotations of the wheels.
Alkali vapour magnetometers are slightly direction-sensitive. Readings
cannot be obtained if the sensor is orientated within a few degrees of either
the direction of the magnetic field or at right angles to it. This is not a
significant limitation in most circumstances, and the rather slow acceptance
of these instruments for ground surveys has had more to do with their
