MAGNETIC METHOD - Field Geophysics - page 74

relatively high cost and the fact that in most geological applications the
high sensitivity is of little use. However, absolute field changes of less than
1 nT may be significant in archaeology, and sensitivity is always important
in gradiometry. The greater tolerance of high field-gradients can also be an
advantage in engineering/environmental surveys.
3.3.3 Fluxgate magnetometers
The sensing elements of fluxgate magnetometers consist of one or more cores
of magnetic alloy, which are magnetised to saturation by alternating current
passed through coils wound around them. The changes in the electrical
properties of the circuits as the core magnetisations pass from the unsaturated
to the saturated state can be converted into voltages proportional to the
external magnetic field along the core axes. Measurements are thus of the
magnetic field component in whichever direction the sensor is pointed. This
is the vertical in most ground surveys, since this is the most easily determined
direction.
Fluxgates do not measure absolute fields and therefore require calibration.
They are also subject to thermal drift, because the magnetic properties of
the cores and, to a lesser extent, the electrical properties of the circuits, vary
with temperature. Early ground instruments sacrificed thermal insulation
for portability and were often accurate to only 10 or 20 nT. Readings were
displayed, rather crudely, by the position of a needle on a graduated dial.
Despite some claims to the contrary, such sensitivity is inadequate for almost
all ground survey work.
One problem with portable fluxgates is that, because they require ori-
entation at each station, the observer must be holding the sensor when a
reading is taken. Ensuring that people are completely non-magnetic is never
easy, and battery packs, in particular, can be sources of noise. Any batteries
supplied by the manufacturer should be completely non-magnetic but they,
and any subsequent replacements, should be carefully checked.
Fluxgates are now used mainly in archaeological surveys, where price
is important and the necessary large numbers of readings must be obtained
more quickly than is possible with proton instruments. Also, measurements
often have to be made close to the ground, which may be difficult with a
proton magnetometer because of its sensitivity to field gradients and elec-
trical interference. Fluxgate sensors are usually paired, with fixed vertical
separations of between 50 and 100 cm, and often only the differences in the
readings are recorded. Such instruments are usually known as gradiome-
ters , but the operation of the inverse-square or inverse-cube laws ensures
that sources that produce measurable anomalies are at depths comparable
with sensor separations. It is easier to understand the resulting maps if the
instruments are thought of as differential magnetometers (Figure 3.5).