Geology Reference
In-Depth Information
voltage in the coils is equal and of opposite sign so that
their combined output is zero. In the presence of an ex-
ternal magnetic field, such as the Earth's field, which has
a component parallel to the axis of the cores, saturation
occurs earlier for the core whose primary field is rein-
forced by the external field and later for the core opposed
by the external field. The induced voltages are now out
of phase as the cores reach saturation at different times
(Fig. 7.10(d)). Consequently, the combined output of
the secondary coils is no longer zero but consists of a
series of voltage pulses (Fig. 7.10(e)), the magnitude of
which can be shown to be proportional to the amplitude
of the external field component.
The instrument can be used to measure
Z
or
H
by
aligning the cores in these directions, but the required
accuracy of orientation is some eleven seconds of arc to
achieve a reading accuracy of ± 1 nT. Such accuracy is
difficult to obtain on the ground and impossible when
the instrument is mobile. The total geomagnetic field
can, however, be measured to ± 1 nT with far less precise
orientation as the field changes much more slowly as a
function of orientation about the total field direction.
Airborne versions of the instrument employ orienting
mechanisms of various types to maintain the axis of
the instrument in the direction of the geomagnetic
field. This is accomplished by making use of the feed-
back signal generated by additional sensors whenever
the instrument moves out of orientation to drive
servomotors which realign the cores into the desired
direction.
The fluxgate magnetometer is a continuous reading
instrument and is relatively insensitive to magnetic field
gradients along the length of the cores. The instrument
may be temperature sensitive, requiring correction.
B
e
(a)
B
p
West
East
(b)
B
e
(c)
(d)
B
e
B
p
Fig. 7.11
Principle of the proton magnetometer.
polarizing field is rapidly removed.The protons return to
their original alignment with
B
e
by spiralling, or precess-
ing, in phase around this direction (Fig. 7.11(d)) with a
period of about 0.5 ms, taking some 1-3 s to achieve
their original orientation.The frequency
f
of this preces-
sion is given by
g
B
pe
2
f
=
p
where
g
p
is the gyromagnetic ratio of the proton, an ac-
curately known constant. Consequently, measurement
of
f
, about 2 kHz, provides a very accurate measurement
of the strength of the total geomagnetic field.
f
is deter-
mined by measurement of the alternating voltage of
the same frequency induced to flow in the coil by the
precessing protons.
Field instruments provide absolute readings of the
total magnetic field accurate to ±0.1 nT although much
greater precision can be attained if necessary.The sensor
does not have to be accurately oriented, although it
should ideally lie at an appreciable angle to the total field
vector. Consequently, readings may be taken by sensors
towed behind ships or aircraft without the necessity of
orienting mechanisms. Aeromagnetic surveying with
proton magnetometers may suffer from the slight disad-
vantage that readings are not continuous due to the finite
cycle period. Small anomalies may be missed since an
aircraft travels a significant distance between the discrete
measurements, which may be spaced at intervals of a few
seconds. This problem has been largely obviated by
modern instruments with recycling periods of the order
7.6.3 Proton magnetometer
The most commonly used magnetometer for both sur-
vey work and observatory monitoring is currently the
nuclear precession
or
proton magnetometer
. The sensing de-
vice of the proton magnetometer is a container filled
with a liquid rich in hydrogen atoms, such as kerosene or
water, surrounded by a coil (Fig. 7.11(a)).The hydrogen
nuclei (protons) act as small dipoles and normally align
parallel to the ambient geomagnetic field
B
e
(Fig.
7.11(b)). A current is passed through the coil to generate
a magnetic field
B
p
50-100 times larger than the geo-
magnetic field, and in a different direction, causing the
protons to realign in this new direction (Fig. 7.11(c)).
The current to the coil is then switched off so that the
