Geology Reference
In-Depth Information
field anomaly is a positive/negative couplet and the
vertical field anomaly is centred over the pole.
The total field anomaly
D
B
is then obtained by substi-
tuting the expressions of equations (7.10) and (7.11) in
equation (7.9), where
a
= 0. If the profile were not
in the direction of magnetic north, the angle
a
would
represent the angle between magnetic north and the
profile direction.
Ferrite core
Primary winding
Secondary winding
Combined output
of secondaries
7.6 Magnetic surveying instruments
(a)
7.6.1 Introduction
Since the early 1900s a variety of surveying instruments
have been designed that is capable of measuring the geo-
magnetic elements
Z
,
H
and
B
. Most modern survey in-
struments, however, are designed to measure
B
only.The
precision normally required is ±0.1 nT which is approxi-
mately one part in 5 ¥ 10
6
of the background field, a
considerably lower requirement of precision than is nec-
essary for gravity measurements (see Chapter 6).
In early magnetic surveys the geomagnetic elements
were measured using
magnetic variometers
. There were
several types, including the torsion head magnetometer
and the Schmidt vertical balance, but all consisted essen-
tially of bar magnets suspended in the Earth's field. Such
devices required accurate levelling and a stable platform
for measurement so that readings were time consuming
and limited to sites on land.
Time
(b)
Time
(c)
Time
(d)
7.6.2 Fluxgate magnetometer
Since the 1940s, a new generation of instruments has
been developed which provides virtually instantaneous
readings and requires only coarse orientation so that
magnetic measurements can be taken on land, at sea and
in the air.
The first such device to be developed was the
fluxgate
magnetometer
, which found early application during the
second world war in the detection of submarines from
the air.The instrument employs two identical ferromag-
netic cores of such high permeability that the geomag-
netic field can induce a magnetization that is a substantial
proportion of their saturation value (see Section 7.2).
Identical primary and secondary coils are wound in op-
posite directions around the cores (Fig. 7.10). An alter-
nating current of 50-1000 Hz is passed through the
primary coils (Fig. 7.10(a)), generating an alternating
magnetic field. In the absence of any external magnetic
Time
(e)
Time
Fig. 7.10
Principle of the fluxgate magnetometer. Solid and
broken lines in (b)-(d) refer to the responses of the two cores.
field, the cores are driven to saturation near the peak of
each half-cycle of the current (Fig. 7.10(b)). The alter-
nating magnetic field in the cores induces an alternating
voltage in the secondary coils which is at a maximum
when the field is changing most rapidly (Fig. 7.10(c)).
Since the coils are wound in opposite directions, the
