Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 113

vertical or horizontal gradient of the magnetic field. For
ground surveys, the two sensors are mounted several
metres apart on a pole. In airborne work, horizontal gradi-
ents are measured by magnetometers on the aircraft

a)

s wing
tips (horizontal gradient) ( Fig. 3.23b ), or at the top and
base of the tail (vertical gradients). Orientation errors can
be large in gradient measurements and they must be
closely monitored and recorded.
Hogg ( 2004 ) provides an extensive discussion about the
advantages and disadvantages of magnetic-gradient meas-
urements. Although there are advantages in making tensor
measurements (see Section 2.2.3 ) of the magnetic field,
equipment to do this routinely is not yet available (Schmidt
and Clark, 2000 ).

'

b)

3.5.2.1 Magnetometers
Three types of magnetic sensor are commonly used in
geophysical surveying, namely the proton-precession, the
Overhauser and the alkali-vapour sensors. The operation
of all three is based on quantum-mechanical properties of
atoms. Importantly, they are sensitive to the strength of the
Earth

c)

s magnetic field, F in Fig. 3.20b , and do not measure
its direction, i.e. they measure the total magnetic intensity
(TMI). Magnetometers using these sensors are equipped
with solid-state memory and can store thousands of meas-
urements, and some can also internally record and store
satellite-derived location information.
The proton-precession sensor makes a measurement of
magnetic field strength over a period of time. Typically,
measurements can be made at the rate of about one per
second, although smaller cycle times are possible but result
in lower accuracy. Modern instruments can measure the
TMI to at least 0.1 nT, and to greater precision with longer
measurement periods. The Overhauser sensor can make a
measurement more quickly, typically to 0.01 nT several
times a second. They can also operate in higher

'

Figure 3.23 (a) Helicopter magnetic survey system. Picture courtesy
of GPX Surveys. (b) Airborne magnetic gradiometer survey system.
Note the magnetic sensors on each wing-tip. Picture courtesy of
Geotech Airborne. (c) Ground magnetic survey system. Picture by
M. Dentith.

effect at the sensor. Usually it is mounted at the end of a
non-magnetic support extending from the vehicle/aircraft.
Sometimes in airborne work the magnetic sensor is towed,
on a long cable, behind and below the aircraft. This is
usually when magnetic data are being collected along with
other types of geophysical measurements, for example,
when making electromagnetic measurements (see Section
5.9 ). Figure 3.23 shows equipment for ground surveying
and some examples of aircraft con gured for aeromagnetic
surveying.
Sometimes the difference in TMI is measured between
one or more pairs of closely spaced sensors forming a
magnetic gradiometer, the reading being a TMI gradient
in a particular direction (see Section 2.2.3 ). The gradi-
ometer is usually oriented so as to measure either the

field

gradients than the proton-precession sensor.
The alkali-vapour sensor is an analogue device produ-
cing a continuous signal so field measurements can be
made rapidly. Measurements of TMI are typically made
to 0.01 nT, and to 1 pT for sensors based on particular
types of alkali-metal vapours. They can operate in higher
field gradients than the proton-precession and Overhauser
instruments. Alkali-vapour magnetometers are routinely
used for most survey work. Their high precision and
greater tolerance of gradients mean that they are the pre-
ferred sensor for high precision gradiometers.