Page 147 - Introduction to Mineral Exploration

130 J. MILSOM

prove to be one of the most useful of the survey every tenth of a second instead of every second
products (see section 9.2). or half second. Both cesium and proton instru-
The requirement for ever smaller ground ments are (within limits) self-orienting and can
clearances and tighter grids is making demands be mounted either on the aircraft or in a towed
on pilots that are increasingly difficult to meet. bird. Sensors on aircraft are usually housed
In the future some airborne surveys, and espe- in specially constructed nonmagnetic booms
cially aeromagnetic surveys, may be flown by (stingers) placed as far from the main aircraft
pilotless drones. sources of magnetic field as possible (Fig. 7.2).
Aircraft fields vary slightly with heading and
must be compensated by systems of coils and
7.3 MAGNETIC SURVEYS permanent magnets. The magnitude of any
residual heading error must be monitored on a
Magnetic surveys are the quickest, and often regular basis.
the cheapest, form of geophysics that can pro- The Earth’s magnetic field is approximately
vide useful exploration information. A few that of a dipole located at the Earth’s center
minerals, of which magnetite is by far the most and inclined at about 10 degrees to the spin
common, produce easily detectable anomalies axis. Distortions covering areas hundreds of
in the Earth’s magnetic field because the rocks kilometers across can be regarded as due to
containing them become magnetized. The a small number of subsidiary dipoles located
magnetization is either temporary (induced) at the core–mantle boundary. The practical
and in the same direction as the Earth’s field, unit of magnetic field for survey work is the
or permanent (remanent) and fixed in direction nanotesla (nT, sometimes also known as the
with respect to the rock, regardless of folding or gamma). At the magnetic poles the field is
rotation. about 60,000 nT and vertical, whereas at the
Since magnetite is a very minor constituent equator it is about 30,000 nT and horizontal
of sediments, a magnetic map generally records (Fig. 7.3). Slow variations in both magnitude
the distribution of magnetic material in the un- and direction, which must be taken into
derlying crystalline basement. Even sediments account when comparing surveys made more
that do contain magnetite have little effect on than a few months apart, are described, together
airborne sensors, partly because the fields from with large-scale variations with latitude and
the randomly oriented magnetite grains typical longitude, by a complicated experimentally
of sediments tend to cancel out and partly be- determined formula, the International Geom-
cause the fields due to thin, flat-lying sources agnetic Reference Field (IGRF). This is gener-
decrease rapidly with height. Because the small ally a reasonable approximation to the regional
pieces of iron scrap that are ubiquitous in field in well-surveyed areas where the control
populated areas strongly affect ground magnet- on its formulation is good, but may be unsatis-
ometers, and also because ground coverage is factory in remote areas.
slow, most magnetic work for mineral explora- As well as long-term variations in field
tion is done from the air. Line separations have strength, there are cyclical daily (diurnal)
decreased steadily over the years and may now changes which are normally of the order of 20–
be as little as 100 m. Ground-clearance may 60 nT in amplitude. Variations are low at night
also be less than 100 m. but the field begins to increase at about dawn,
Magnetic surveys are not only amongst the reaches a peak at about 10 a.m., declines rap-
most useful types of airborne geophysics but idly to a minimum at about 4 p.m., then rises
are also, because of the low weight and sim- more slowly to the overnight value. Although
plicity of the equipment, the cheapest. The this pattern is repeated from day to day, the
standard instrument is now the high sensitiv- changes are not predictable in detail and must
ity cesium vapor magnetometer. Proton mag- be determined by actual measurement. Diurnal
netometers are still occasionally used but the effects are especially important in airborne
cesium magnetometer is not only a hundred surveys, which now typically use measuring
times more sensitive but provides readings precisions of the order of 0.1 nT and contour