Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 107

a) b)

105°10´

>0.5 ppm Ag
>0.1 ppm Au
>10 K 2 O/Na 2 O)

38°45´ 38°45´

Mines

Latite/phonolite/syenite/basalt
Phonolite/latite breccia
Granitoids (Precambrian)

0 20

105°10´

Kilometres

c) d) 9,793,200

- 2100

gu

gu

- 2250

9,791,80 0

38°45´ 38°45´

Gravity
stations

105°10´ 105°10´

Figure 3.18 Geological and geophysical data from Cripple Creek. (a) Simpli Te mineralisation and

associated hydrothermal potassic alteration, (c) gravity before correction for height and topography, and (d) complete Bouguer anomaly.
The gravity images were created from open-file gravity data. Geological data based on diagrams in Kleinkopf et al.( 1970 ).

ed geology, (b) distribution of Au - Ag -

variations of the field due to magnetic materials in the
crust that are of interest in mineral exploration. The
average strength of the Earth

direction and strength over the Earth and at time scales
which are signi cant for exploration surveys. These

short-term temporal variations are a source of noise during
magnetic surveying, and corrections can be applied to
compensate for them.
The instrument used to measure the magnetic field is
called a magnetometer. Magnetometers used in exploration
make absolute measurements, although only relative dif-
ferences are actually required, and these are usually the
scalar strength of the field (TMI). The strength of the field
in a particular direction, i.e. a component of the field, is
made with a vector magnetometer. The instruments are
relatively small and lightweight, and measurements are
routinely made from the air, on the ground and downhole.
Magnetic surveys conducted in the air with a fixed-wing

field is about
50,000 nT and variations of geological origin may exceed
10,000 nT, which is about 20% of the field strength, so
variations can be extremely large compared with the
very small in uence that local geological features have on
the gravity field. However, these large variations are rare.
More common are variations of

' s magnetic

tens or hundreds of

nanoteslas.
Unlike the gravity field, the Earth s magnetic field is

fundamental in determining the strength and shape of
crustal magnetic anomalies. Without it there would be no
magnetic anomalies and no formation of remanent mag-
netism. The magnetic field changes significantly in both

'