Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 85

CHAPTER

3

Gravity and magnetic methods

3.1 Introduction

The gravity and magnetic methods measure spatial vari-
ations in the Earth's gravity andmagnetic fields ( Fig. 3.1 ).
Changes in gravity are causedby variations in rock density
and those in the magnetic field by variations in rock
magnetism, which is mostly controlled by a physical prop-
erty called magnetic susceptibility. Gravity and magnetic
surveys are relatively inexpensive and are widely used for
the direct detection of several different types of mineral
deposits and for pseudo-geological mapping.
Magnetic measurements made from the air, known as
aeromagnetics

Field
strength

Gravity field

Magnetic field

Response
from body #2

Response
from contact

Response
from body #1

, are virtually ubiquitous inmineral explor-
ation for wide-area regional surveying, for detailedmap-
ping at prospect scale and for target detection. In areas
where exposure is poor, aeromagnetics has become an
indispensable component of exploration programmes.
Gravity measurements are also used for regional and
prospect-scale mapping but, historically, measurements
of sufficient accuracy and resolution for mineral explor-
ation could only be made on the ground. The develop-
ment of airborne gravity systems, known as

Location

#1 #2

Density/magnetism

,
with precision suitable for mineral targeting, means that
aerogravity in mineral exploration is likely to become as
common as aeromagnetics.
Downhole gravity measurements for mineral applica-
tions are rare, but the recent development of instruments
for use in the small diameter drillholes used by the min-
erals industry adds a new dimension to geophysical
exploration. Downhole magnetic measurements, whilst
not common, find application in mining for delineating

aerogravity

Lowest Highest

Figure 3.1 Schematic illustration showing variations in the strength
of the Earth

'

fields due to variations in crustal
density and magnetism, respectively. Note the simpler form of the
variations in gravity compared with those in the magnetic

s gravity and magnetic

field.

highlymagnetic targets. Downhole logging of rock dens-
ity and magnetic susceptibility is routinely undertaken.
The gravity and magnetic methods have much in
common, but they also have important aspects that