Page 144 - Introduction to Mineral Exploration

7

Geophysical Methods

John Milsom

7.1 INTRODUCTION dense and electrically conductive, is negligible
in deposits suitable for large-scale mining
Exploration geophysicists use measurements because of the very low concentrations (see
of physical quantities made at or above the Corbett 1990). Diamonds (the other present-
ground surface or, more rarely, in boreholes day “high profile” targets) are also present in
to draw conclusions about concealed geology. deposits in very low concentration and have,
Lines may have to be surveyed and cleared, moreover, no outstanding physical properties
heavy equipment may have to be brought on (see Chapter 17). In these and similar cases,
site, and detectors and cables may have to be geophysicists must rely on detecting associ-
positioned, so geophysical work on the ground ated minerals or, as in the use of seismic reflec-
is normally rather slow. Airborne geophysical tion to locate offshore placers, and magnetics
surveys, on the other hand, provide the quick- and electromagnetics to locate kimberlites
est, and often the most cost-effective, ways of (see Macnae 1995), on defining favorable
obtaining geological information about large environments.
areas. In some cases, as at Elura in central New Geophysical methods can be classed as either
South Wales (Emerson 1980), airborne indica- passive (involving measurements of naturally
tions have been so clear and definitive that existing fields) or active, if the response of the
ground follow-up work was limited to defining ground to some stimulus is observed. Passive
drill sites, but this is unusual. More often, methods include measurements of magnetic
extensive ground geological, geochemical, and and gravity fields, naturally occurring alpha and
geophysical surveys are required to prioritize gamma radiation, and natural electrical fields
airborne anomalies. (static SP and magnetotellurics). All other elec-
For a geophysical technique to be useful in trical and electromagnetic techniques, seismic
mineral exploration, there must be contrasts in methods, and some downhole methods that
the physical properties of the rocks concerned use artificial radioactive sources are active.
that are related, directly or indirectly, to the In the discussion that follows, the general
presence of economically significant minerals. principles of airborne surveys are considered
Geophysical anomalies, defined as differences before describing the individual methods in
from a constant or slowly varying background, detail. The final sections are concerned with
may then be recorded. Anomalies may take practicalities, and particularly with the role of
many different forms and need not necessarily nonspecialist geologists in geophysical work.
be centered over their sources (Fig. 7.1). Ideally,
they will be produced by the actual economic
minerals, but even the existence of a strong 7.2 AIRBORNE SURVEYS
physical contrast between ore minerals and the
surrounding rocks does not guarantee a signifi- Magnetic, electromagnetic, gamma-ray, and
cant anomaly. The effect of gold, which is both more recently gravity, measurements do not