Page 167 - Introduction to Mineral Exploration

150 J. MILSOM

from each hole, and the electrical character- not been shown to be anomalous from the air
istics of the rocks surrounding a borehole can but which appeared geologically or geochemic-
often be better guides to the significance of ally promising. Once a drilling program has
an ore intersection, or of the presence of ore begun, not only can the new information be fed
nearby, than the information contained in a back into the geophysical interpretation but
few centimeters of core. the holes can be used for electrical or radiome-
tric logging or, if there are ore intersections,
for mise-à-la-masse. Work of this sort may well
7.15 GEOPHYSICS IN EXPLORATION continue into the development stage.
PROGRAMS Not all geophysical exploration follows this
pattern. Sometimes only one or two methods
A logically designed exploration program pro- have any chance of success. Seismic tech-
gresses through a number of stages, from niques, whether reflection or refraction, tend
regional reconnaissance to semi-detailed to be used alone, being applicable where, as in
follow-up and thence to detailed evaluation. the search for placer deposits, other methods
The geophysical component will tend to pass fail, and virtually useless in dealing with the
through the same stages, reconnaissance often complex geologies encountered in hard rock
being airborne and the final evaluation perhaps mining. Lateral thinking has also produced
involving downhole techniques. The exact some quite surprising specialized applications
route followed will depend on the nature of the of geophysical methods, e.g. phosphate grades
target, but a typical base metal or kimberlite can in some cases be estimated by radiometric
exploration program would have, as its first logging of uncored boreholes.
stage, the assembling of all the geophysical
data already available. Lease conditions may in
some cases require the integration of certain 7.16 INTEGRATION OF GEOLOGICAL AND
types of survey data into national databases, GEOPHYSICAL DATA
and most countries have at least partial mag-
netic and gravity coverage and there may have Making good use of data from a variety of very
been local surveys of these or other types. With different sources is the most testing part of
gravity and magnetics it is usually worthwhile an exploration geologist’s work, requiring the-
going beyond the published maps, which often oretical knowledge, practical understanding,
contain errors, to the original data. and a degree of flair and imagination. All these
For sulfide ores or kimberlites, the next stage will be useless if information is not available in
might involve a combined airborne magnetic convenient and easily usable forms. The most
and electromagnetic survey, using the smallest effective way of recognizing correlations is to
possible terrain clearance and very closely overlay one set of data on another (see sections
spaced lines. Semi-regional magnetic maps 5.1.8. & 9.2), and in a paper-based exploration
would be produced and, with luck, a number of program decisions on map scales and map
interesting electromagnetic anomalies might boundaries have to be made at an early stage
be located. Some sources might be obvious and adhered to thereafter. Computer-based
and irrelevant (e.g. powerlines, pipelines, geographical information systems (GIS) offer
corrugated-iron barns) but any other anomalies greater flexibility, although most require a con-
would have to be checked on the ground, with siderable investment of time and effort on the
magnetic and electromagnetic surveys to loc- part of the users before their advantages can be
ate the anomaly precisely, geological mapping realized (discussed in detail in section 9.2).
to determine rock type, and perhaps a gravity Presenting different types of data together
survey if density differences are probable on a single map is generally useful if they are
between ore and country rock. Other electrical shown as separate entities that can still be
methods, such as IP and even resistivity, may clearly distinguished. Combining data of differ-
also be used to define drilling targets. Ground ent types before they are plotted can be danger-
geophysics might also be used in areas that had ous. This approach is common in exploration