Environmental Engineering Reference
In-Depth Information
Radiometric Surveys
Radiometric methods rely on the use of portable Geiger-Muller apparatus for i eld detec-
tion of emission counts in the search for radioactive minerals. Many rocks and minerals
are naturally radioactive. Radiometric surveys measure variations in the natural radio-
activity of an area. Modern spectrometers allow the detection of radioactivity at very low
levels that were not previously detectable. Surveys of this type are undertaken from the air
with follow-up ground work wherever anomalies are identii ed.
Down-hole Methods
Down-hole geophysical surveying is usually a secondary exploration method used to maxi-
mize the value of a drill hole. It increases the effective search area of a hole and may detect
an ore deposit narrowly missed by drilling. Rock properties such as natural radioactivity,
density, conductivity or other electrical or magnetic properties may be measured by low-
ering a probe down the hole. Alternatively, a receiver may be placed in the hole, and a
transmitting loop on the surface. This coni guration can be used to measure the rock prop-
erties between the surface and the drill hole. Holes are rarely drilled purely for geophysi-
cal exploration, but as technology improves this is likely to become more common, as will
measurement between drill holes. Down-hole methods are also extensively used in oil and
gas exploration and in some groundwater investigation programmes.
Trenching and Pitting
Trenching or costeaning and pitting are traditional methods of exposing concealed bed-
rock or taking samples for analysis. In the early days excavations were usually undertaken
using hand tools. The practice is still carried out today, particularly in hard to reach areas,
but to a much lesser extent. Costeaning to map and sample the bedrock is normally under-
taken with an excavator. Larger-scale excavations may be used to provide bulk samples of
known deposits for detailed testing prior to mining. Prior approval is usually required for
excavations of this nature, with strict requirements governing the back-i lling and surface
rehabilitation of any such approved works after sampling has been completed.
Exploration Drilling
The exploration methods described so far are generally applied over relatively large areas
of land and have only a very minor impact, if any, on the surface. They are designed to
indicate the possible presence of a mineral deposit, but cannot physically dei ne it. Once all
the data from these surveys have been collected, they are plotted on various maps, with the
assistance of sophisticated computers, and then interpreted by geoscientists. The only way
to test for the possibility of a concealed mineral deposit is to gather data from beneath the
surface. This is mainly achieved by drilling.
The i rst stage of drilling is usually termed reconnaissance drilling. If the results from
this drilling are encouraging, a further programme of follow-up drilling may take place. If
the results of this additional drilling are also encouraging, geologists may embark upon a
programme of detailed or deposit delineation drilling. The selection of the drilling method
and equipment is determined by such factors as the nature and depth of the target deposit,
location and access factors associated with the drill site, and cost. Only a few exploration
holes will intersect an ore body. Within gold deposits, a discovery hole may be one in one
The only way to test for the
possibility of a concealed mineral
deposit is to gather data from
beneath the surface. This is
mainly achieved by drilling.
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