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7: GEOPHYSICAL METHODS 135

is proportional to the total excess mass of the of the use of modeling in gold exploration see
source body. Any errors or uncertainties are section 14.5.4.
due to the difficulty, under most circum-
stances, of defining background, and are not
inherent in the method. The true total mass 7.5 RADIOMETRICS
of the body is obtained by adding in the mass
of an equivalent volume of country rock, so Natural radioactive decay produces alpha par-
some knowledge of source and country-rock ticles (consisting of two neutrons and two
densities is needed. protons bound together), beta particles (high-
Very much less can be deduced about either energy electrons), and gamma rays (very high
the shapes or the depths of sources. A variety of frequency electromagnetic waves which quan-
rule of thumb methods exist, all relying on the tum theory allows us to treat as particles).
rough general relationship between the lateral Alpha and beta radiations are screened out by
extent of an anomaly and the depth of its one or two centimeters of solid rock, and even a
source (e.g. Milsom 2002). A deeper body will, little transported soil may conceal the alpha
other things being equal, give rise to a broader and beta effects of mineralisation. Gamma rays
(and flatter) anomaly. Strictly speaking, only are more useful in exploration but even they
homogeneous spherical bodies can be approxi- have ranges of only one or two meters in solid
mated by the point sources on which most of matter. The traditional picture of a radiometric
the rules are based, and the widths of real survey is of the bearded prospector, with or
anomalies obviously depend on the widths of without donkey, plodding through the desert
their sources, but rough guides are obtained and listening hopefully to clicking noises com-
to the depths of centers of mass in many cases. ing from a box on his hip. The Geiger counter
Also, the peaks of gravity anomalies are gen- he used was sensitive only to alpha particles
erally located directly above the causative and is now obsolete, but tradition dies hard.
bodies, which is not the case with many of the Modern gamma rays detectors (scintillometers)
other geophysical methods. However, all inter- have dials or digital readouts, but ground
pretation is subject to a fundamental limita- survey instruments can usually also be set to
tion because the field produced by a given body “click” in areas of high radioactivity. For an
at a given depth can always also be produced by example of a carborne survey see Fig. 4.8.
a laterally more extensive body at a shallower Radiometric methods are geophysical odd-
depth. No amount of more detailed survey or ities because the measurements are of count
increased precision can remove this ambiguity. rates, which are subject to statistical rules,
Full quantitative interpretations are usually rather than of fields with definite, even if diur-
made by entering a geological model into a nally variable, values. Survey procedures must
computer program that calculates the corres- strike a balance between the accuracy obtain-
ponding gravity field, and then modifying the able by observing for long periods at each sta-
model until there is an acceptable degree of tion and the speed of coverage demanded by
fit between the observed and calculated fields. economics. Statistics can be improved by using
This process is known as forward modeling. very large detector crystals, and these are es-
Several packages based on algorithms published sential for airborne work, but their cost rises
by Cady (1980) are available for modeling dramatically with size. The slow speeds needed
fields due to 2D bodies (which have constant to obtain statistically valid counts in the air
cross-section and infinite strike extent) and to generally necessitate helicopter installations.
1
2 /2D bodies, which are limited in strike length. Gamma ray energies can be measured by
Full three-dimensional modeling of bodies of spectrometers, allowing different sources of
complex shape is now becoming more com- radiation to be identified. Terrestrial radia-
40
mon as desktop computers become faster and tion may come from the decay of K, which
memory sizes increase. The reliability of any makes up about 0.01% of naturally occurring
interpretation, no matter how sophisticated potassium, or from thorium or uranium. Radia-
the technique, depends, of course, on the valid- tion arises not only from decay of these long-
ity of the input assumptions. For an example lived primeval elements but also from unstable