Page 155 - Introduction to Mineral Exploration

138 J. MILSOM

and also to compensate for any natural currents needed for four! Multiple layers were therefore
and voltages, directions of current flow are usually interpreted by matching successive
reversed periodically, usually at intervals of the curve segments using two-layer curves together
order of a second. Voltage electrodes that do with auxiliary curves that constrained the two-
not polarize can be made by immersing copper layer curve positions. This technique is now
rods in saturated copper sulfate solution con- largely obsolete, having been replaced by inter-
tained in porous pots. Contact with the ground active computer modeling, but the insights
is made by solution that leaks through the provided by type curves are still valuable. More
bases of the pots. These electrodes are messy sophisticated computer programs now allow
and inconvenient and are used only when, as in the results obtained from multiple Wenner
induced polarization surveys (section 7.8), they traverses at different electrode separations
are absolutely essential. along single lines to be inverted to cross-
For studying lateral variations in resistiv- sections approximating the actual subsurface
ity, the Wenner, pole–pole, and gradient arrays distribution of resistivity. The fieldwork is
(Fig. 7.7a,b,d) are the most convenient. In much more time-consuming than simple
Wenner traversing, the leading electrode is single-expansion depth sounding, but the extra
moved one electrode interval along the line for effort is almost always justified by the im-
each new station and each following electrode proved results.
is moved into the place vacated by its neighbor.
With the gradient array, which requires very
powerful current generators, the outer elec- 7.7 SPONTANEOUS POLARIZATION (SP)
trodes are fixed and far apart and the inner
electrodes, separated by only a few meters, are Natural currents and natural potentials exist
moved together. The pole–pole array also has and have exploration significance. In particu-
the advantage of requiring only two moving lar, sulfide orebodies may produce negative
electrodes but the very long cables needed to anomalies of several hundred millivolts. It was
link these to the fixed electrodes “at infinity” originally thought that these potentials were
may be inconvenient. maintained by oxidation of the ore itself, but it
Resistivity surveys are also used to investi- is now generally agreed that the ore acts as a
gate interfaces, such as water tables or bedrock passive conductor, focussing currents produced
surfaces, that are approximately horizontal. by the oxidation-reduction reactions that take
Current can be sent progressively deeper into place across the water table. For a voltage to
the ground by moving electrodes further apart. be produced, the conductor must straddle the
Resistivities can thus be estimated for pro- water table and the method, although quick and
gressively deeper levels, although, inevitably, simple, requiring only a high-impedance volt-
resolution decreases as electrode separations meter, some cables, and a pair of nonpolarizing
increase. The Wenner array is often used, as electrodes, is often rejected because economic-
is the Schlumberger array (Fig. 7.7c), which ally exploitable bodies do not necessarily pro-
is symmetrical but has outer electrodes very duce anomalies even when highly conductive.
much further apart than the inner ones. All four
electrodes must be moved when expanding the
Wenner array but with the Schlumberger array, 7.8 INDUCED POLARIZATION (IP)
within limits, the inner electrodes can be left
in the same positions. Traditionally, the results Flow of electric current in a rock mass can
obtained with either array were interpreted cause parts of it to become electrically polar-
by plotting apparent resistivity against array ized. The effect is almost negligible in sand-
expansion on log–log paper and comparing the stones, quite marked in clays, in which small
graphs with type curves. A single sheet of pore spaces and electrically active surfaces
curves was sufficient if only two layers were impede ionic flow, and can be very strong at the
present (Fig. 7.8), but a book was needed for surfaces of grains of electronic conductors such
three layers and a library would have been as graphite and metallic sulfides. If current