Geology Reference
In-Depth Information
7
SP AND IP
Natural, unidirectional currents flow in the ground and produce voltage
(self-potential, or SP) anomalies that can amount to several hundreds of
millivolts. They have applications in exploration for massive sulphides, and
in some engineering and environmental work.
Artificial currents flowing in the ground can cause some parts of the rock
mass to become electrically polarised. The process is analogous to charging
a capacitor or a car battery, and both capacitative and electrochemical effects
are involved. If the current suddenly ceases, the polarisation cells discharge
over periods of several seconds, producing currents, voltages and magnetic
fields that can be detected at the surface. Disseminated sulphide minerals can
produce large effects of this type, and induced polarisation ( IP ) techniques
are therefore widely used in exploring for base metals. Arrays are similar
to those used in conventional resistivity work. The gradient and dipole-
dipole arrays are especially popular (for reconnaissance and detailed work
respectively) because current and voltage cables can be widely separated to
minimise electromagnetic inductive noise ( cross-talk ). More exotic systems,
such as the pole-dipole array, are also used.
7.1 SP Surveys
SP surveys were at one time popular in mineral exploration because of their
low cost and simplicity. They are now little used, because some near-surface
ore bodies that are readily detected by other electrical methods produce no
SP anomaly. In contrast, the popularity of SP methods in detecting water
seepage pathways through containment structures is growing.
7.1.1 Origins of natural potentials
Natural potentials of as much as 1.8 V have been observed where alunite
weathers to sulphuric acid, but the negative anomalies produced by sulphide
ore bodies and graphite are generally less than 500 mV. The conductor
should extend from the zone of oxidation near the surface to the reducing
environment below the water table, thus providing a low-resistance path for
oxidation-reduction currents (Figure 7.1).
 
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