Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 268

5.6.1 Electric fields and currents in
the subsurface

Hole-to-surface
(applied potential)
survey

Surface (conventional)
survey

Logging

- + I V V - + I

To assist in visualising the subsurface electric field and
associated current flow we make use here of the plumbing
analogy of electricity introduced in Section 5.2.1 .
Figures 5.34a to c show the situation for one current
electrode in isolation located on the surface of an electric-
ally homogeneous subsurface, known as a half-space. The
hydraulic equivalent is a shallow well injecting water into a
homogenous

V

-

V - + I

flat topped aquifer ( Fig. 5.34d ). The surfaces
of equal electrical-potential/water-pressure are hemispher-
ical with the electric

flow lines
diverging from the electrode/injection point. The eld
resembles that of an isolated electrical charge (see
Figs. 5.2a and b ) . Introducing the other current electrode,
which has opposite polarity, distorts the hemispherical
surfaces to form the dipole field shown in Figs. 5.34e to g ,
i.e. the half-space field of a current dipole. It resembles the
case of two electric charges of opposite sign (see Fig. 5.2d ) .
It shows the paths taken by the current flow between a pair
of current electrodes located on the surface of a half-space.
Note that the current flow lines are always everywhere
perpendicular to the equipotential surfaces. For this case
the hydraulic equivalent is a pair of wells penetrating the
aquifer, one used for injection and the other for extraction
( Fig. 5.34h ). The equipotential surfaces and flow lines are
identical in the two cases. Note how the water/current flow
lines spread out widely between the wells/electrodes to
occupy a large volume of material. Clearly then, electrical
properties inferred from the measurements are an

field and current/water

Potential electrodes Current electrodes

Figure 5.33 Electrode con gurations commonly used for resistivity/

IP surveying.

resistivity logging, described in Section 5.6.8 . Here we
primarily discuss

surface (conventional) surveys and

logging.
Resistivity/IP measurements are in uenced by the elec-
trical properties of the large volume of rock through which
the current passes, so they are not necessarily indicative
of the electrical properties of the material immediately
below the measurement point. Furthermore, they are not
displayed in geology-like form, so complex interpretation
and modelling techniques are required to transform the
measurements into electrical models of the subsurface.
Inversion modelling of 3D data volumes is becoming more
common with the increasing mass-acquisition of detailed
3D resistivity/IP data volumes. These techniques produce
results in a geology-like form, following a similar trend to
the potential

' average '

of a large volume of subsurface material.
Variations in the subsurface resistivity/conductivity alter
the shape of the equipotential surfaces which in turn alter
the path of the current flow. Referring to Fig. 5.35 , the
equipotential surfaces conform to the shape of a conduct-
ive body, causing the current to follow the line of least
resistance, i.e. the flow lines are de ected toward conduct-
ive bodies. This is an illustration of current gathering or
current channelling (see Section 5.7.2.4 ). Contrastingly,
current

field and EM methods.
Instead of measuring potential differences associated
with the subsurface current flow, the magnetic fields asso-
ciated with these currents can be measured by a class of
surveying techniques referred to here as magnetometric
methods. These include magnetometric resistivity (MMR)
and magnetic induced polarisation (MIP), and a set of
related techniques known as sub-audio magnet-

ics (SAM). There are some advantages to measuring mag-
netic fields: for example, they are less affected by
conductive overburden. Also, magnetometric methods are
responsive to both highly conductive targets and to weakly
conductive, electrically connected mineralisation, such as
disseminated mineralisation and sphalerite-rich ores,
which can be poor targets for conventional electrical and
EM measurements. These methods are described in online
Appendix 3 .

' total- eld '

resistive bodies. Groundwater
behaves in an equivalent way, with flow being concentrated
in porous and permeable formations. The diagrams in
Fig. 5.35 are equally valid whether treated as maps or
cross-sections. Clearly then, measuring potential variations
on the surface or downhole will allow zones of anomal-
ously high or low electrical conductivity/resistivity to be
detected.

ows ' around '