Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 263

SP is one of the quickest, simplest and cheapest forms of
downhole logging, although measurements can only be
made in uncased, water- lled, drillholes. If necessary, the
local base can be tied to other surface or downhole surveys
allowing the creation of potential-variation cross-sections
or even volumes.

amplitude positive peak in the dip direction related to the
deeper pole of positive polarity: see for example Fig. 5.29c
and f .
The overall anomaly shape on the map re ects the
overall shape of the source. Assuming a particular shape
for the source, e.g. a polarised sphere, rod or thin sheet (the
latter two polarised in the direction of dip) information
about the source, such as its depth and dip, can be deter-
mined. Responses for these simple source models can be
computed to model survey data. El-Araby ( 2004 ) describes
a method where the most appropriate source-shape is
estimated, and also provides a comprehensive reference list
covering methods of inverse modelling SP responses.
Figure 5.29 demonstrates the general characteristics of
SP anomalies with computed responses and

5.5.3 Display and interpretation of SP data

Self-potential maps can often be very complicated and
difficult to interpret in terms of the geology, owing to the
poor understanding of the origins of the potential vari-
ations. Responses of interest are often superimposed on
regional trends, for example caused by streaming poten-
tials or telluric currents (see Atmospheric noise in Section
5.4.2.1 ) . Removal of the regional response is done using the
same methods as applied to other types of geophysical data
(see Section 2.9.2 ).

field data from
various mineral deposits for the widely applicable thin-
sheet source geometry. An actual example of this kind of
source is the Joma pyrite deposit in Trøndelag, Norway
(Logn and Bølviken, 1974 ). The mineralised body com-
prises predominantly pyrite, with some chalcopyrite, pyr-
rhotite and sphalerite within a carbonate-bearing
greenstone unit. A potential cross-section across it is
shown in Fig. 5.29g . The downhole data, which have been
smoothed to emphasis the effects of the electrochemical
processes (see Fig. 2.37 ), clearly de ne a negative pole near
the surface and a positive pole at depth.
Sometimes the observed SP anomaly reflects polarisation
of only that part of a larger mass exposed to the water table
and the zone of oxidation. Anomaly analysis often shows
that the centre of the polarised source coincides with the
base of the regolith or weathered horizon, consistent with
electrochemical theories for SP mechanisms. Although
anomaly width increases with increasing depth of the
source, a wide anomaly in an area with the limited depth
of oxidation may indicate a wider rather than a deeper
source. In areas with severe topography the SP anomaly
may be laterally offset, downslope, from its source.

5.5.3.1 Surface data
SP responses of economic signi cance are usually large in
amplitude compared with other responses, so they are easy
to identify in a contour map or basic image of the meas-
ured potential.
It is helpful to interpret surface SP data in conjunction
with other types of data in order to identify spurious
responses. Topographic data help in identifying streaming
potentials; geological maps, magnetic data and satellite
spectral measurements help in recognising effects related
to changes in geology or vegetation. Since faults may act as
conduits for groundwater flow, these may be associated
with SP responses. Graphite is an excellent target for SP
surveys because of its conductivity. For the same reason,
black shales and other conductive lithologies may produce
strong responses, but these may be of no economic signifi-
cance. Conductive cover can attenuate or even completely
mask bedrock responses, and changes in the thickness or
type of cover may control groundwater ow and produce
variations in electrical potential.
The subsurface electrochemical mechanisms that give
rise to economically signi cant anomalies are dipolar (i.e.
the sources have both positive and negative electrical
poles), so both positive and negative responses can be
expected in SP anomalies. A general characteristic of these
anomalies, regardless of source shape, is that vertically
dipping bodies have a single negative anomaly located over
their negatively polarised shallow portions ( Fig. 5.29a and
d ), and dipping bodies also exhibit an additional lower-

5.5.3.2 Downhole data
The downhole SP log is one of the standard logs used by
the petroleum industry, its main use being the qualitative
discrimination of permeable and impermeable strata and
the calculation of the electrical resistivity of the pore waters
in the rocks surrounding the drillhole. An excellent intro-
duction to SP logging as used by the petroleum sector is
given by Rider ( 1996 ) . Examples of SP logs from various
geological environments and types of mineralisation are
shown in Fig. 5.30 .