Geoscience Reference
In-Depth Information
The major failing of the Sato and Mooney model, and
noted by them, is its inability to explain the magnitude of
observed potential variations. The model predicts that
potential differences will not exceed 1000 mV, depending
on the type of mineralisation. For example, the theoretical
maximum for graphite is 780 mV and for pyrite 730 mV,
but anomalies much greater than these have been meas-
ured in many areas. Another problem is that SP anomalies
are known to be associated with apparently poorly con-
ducting bodies, including disseminated mineralisation. The
shortcomings of the Sato and Mooney model have long
been recognised, and although developments and improve-
ments have been suggested, no profoundly different alter-
native models have been proposed.
The failure to identify a consistent set of geo-electrical
phenomena for bodies associated with SP anomalies
3
X
V
+
2
X
X
V
+
V
+
a)
Base station
X
X
V
X
V
+
V
+
+
b)
Base station
V
c)
-
+
considering the almost
nite variety of geometrical
and electrical properties of mineralisation, and chemical
and electrical properties of the surrounding geology and
groundwater
in
Base station
suggests that there are various mechanisms
responsible for the observed potential variations. The
source of each anomaly may be unique to the body of
mineralisation, as suggested by the observation that where
there are adjacent and apparently identical bodies of min-
eralisation one may produce an SP anomaly whilst the
other does not.
-
Figure 5.28
Electrode configurations for SP surveying. Surface
surveying using the (a) fixed-base and (b) gradient modes. (c)
Downhole survey modes. Potential differences are measured relative
to a base station.
5.5.2
Measurement of self-potential
differences between the electrodes are recorded. The pro-
cedure is repeated for each survey line, and a tie line,
intersecting the survey lines at their
The SP method requires only the simplest of field appar-
atus: a pair of non-polarising electrodes (see
Section 5.4.1
),
a reel of insulated wire, and a high-impedance voltmeter
with a resolution of at least 1 mV. Alternatively, measure-
ments may be made using a modern resistivity/IP receiver,
which offers the convenience of digital storage.
base stations,
is surveyed in the same manner. All the measurements are
then referenced to a
'
local
'
base station located in an area
of low potential gradient, and usually assigned a potential
of zero volts.
The second survey procedure measures potential differ-
ences with a constant electrode separation, usually about
10 m. Both electrodes are moved along the survey line and
the potential differences recorded. The total potential vari-
ation can be obtained by summing the measured differ-
ences between stations, although measurement errors are
cumulative.
'
master
'
5.5.2.1
Surface surveys
Surface SP surveys usually comprise a series of parallel
lines oriented perpendicular to strike and spaced to suit
the resolution required, but generally 50 to 100 m apart for
compact 3D targets. Station spacing varies from a few
metres to a few tens of metres.
Surveys are conducted in one of two ways (
Fig. 5.28a
at the base station and connected to the negative terminal
of the voltmeter. The second (roving) electrode, connected
to the positive terminal of the voltmeter through the reel of
wire, is moved along the survey line and the potential
5.5.2.2
Downhole surveys
For downhole SP surveys (
Fig. 5.28c
)
the local base station
is some arbitrary point on the surface and measurements
are made at various depths downhole, commonly with
measurement spacing of a few centimetres to a few metres.
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