Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 260

the horizontal component of the sferic noise is usually
stronger than the vertical component. Being pulses, sferics
have a broad frequency bandwidth (see Appendix 2 ) and
so can be dif cult to remove from the survey data. Usually
some kind of de-spiking algorithm is used (see Section
2.7.1.1 ) . An example of sferics in EM data, and the removal
of this noise, is discussed in Section 2.7.4.5 .
Another common type of environmental noise is
motion of receiver coils and associated wiring in the
Earth

well away from the noise sources or arrange to have them
turned off, if that is possible.

5.5 Self-potential method

The self-potential or spontaneous-potential (SP) method is
a passive form of geophysical surveying that measures
naturally occurring variations in electrical potential due
to spontaneous polarisation, the sources of which are many
and diverse. The method smagneticeld. The motion induces spurious

time-varying responses into the data. In ground surveys
the motion is usually caused by the wind and in airborne
surveys it is the continuous turbulent movement of the
system.

' s value to mineral exploration lies

in its ability to detect natural potential variations associ-
ated with metal sulphide, metal oxide and carbonaceous
mineralisation.
SP surveys are usually conducted on the ground surface
to create a map, or downhole to produce an SP log. Despite
being one of the simplest and cheapest methods of geo-
physical surveying, the method has found only limited
application in mineral exploration. This is due to the
incomplete understanding of the mechanisms causing nat-
ural potentials; the fact that responses from blind mineral-
isation are easily masked by overburden; and the presence
of large potentials produced by water movement, ubiqui-
tous in some environments.
In this section, sources of potential variations in the
natural environment, relevant to SP surveying, are
described first. This is followed by a description of SP data
acquisition and processing, interpretation methods, and
finally some examples of SP responses from mineral
deposits.

'

Cultural noise
The main source of man-made, or cultural, noise is the
electric power transmission systems operating at a funda-
mental frequency of 50 Hz (60 Hz in some countries)
( Fig. 5.26 ). Various electrical devices associated with the
power transmission system generate currents at multiples
of the fundamental frequency. These ow through the
powerlines and, along with the 50/60 Hz currents, they
electromagnetically induce electric currents in nearby con-
ductors, e.g. wire fences, pipelines, railway lines, metal
buildings and borehole casing, and also EM receiving coils.
In some areas the return current- ow in the power trans-
mission system is via the ground, and these ground cur-
rents can be so strong as to render electrical and EM
measurements useless.
Radio transmitters are another common source of inter-
ference. The naval VLF communication stations, operating
in the 20 kHz region, are a significant noise source for
many geophysical EM systems ( Fig. 5.26 ) .
Cultural features are also a source of spurious anomal-
ies. The responses are strong, narrow and coincident with
the highly conductive feature causing them. They appear
in survey data either as

5.5.1 Sources of natural electrical potentials

Natural electrical potentials are caused by a number of
mechanisms involving movement of charge carriers,
mostly ions in groundwater. The moving charge carriers
comprise an electric current so there is an associated
potential difference (see Section 5.2.1.2 ) . The physical
and chemical processes involved are not well understood.
Electrical potentials produced by sources other than min-
eralisation are a form of environmental noise in mineral
exploration SP surveys.

anomalies associated
with spot features such as metal drill casing and metal
buildings, or as linear anomalies extending across
several survey lines and related to powerlines, pipelines,
railway lines etc. They are not normally masked by the
response of conductive overburden. The sources can
usually be identi ed from surface maps and aerial
photographs.
Cultural noise is a serious problem for all electrical and
EM methods when surveying in mine environments and
close to built-up areas. It is sometimes necessary to work

' one-line '

5.5.1.1 Non-mineralised sources
Electro ltration or streaming potentials are associated with
the flow of groundwater through porous media. The
potentials are produced by many mechanisms, which
include interactions between ions and the pore walls, and
the different flow velocities of the different types of ions,