Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 258

strength of the magnetic field, or the rate of change of the
strength of the magnetic eld.
The various detectors are connected to the recording
unit within which some basic data processing may take
place to enable data to be viewed in the field. In electrical
surveys the detectors are the receiving or potential elec-
trodes and are also in electrical contact with the ground.
As the name suggests, they detect differences in electrical
potential (it is difficult to measure absolute electrical
potentials in the field). Special non-polarising electrodes
are required for some types of survey (see Section 5.4.1 ).
Detectors for EM surveys may be magnetometers or wire
antennae (sometimes loops, but usually coils) which are
normally referred to as the receiver (Rx). Wire antennae
consist of small portable multi-turn coils of wire wound
either on rigid air-cored forms of a metre or two in
diameter, as used in airborne EM, or as small compact
forms wound on ferrite or metal cores suitable for ground
and downhole EM surveying. In electrical surveys, the
term receiver tends to be used to describe the actual
recording unit.
In the past, different controllers and recorders were used
for different kinds of survey, but modern digital survey
equipment has the capability to produce and record signals
for all the common types of electrical and EM surveys.
Accurate time synchronisation between the controller and
recording systems is required. This may be achieved by a
wire connection between them or synchronisation of their
internal clocks, which are automatically synchronised to
the time signals broadcast by the constellations of satellites
forming global positioning systems.
Recording the orientation of the transmitter and receiver
antennae and magnetometers is imperative in EM methods
because the parameters measured are vector quantities (see
Section 2.2.2 ). Individual sensors may include spirit-bubble
levels to allow manual orientation, or electronic means of
recording orientations, as in airborne and downhole equip-
ment. The positional and orientation data are recorded,
along with the electrical and electromagnetic signals, by the
recording unit, these essential ancillary data being required
in the analysis of the survey data.

This may be improved by increasing the electrode s surface

area, which is achieved by using aluminium foil sheeting
lining a small pit at least 1 m 2 in area, dug into the soil to a
depth of at least 50 cm. A large volume of water is applied,
and often common salt and detergent are added. The foil is
covered with wet soil. In sandy porous soils and arid
environments it is usually necessary to maintain the mois-
ture content of each electrode during the course of the
survey to prevent them becoming resistive.
A voltmeter connected across a pair of metal stakes
pushed into the soil will measure a voltage that quickly
drifts with time. Electrochemical reactions occurring
between the electrodes and chemical salts in the soil polar-
ise the electrodes with charges, the drift representing the
progression of the reactions. Accurate measurement of
difference in potential requires the use of non-polarising
electrodes. These consist of a metal immersed in a solution
of one of its salts in a porous container so that the solution
leaks to the ground. The metal is connected to the volt-
meter, and the solution maintains the electrical connection
between the metal and the ground. The typical combin-
ation is copper in saturated copper sulphate solution con-
tained in a porous ceramic pot.
For downhole surveys, the downhole transmitting elec-
trode can be a copper pipe about 20 mm in diameter and
2 m in length. The non-polarising downhole potential
electrodes can be pieces of oxidised lead forming a
Pb

'

- PbO electrode.

5.4.2 Electrical and electromagnetic noise

All electrical and electromagnetic systems are susceptible
to electrical interference from both environmental and
methodological noise (see Section 2.4 ). Sources of meth-
odological noise include variations in the arrangement of
the electrodes, EM loops, coils and magnetic sensors
during the survey which cause variations in coupling (see
Section 5.7.1.3 ) with the signal being measured. Great
efforts are taken to minimise this form of system noise in
both ground and airborne systems.
In general, environmental noise levels are high when
compared with the geological responses, and much care
is needed during data acquisition to reduce the noise level
and maximise the signal-to-noise ratio (see Section 2.4 ).
Noise levels are often the limiting parameter in recognising
a target response in survey data.
The various electrical and EM methods make measure-
ments at different frequencies ( Fig. 5.1 ), and noise levels in

5.4.1 Electrodes

Transmitting electrodes for electrical surveys may be noth-
ing more than a metal stake driven into the ground, but
when the ground is highly resistive or the signal is weak
this may not lead to sufficient current flow into the ground.