Geology Reference
In-Depth Information
9
Electromagnetic surveying
aircraft or towed behind them. Airborne EM methods
are widely used in prospecting for conductive ore bodies
(see Section 9.8).
All anomalous bodies with high electrical conductiv-
ity (see Section 8.2.2) produce strong secondary electro-
magnetic fields. Some ore bodies containing minerals
that are themselves insulators may produce secondary
fields if sufficient quantities of an accessory mineral with
a high conductivity are present. For example, electro-
magnetic anomalies observed over certain sulphide ores
are due to the presence of the conducting mineral
pyrrhotite distributed throughout the ore body.
9.1 Introduction
Electromagnetic (EM) surveying methods make use of
the response of the ground to the propagation of electro-
magnetic fields, which are composed of an alternating
electric intensity and magnetizing force. Primary elec-
tromagnetic fields may be generated by passing alternat-
ing current through a small coil made up of many turns
of wire or through a large loop of wire. The response of
the ground is the generation of secondary electromag-
netic fields and the resultant fields may be detected by
the alternating currents that they induce to flow in a re-
ceiver coil by the process of electromagnetic induction.
The primary electromagnetic field travels from the
transmitter coil to the receiver coil via paths both above
and below the surface.Where the subsurface is homoge-
neous there is no difference between the fields propa-
gated above the surface and through the ground other
than a slight reduction in amplitude of the latter with
respect to the former. However, in the presence of a
conducting body the magnetic component of the
electromagnetic field penetrating the ground induces
alternating currents, or eddy currents, to flow in the
conductor (Fig. 9.1). The eddy currents generate their
own secondary electromagnetic field which travels to
the receiver. The receiver then responds to the resultant
of the arriving primary and secondary fields so that the
response differs in both phase and amplitude from the
response to the primary field alone. These differences
between the transmitted and received electromagnetic
fields reveal the presence of the conductor and provide
information on its geometry and electrical properties.
The induction of current flow results from the mag-
netic component of the electromagnetic field. Conse-
quently, there is no need for physical contact of either
transmitter or receiver with the ground. Surface EM sur-
veys can thus proceed much more rapidly than electrical
surveys, where ground contact is required. More impor-
tantly, both transmitter and receiver can be mounted in
9.2 Depth of penetration of
electromagnetic fields
The depth of penetration of an electromagnetic field
(Spies 1989) depends upon its frequency and the electri-
cal conductivity of the medium through which it is
propagating. Electromagnetic fields are attenuated dur-
ing their passage through the ground, their amplitude
decreasing exponentially with depth. The depth of
penetration
d
can be defined as the depth at which the
amplitude of the field
A
d
is decreased by a factor e
-1
com-
pared with its surface amplitude
A
0
AA
d
=
e
-
1
(9.1)
0
In this case
503 .
s
d
=
(9.2)
f
where
d
is in metres, the conductivity
s
of the ground is
in S m
-1
and the frequency
f
of the field is in Hz.
The depth of penetration thus increases as both the
frequency of the electromagnetic field and the con-
ductivity of the ground decrease. Consequently, the
