Geoscience Reference
In-Depth Information
Their strength and duration depends on the electrical
properties and geometry of the conductor, so monitoring
their decay provides information about the subsurface
conductivity. In our description of TDEM methods we
describe the induction processes at turn-off.
In the frequency domain a continuous sinusoidal (a.c.)
current is used to produce a continuous sinusoidally vary-
ing primary magnetic field, and usually at several frequen-
cies, inducing sinusoidally varying eddy currents into a
conductor. They circulate in the conductor and are out of
phase with respect to the primary field. Their strength and
phase depend on the properties of the conductor, so by
measuring these properties information can be obtained
about the subsurface conductivity.
FDEM systems measure the weak secondary field in the
presence of the stronger primary
Section 5.2.2.1 ). The strength of the magnetic field is
quanti ed by the magnetic dipole moment (m) given by:
m
¼
nIA
ð
5
:
21
Þ
where I is the current in amperes, n the number of turns in
the coil or loop and A the area of the coil/loop in square
metres. The dipole moment has units of A m 2 .
Increasing the number of turns, and/or increasing the
current in a loop, increases its dipole moment. The larger
the transmitter
'
is dipole moment the stronger is the pri-
mary field and the stronger the eddy currents it induces.
This improves the signal-to-noise ratio of the secondary
field measurement. There are several advantages in using
large loops as transmitters. Equation (5.21) shows that a
large loop has a larger dipole moment than a smaller loop,
and increasing the size of the loop spreads the field over a
larger volume of the ground. The strength of the field of
small coils and loops decreases as 1/distance 3
field. Features of the
resultant of the primary and secondary
fields combined
from the
are measured, which reduces the system
s sensitivity to
small variations in the secondary field. Furthermore,
variations in the orientation and position of the receiver
with respect to the transmitter cause variations in the
strength of the primary field at the receiver and are a
signi cant source of noise. In contrast, TDEM measure-
ments are, in principle, made when the primary field is
turned off (variations to this arrangement are described in
Sections 5.7.1.6 and 5.7.1.7 ), so the receiver sensitivity is
maximised to detect the weaker secondary signals. In
addition, variations in the orientation and location of
the receiver with respect to the transmitter are not a
source of noise. TDEM systems offer superior perform-
ance over FDEM systems.
FDEM systems were developed before TDEM systems,
but because of inferior performance in resolving conduct-
ors in host rocks with variable conductivity and beneath
conductive cover, they have been almost completely
replaced in mineral exploration by TDEM systems. For
this reason, our description of EM methods is almost
entirely focused on time domain methods. Frequency
domain FDEM systems still find application in shallow
engineering, archaeological and groundwater investiga-
tions. Their use in mineral exploration is limited to air-
borne surveys to map shallow conductive targets, e.g.
kimberlites (Reed and Witherly, 2007 ) and magnetotelluric
methods, as described in Appendix 4 .
'
loop, but as approximately 1/distance 2
for a large loop, so
deeper targets can be detected.
Transmitter currents of up to several hundred amps are
used and are limited by the level of the back emf produced
at the instant of turn-off (see Section 5.2.2.2 ) . The back emf
is determined by the rate of turn-off of the current and the
inductance of the loop, which is proportional to the square
of its dimensions (L 2 ) and the number of turns. A large
multi-turn loop produces a large back emf, preventing the
instantaneous step turn-off of the magnetic field. The
current must be turned on, or off, at a slower rate over a
period known as the ramp time. The magnetic field
changes at a less desirable rate and has a detrimental effect
on the EM response.
5.7.1.3 Primary-field to conductor coupling
The concept of coupling is illustrated in Fig. 5.69 where the
large current-carrying loop represents the transmitter, pro-
ducing the primary
field, and the conductor is represented
by a small loop of wire. The only possible
flow path for the
induced current is around the small loop, and the current
is greatest when the plane of the loop is perpendicular to
the primary
field (see Section 5.2.2.2 ), a relationship
labelled as
in the figure. There is no
current flow in the small loop where the magnetic field is
parallel to the plane of the loop, i.e. where the field does
not cut the loop. The loop is then said to be null coupled
with the field. Note how the coupling changes around the
transmitter loop for various locations and orientations of
the small loop.
'
very well coupled
'
5.7.1.2 Creating the primary magnetic field
The primary magnetic field in EM methods is usually
created by using a large loop of wire (see Fig. 5.7 and
 
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