Geology Reference
In-Depth Information
The electrical properties of the conductor cause a
further phase lag f ,
(a)
2
π
2 fL
r
p
Ë
¯
=
tan
-
1
f
(9.4)
θ
where f is the frequency of the electromagnetic field, L
the inductance of the conductor (its tendency to oppose
a change in the applied field) and r the resistance of the
conductor. For a good conductor f will approach p /2
while for a poor conductor f will be almost zero.
The net effect is that the secondary field S produced
by the conductor lags behind the primary with a phase
angle of ( p /2 + f ).The resultant field R can now be con-
structed (Fig. 9.8(b)).
The projection of S on the horizontal (primary field)
axis is S sin f and is an angle p out of phase with P . It
is known as the in-phase or real component of S .The verti-
cal projection is S cos f , p /2 out of phase with P , and
is known as the out-of-phase , imaginary or quadrature
component.
Modern instruments are capable of splitting the sec-
ondary electromagnetic field into its real (Re) and imag-
inary (Im) components.The larger the ratio Re/Im, the
better the conductor. Some systems, mainly airborne,
simply measure the phase angle f .
Classical phase measuring systems employed a fixed
source, usually a very large loop of wire laid on the
ground.These systems include the two-frame , compensator
and turam systems.They are still in use but are more cum-
bersome than modern systems in which both transmitter
and receiver are mobile.These latter systems are referred
to as twin-coil or slingram systems.
A typical field set is shown in Fig. 9.9.The transmitter
and receiver coils are about 1 m in diameter and are usu-
ally carried horizontally, although different orientations
may be used.The coils are linked by a cable which carries
a reference signal and also allows the coil separation to be
accurately maintained at, normally, between 30 m and
100 m.The transmitter is powered by a portable AC gen-
erator. Output from the receiver coil passes through a
compensator and decomposer (see below). The equip-
ment is first read on barren ground and the compen-
sator adjusted to produce zero output. By this means,
the primary field is compensated so that the system
subsequently responds only to secondary fields.
Consequently, such EM methods reveal the presence of
bodies of anomalous conductivity without providing in-
formation on absolute conductivity values. Over the
survey area the decomposer splits the secondary field
(b)
R
P
S
R
= primary
= secondary
= resultant
S cos
φ
S
φ
P
S sin
φ
Fig. 9.8 (a) The phase difference q between two waveforms.
(b)Vector diagram illustrating the phase and amplitude
relationships between primary, secondary and resultant
electromagnetic fields.
cheap and the technique is rapid to employ. However,
they provide little quantitative information on the con-
ductor. More sophisticated EM surveying systems mea-
sure the phase and amplitude relationships between
primary, secondary and resultant electromagnetic fields.
The various types of system available are discussed in
McCracken et al. (1986).
An alternating electromagnetic field can be repre-
sented by a sine wave with a wavelength of 2 p (360°)
(Fig. 9.8(a)). When one such wave lags behind another
the waves are said to be out-of-phase. The phase differ-
ence can be represented by a phase angle q correspond-
ing to the angular separation of the waveforms. The
phase relationships of electromagnetic waves can be rep-
resented on special vector diagrams in which vector
length is proportional to field amplitude and the angle
measured counterclockwise from the primary vector to
the secondary vector represents the angular phase lag of
the secondary field behind the primary.
The primary field P travels directly from transmitter
to receiver above the ground and suffers no modification
other than a small reduction in amplitude caused by geo-
metric spreading. As the primary field penetrates the
ground it is reduced in amplitude to a greater extent but
remains in phase with the surface primary. The primary
field induces an alternating voltage in a subsurface
conductor with the same frequency as the primary but
with a phase lag of p /2 (90°) according to the laws of
electromagnetic induction. This may be represented on
the vector diagram (Fig. 9.8(b)) by a vector p /2 counter-
clockwise to P .
 
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