Geology Reference
In-Depth Information
are determined.The latter ratio is readily converted into
conductor depth.
l
corresponds to 10
7
/
s
ft
, where
s
is
the conductivity of the sheet and
f
the frequency of the
field. Since
a
and
f
are known, the product
s
t
can be
determined. By performing measurements at more than
one frequency,
s
and
t
can be computed separately.
Much electromagnetic interpretation is, however,
only qualitative, particularly for airborne data. Contour
maps of real or imaginary components provide informa-
tion on the length and conductivity of conductors while
the asymmetry of the profiles provides an estimate of the
inclination of sheet-like bodies.
A
φ
'
φ
(a)
(b)
Conductivity
¥
frequency
9.10 Limitations of the electromagnetic
method
The electromagnetic method is a versatile and efficient
survey technique, but it suffers from several drawbacks.
As well as being caused by economic sources with a high
conductivity such as ore bodies, electromagnetic anom-
alies can also result from non-economic sources such as
graphite, water-filled shear zones, bodies of water and
man-made features. Superficial layers with a high con-
ductivity such as wet clays and graphite-bearing rocks
may screen the effects of deeper conductors. Penetration
is not very great, being limited by the frequency range
that can be generated and detected. Unless natural fields
are used, maximum penetration in ground surveys is
limited to about 500 m, and is only about 50 m in
airborne work. Finally, the quantitative interpretation
of electromagnetic anomalies is complex.
Fig. 9.18
The relationship between the phase/amplitude of a
secondary electromagnetic field and the product of conductivity
and frequency. A given phase shift
f
¢ could result from a poor
conductor (a) or a good conductor (b).
9.9 Interpretation of electromagnetic data
As with other types of geophysical data an indirect
approach can be adopted in the interpretation of
electromagnetic anomalies. The observed electromag-
netic response is compared with the theoretical response,
for the type of equipment used, to conductors of various
shapes and conductivities. Theoretical computations of
this type are quite complex and limited to simple geo-
metric shapes such as spheres, cylinders, thin sheets and
horizontal layers.
If the causative body is of complex geometry and
variable conductivity, laboratory modelling may be used
(Chakridi & Chouteau 1988). Because of the com-
plexity of theoretical computations, this technique is
used far more extensively in electromagnetic interpreta-
tion than in other types of geophysical interpretation.
For example, to model a massive sulphide body in a well-
conducting host rock, an aluminium model immersed
in salt water may be used.
Master curves are available for simple interpretation of
moving source-receiver data in cases where it may be as-
sumed that the conductor has a simple geometric form.
Figure 9.20 shows such a set of curves for a simple sheet-
like dipping conductor of thickness
t
and depth
d
where
the distance between horizontal, coplanar coils is
a
.The
point corresponding to the maximum real and imagi-
nary values, expressed as a percentage of the primary
field, is plotted on the curves. From the curves coincid-
ing with this point, the corresponding
l
/a
and
d/a
values
9.11 Telluric and magnetotelluric
field methods
9.11.1 Introduction
Within and around the Earth there exist large-scale, low-
frequency, natural magnetic fields known as
magneto-
telluric fields
.These induce naturalalternatingelectric cur-
rents to flow within the Earth, known as
telluric currents
.
Both of these natural fields can be used in prospecting.
Magnetotelluric fields are believed to result from the
flow of charged particles in the ionosphere, as fluctua-
tions in the fields correlate with diurnal variations in the
geomagnetic field caused by solar emissions. Magneto-
telluric fields penetrate the ground and there induce
telluric currents to flow. The fields are of variable
