Geology Reference
In-Depth Information
secondary field induced by the primary is only measured
during the interval when the primary is absent.The eddy
currents induced in a subsurface conductor tend to dif-
fuse inwards towards its centre when the inducing field is
removed and gradually dissipate by resistive heat loss.
Within highly conductive bodies, however, eddy cur-
rents circulate around the boundary of the body and
decay more slowly. Measurement of the rate of decay of
the waning eddy currents thus provides a means of locat-
ing anomalously conducting bodies and estimating their
conductivity. The analysis of the decaying secondary
field is equivalent to analysing the response to a continu-
ous EM wave at a number of frequencies.TDEM conse-
quently bears the same relationship to continuous-wave
EM as, for example, time-domain IP does to frequency-
domain IP. INPUT
®
(Section 9.8.1) is an example of an
airborne TDEM system.
In ground surveys, the primary pulsed EM field is
generated by a transmitter that usually consists of a large
rectangular loop of wire, several tens of metres across,
which is laid on the ground. The transmitter loop can
also be utilized as the receiver, or a second coil can be
used for this purpose, either on the ground surface or
down a borehole (Dyck & West 1984).The transient sec-
ondary field produced by the decaying eddy currents can
last from less than a millisecond for poor conductors to
more than 20 ms for good conductors.The decaying sec-
ondary field is quantified by measuring the temporal
variation of the amplitude of the secondary at a number
of fixed times (channels) after primary cut-off (Fig.
9.11). In good conductors the secondary field is of long
duration and will register in most of the channels; in
poor conductors the secondary field will only register in
the channels recorded soon after the primary field be-
comes inactive. Repeated measurements can be stacked
in a manner analogous to seismic waves (see Section 4.3)
to improve the signal-to-noise ratio. The position and
attitude of the conductor can be estimated from the
change in amplitude from place to place of the secondary
field in selected channels, while depth estimates can be
made from the anomaly half-width. More quantitative
interpretations can be made by simulation of the anom-
aly in terms of the computed response of simple geomet-
ric shapes such as spheres, cylinders or plates, or more
simply by using the concept of equivalent current fila-
ments which models the distribution of eddy currents in
the conductor. Limited two-dimensional modelling
(Oristaglio & Hohmann 1984) is also possible using a
finite-difference approach.
A form of depth sounding can be made utilizing
Compensator +
decomposer
Generator
Re + Im
Cable
30 - 100 m
Transmitter
Receiver
Fig. 9.9
Mobile transmitter-receiver EM field equipment.
into real and imaginary components which are usually
displayed as a percentage of the primary field whose
magnitude is relayed via the interconnecting cable. Tra-
verses are generally made perpendicular to geological
strike and readings plotted at the mid-point of the sys-
tem. The maximum detection depth is about half the
transmitter-receiver separation.
Fieldwork is simple and requires a crew of only two or
three operators.The spacing and orientation of the coils
is critical as a small percentage error in spacing can pro-
duce appreciable error in phase measurement.The coils
must also be kept accurately horizontal and coplanar as
small relative tilts can produce substantial errors.The re-
quired accuracy of spacing and orientation is difficult to
maintain with large spacings and over uneven terrain.
Figure 9.10 shows a mobile transmitter-receiver
EM profile across a sheet-like conductor in the
Kankberg area of northern Sweden. A consequence of
the coplanar horizontal coil system employed is that
conducting bodies produce negative anomalies in both
real and imaginary components with maximum ampli-
tudes immediately above the conductor.The asymmetry
of the anomalies is diagnostic of the inclination of the
body, with the maximum gradient lying on the downdip
side. In this case the large ratio of real to imaginary com-
ponents over the ore body indicates the presence of a
very good conductor, while a lesser ratio is observed over
a sequence of graphite-bearing phyllites to the north.
9.6 Time-domain electromagnetic surveying
A significant problem with many EM surveying tech-
niques is that a small secondary field must be measured in
the presence of a much larger primary field, with a con-
sequent decrease in accuracy. This problem is overcome
in
time-domain electromagnetic surveying
(TDEM), some-
times called
pulsed
or
transient-field EM
, by using a pri-
mary field which is not continuous but consists of a series
of pulses separated by periods when it is inactive. The
