Page 161 - Introduction to Mineral Exploration

144 J. MILSOM

Skin depth = 500 ρ /f
500
300 1250 Hz 20 kHz 80 kHz
Skin depth (m) 100 5 kHz

50
Variation of skin depth
FIG. 7.13
30
with resistivity and frequency in
homogenous media. The signal
10 amplitude falls to about one third
(actually 1/e = 1/2.718) of its
ρ
10 100 1000 10,000 ρ (ohm-meters) original value in one skin depth.

natural conductors existed of such quality that domain. A magnetic field can be suddenly col-
the quadrature anomalies would be neglig- lapsed by cutting off the current producing
ible. The question never seems to have been it, and the magnetic effects of the currents
formally answered, there being commercial induced in ground conductors by this abrupt
vested interests on both sides, but it does seem change can be observed at times when no pri-
to be generally agreed that some good targets mary field exists. In the case of a fixed-wing air-
may produce only very small phase shifts. craft, the transmitter coil may extend from the
In any case, as noted above, the in-phase to tailplane to the nose via the wingtips (Fig. 7.2).
quadrature ratio is an important interpretation The receiver coil may be mounted in a towed
parameter. For CWEM surveys the transmitter bird, the variation in coupling to the primary
and receiver coils are now rigidly mounted field being irrelevant since this field does not
either on the aircraft frame or in a single large exist when measurements are being made.
towed bird. Birds are now used almost exclu- Helicopter TEM is becoming steadily more
sively from helicopters, as civil aviation au- popular, with a number of new systems com-
thorities enforce increasingly stringent safety peting in the market. Induced currents attenu-
regulations on fixed wing aircraft. ate quite rapidly in poorly conductive country
Originally almost all airborne systems used rock but can persist for considerable periods
the vertical-loop coaxial maximum-coupled in massive sulfide orebodies (and often in
coil configuration, but multi-coil systems are graphitic shales). Signals associated with these
now general. Because the source–receiver currents were originally observed at four delay-
separations in airborne systems are inevitably times only (Fig. 7.14), but the number of chan-
small compared to the distances to the con- nels has steadily increased, up to 256 in some
ducting bodies (even though surveys with sen- recent instruments.
sors only 30 m above the ground are becoming The TEM method was first patented by
commonplace), anomalies generally amount to Barringer Research of Toronto as the INduced
only a few parts per million of the primary PUlse Transient system (INPUT) and was used
field. Inevitably, there is some degree of flexure almost exclusively for airborne surveys. The
in even the most rigid of coil mountings and success of INPUT (and the skills of its patent
this produces noise of similar amplitude. In lawyers) delayed development of ground-based
some systems flexure is monitored electronic- transient systems outside the communist bloc
ally and corrections are made automatically. for some years, but by 1968 a Russian instru-
ment was being used for surveys in regions
of high-conductivity overburden around
7.10 TRANSIENT ELECTROMAGNETICS (TEM) Kalgoorlie. The SIROTEM system was then
developed in Australia and, at about the same
One solution to the problem of variable source– time, the Crone PEM system was designed in-
receiver coupling is to work in the time- dependently in North America. Subsequently