Geology Reference
In-Depth Information
8.4.3 TEM and CWEM
Transient electromagnetic and CWEM methods are theoretically equivalent
but differ in their advantages and disadvantages because the principal sources
of noise are quite different.
Noise in CWEM surveys arises mainly from variations in the coupling
between transmitter and receiver coils; hence, the separations and relative
orientations of the coils must either be kept constant or, if this is not possible,
must be very accurately measured. The receiver circuitry must be very stable,
but even so it is difficult to ensure that the initial 100% (for the in-phase
channel) and 0% (for the quadrature channel) levels do not drift significantly
during the course of a day. The primary field is the ultimate source of all
these forms of noise, and the situation can therefore not be improved merely
by increasing transmitter power. In TEM surveys, on the other hand, the
secondary fields due to ground conductors are measured at times when no
primary field exists, and coupling noise is therefore negligible. Moreover,
the very sharp termination of transmitter current provides a timing reference
that is inherently easier to use than the rather poorly defined maxima or
zero-crossings of a sinusoidal wave.
The most important sources of noise in TEM surveys are external natural
and artificial field variations. The effect of these can be reduced by increas-
ing the strength of the primary field and by
N
-fold repetition to achieve a
√
N
improvement in signal to noise ratio (see Section 1.5.7). There are,
however, practical limits. Transmitter-loop magnetic moments depend on
current strengths and loop areas, neither of which can be increased indef-
initely. Safety and generator power, in particular, set fairly strict limits to
usable current magnitudes. Multiple repetitions are not a problem in shallow
work, where virtually all the useful information is contained in the first few
milliseconds of the decay curve, but can be time-consuming in deep work,
where measurements may have to be extended to time delays of as much as
half a second. Moreover, repetition rates must be adjusted so that power-line
noise (which is systematic) is cancelled and not enhanced, and the number
of repetitions must be adequate for this purpose. It may take more than
ten minutes to obtain satisfactory data at a single point when sounding to
depths of more than a hundred metres. This does, of course, compare very
favourably with the time needed to obtain soundings to similar depths with
Wenner or Schlumberger arrays, but the equipment required is considerably
more expensive.
In Slingram surveys, resolution is determined by the spacing between the
transmitter and receiver coils. Both CWEM and TEM surveys may use the
Slingram configuration but with TEM it is also possible to co-locate
the receiver and transmitter coils, giving very high resolving power. TEM
is thus much more suitable than CWEM for precisely locating very small
