Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 316

5.7.3.3 EM survey design
Detectability and resolution of a discrete buried conductor
depend on the geological environment and the configuration
of the EM system used. Parameters that need to be con-
sidered include whether the target is a massive conductor or
disseminated, the possible range of conductivity, its geometry
and orientation, depth of burial and whether it is likely to be
in close association with other local or regional conductors.
For example, in a particular host environment, a horizontal
plate-like conductor may be detectable at greater depths than
a similar vertically dipping conductor depending upon the
orientation of the primary field at the conductor ( Fig. 5.70 )
and the orientation and location of the receiver etc.; and
conductivity of the environment can signi cantly affect
detectability (see Section 5.7.6.1 ) . Despite the fact that the
explorer generally has only limited knowledge about the
electrical properties of the area to be surveyed, computer
modelling techniques can be applied during survey design
to estimate the system

strength and the normalised measurement has units T/A
(where T is teslas and A is amps). For coil sensors, the
induced emf is also dependent on the sensitivity of the coil,
so it is necessary to normalise the measured voltage for the
moment of the coil (see Section 5.7.1.2 ) . The fully normalised
measurement has units V(A N m 2 / (where V is volts, A is
amps, N is number of turns, m 2 is square metres). Normal-
isation allows the measurements to be compared with those
obtained with other sensors of different sensitivities.
The decrease in primary field strength with distance
from the loop in a fixed-loop survey causes the strength
of the secondary field to decrease progressively away from
the loop, distorting the shape of anomalies. The effect can
be reduced by normalising the decay channel amplitudes
by the strength of the primary field measured, either at a
particular reference station over the target anomaly/
conductor or at each survey station. The former is known
as point normalisation and useful for analysing anomaly
shape, and the latter is known as continuous normalisation
and useful for analysing anomaly amplitude. The actual
procedure used varies between the different EM systems,
making comparisons of their data dif cult.

s depth of investigation and lateral
resolution, and the detectability of a particular target in a
particular environment for a range of system parameters.

'

5.7.4 Processing and display of EM data

5.7.4.2 Decay channel amplitudes
Amplitudes of the channel values can be displayed as
1D multichannel pro les showing their variations as a
function of location along the survey lines ( Fig. 5.83a ).
For in-loop con guration this is the centre of the loop
and for the fixed-loop configuration it is the receiver
location. For the separated-loop array it is the midpoint
between the loop and receiver. For downhole surveys it is
distance measured along the drillhole trajectory. This kind
of display is becoming less common for ground and
especially AEM data but remains the only way to display
DHEM data (see Section 5.8.2 ) .
Decay channel amplitude data typically have large
variations (particularly impulse response data) and are
usually displayed using a combined logarithmic and linear
amplitude scale. Logarithmic scaling suppresses large-scale
variations and is used for amplitudes whose absolute value
is greater than 1. Linear scaling helps resolve small ampli-
tude variations, useful for the lower-amplitude late-time
channels, and is normally used to display amplitudes
between +1 and

Since the decaying secondary field is measured at multiple
delay times at each survey station ( Fig 5.73 ) , the results
from an EM survey comprise a multichannel dataset.
Ground and downhole EM surveys are usually of compara-
tively limited extent so the multichannel data volumes
obtained are not large. However, AEM datasets can involve
signi cantly large volumes of data.
Reduction of EM data involves normalising the meas-
urements for variations in the strength of the primary field.
The data are displayed in two basic ways: as profile plots of
variations in channel amplitudes and as displays of con-
ductivity variations in the subsurface.

5.7.4.1 Amplitude normalisation
Among other factors, the strength of the secondary field also
depends upon the strength of the primary field, so it is
essential that the measurements be corrected for variations
in the amplitude of the primary field. The amplitude of each
channel is divided, or normalised, by the transmitter current
measured at the time of the pulse. Sometimes the measure-
ments are further normalised for the moment of the trans-
mitter loop (see Section 5.7.1.2 ), the usual practice in AEM
systems. For B- eld sensors (including the recovered B- eld
of AEM systems) the measurement is a magnetic

1.
Pro le plots have the advantage of being a true repre-
sentation of the measurement and they can be rapidly
scanned for anomalous responses. Their disadvantage is
that they are hard to interpret

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eld ' geologically ' since the