Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 333

is possible to uniquely locate a conductor with just one
transmitter loop, assuming the primary field couples with
the conductor. Not having to survey the drillhole with
several transmitter loop positions signi cantly reduces
the time and cost of DHEM surveying. Nevertheless, it is
good practice to survey the drillhole with several transmit-
ter loops, and to survey neighbouring drillholes with the
same transmitter loop, to con rm the interpretation.
The orientations of the three measured field compon-
ents are fundamental to the analysis of the data. In order
that the data can be analysed in a consistent reference
frame, changes in the orientation of the drillhole and the
probe must be monitored and their effects corrected for.
Magnetic sensors and accelerometers in the probe provide
the necessary information. The axis of the drillhole pro-
vides the reference for the analysis of the three components
of the EM

with the local stratigraphy and any concordant conductors,
although coupling and access considerations may dictate
otherwise. Loop size is determined using the same criteria
described for ground surveys (see Transmitter loop size in
Section 5.7.3.2 ).
Drillhole locations, orientations and depths are usually
known when designing DHEM surveys, as are the most
likely locations and attitudes of targets. Forward-modelling
techniques can therefore be very effective in optimising the
transmitter loop size, the loop layouts and the number of
loops required. The primary field from the loops can be
computed and plotted to show their coupling with known
conductors (see Fig. 5.70 ) and expected conductors at
other locations around the drillhole; for example, see
Bishop ( 1996 ). Anomaly shapes and polarities can be pre-
dicted for each component of the secondary field, and

survey procedures optimised to record the expected
responses. Where multiple drillholes are available, those
offering the optimum response from the target conductors
can be identi ed and selected for surveying.
In the event that the direction to the target is unknown,
a transmitter loop centred over the drillhole ( Fig. 5.91b ) to
energise the ground equally around it is often the best
starting point. Additional loops can be deployed based on
results obtained from the starting loop.

field, which are termed A, U and V. The axial
component (A) is always oriented along the drillhole axis,
reckoned positive upwards ( Fig. 5.92 ). The radial (U) com-
ponent lies in the plane of the drillhole and perpendicular
to the drillhole axis, and is reckoned positive upward. It is
vertical in a horizontal drillhole, and in a vertical drillhole
it is horizontal and can point in a direction to suit the
interpreter. The other radial component (V) is perpendicu-
lar to the U component and horizontal, with positive
polarity as shown in ( Fig. 5.92 ) . The measured EM eld
is visualised and interpreted in the A, U, V coordinate
space. Some workers refer to the A, U, and V axes as Z, X
and Y, respectively, and may change the polarity (sign) of
the Y axis.
Clearly the orientation of the A, U and V reference
frame varies in absolute direction depending upon the
azimuth and inclination of the drillhole. The EM data
can be further rotated through the inclination and azimuth
of the drillhole to resolve the measured EM

5.8.2 Display and interpretation of DHEM data

DHEM surveys are characterised by large numbers of
multichannel profiles of three-component data acquired
from a multiplicity of drillholes and transmitter loops
potentially located anywhere within the rock volume of
interest. This contrasts with data from ground and air-
borne EM surveys which are in the form of an ordered
series of parallel survey lines with measured responses
from targets located below the survey surface, and which
can be displayed as 2D conductivity maps and sections. In
DHEM it is generally not possible to make composite
conductivity images of multiple drillhole data, so special-
ised presentation, visualisation and modelling software is
required. The data are still presented in pro le form (see
Pro le analysis in Section 5.7.5.3 ) but within a 3D volume
display which allows the relative positions of the relevant
drillholes, conductors and transmitter loops to be dis-
played ( Fig. 5.93 ) .
In DHEM surveying, conductors can be located any-
where around the receiver producing a wide range of
responses. The data often have a large range in amplitude,

field in an
absolute reference frame, such as relative level (RL), easting
and northing (X, Y, Z in Fig. 5.92 ) . This can be useful when
working with data from several drillholes in the vicinity. At
prospect scale, drillholes are often drilled in the same
direction (azimuth) and the transmitter loops are usually
laid out along this direction, so the A, U and V coordinate
system is usually adequate for a small survey area.

5.8.1.4 DHEM survey practice
In DHEM surveying two sides of the rectangular transmit-
ter loops are usually oriented parallel to the drillhole azi-
muth ( Fig. 5.91 ). Given that most drilling is directed across
strike, it means that the transmitter loops will be aligned