Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 334

Urquhart Shale (20 S)

Overburden (15 S)

Y

X

Urquhart
Shale - top (20 S)

Z

Urquhart Shales
West Contact (10 S)

Fine grained pyrite (2000 S)

Paroo fault (10 S)

Off-hole
targets (3000 S)

0 ~500

Metres

1400
1600

1800

2000

Fine-grained
pyrite (3000 S)

2200

Secondary field
continuously
normalised
(%)

Secondary field
continuously
normalised
(%)

180 180

Observed Calculated

160 160

120 120

Figure 5.93 Presentation of DHEM data in 3D as used for
data modelling. The locations of conductors relative to the
drillhole trajectory are shown along with the observed and
calculated data. Data are from the Mount Isa Cu mine.
Redrawn, with permission, from Jackson et al.( 1996 ) .

80 80

40 40

0 0

1400 1600 1800 2000 2200 1400 1800

Downhole depth (m)

1600 2000 2200

-40 -40

Downhole depth (m)

-60 -60

with well-coupled conductors close to the drillhole produ-
cing particularly strong responses. The measurements may
also include complex responses due to closely spaced and
interacting conductors, and artefacts due to deviations in
the drillhole trajectory (probe orientation changing). Inter-
pretation proceeds by identifying the responses of in-hole
and off-hole conductors (see Section 5.8.2.1 ). A series of
computed downhole profiles for all three components from
one or more drillholes located at various locations around
and intersecting a model conductor is an essential inter-
pretation aid.

component show symmetry and change in polarity around
the conductor. The distribution of the current flow in the
conductor depends on the size, shape and orientation of
the induced current system at any particular delay time
(see Section 5.7.1.4 ). For in-hole conductors the responses
vary according to whether the drillhole has intersected the
central region of the conductor or its edge, because this
means the hole is in a different location relative to the
migrating eddy currents.
Based on the relationships between component
responses and their polarities, the location, orientation
and size of the source conductor with respect to the drill-
hole can be interpreted. The computed model responses
shown in Fig. 5.94 illustrate the key characteristics of the
axial (A) and radial (U) component responses (see Section
5.8.1.3 ) , in four vertical drillholes, obtained with a single
surface transmitter loop located directly above a horizontal
finite thin plate conductor. The direction of U is ambigu-
ous in a vertical drillhole ( Fig. 5.92 ) ; it is taken as positive

5.8.2.1 In-hole and off-hole responses
Conductors detected by DHEM are classi ed as either in-
hole conductors, those intersected by the drillhole, or off-
hole conductors, those located away from the drillhole. The
three-component response is diagnostic of these two
classes and their identi cation is fundamental in the inter-
pretation of DHEM data ( Fig. 5.94 ). Profiles of each