Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 331

Transmitter loop
(Tx)

I

Recorder

Drillhole

Primary
magnetic
field

Figure 5.90 Typical time domain DHEM survey system
using a large transmitter loop (Tx) located on the surface.
Directions of the eddy current

Receiver
(Rx)

Eddy
currents

flow and its associated

Secondary
magnetic
field

secondary magnetic field are shown for the period

immediately after turn-off of the primary field shown.

Redrawn, with permission, from Killeen ( 1997a ).

Conductive
mineralisation

of rugged terrain (Mudge, 1996 ). Where there are adjacent
mine workings, it may be possible to locate the loop within
the workings (Doe et al., 1990 ). The receiver is usually
lowered down the drillhole, although for in-mine surveys
it may be moved along drillholes of almost any orientation,
including upward oriented drillholes. These arrangements
are very useful because, in principle, there are no spatial
restrictions on the location of conductors relative to the
DHEM system, so optimal coupling with a target con-
ductor can often be achieved ( Figs. 5.69 and 5.70 ). This is
unlike surface and airborne surveying where both the
transmitter loop and receiver are usually co-planar and
parallel to the ground surface, and the conducting regions
lie below the EM system.
DHEM probes are usually capable of operating to depths
exceeding 1 km, with the most robust probes capable of up
to 3 km depth. Transmission of the primary field signal
and data recording are achieved using the same equipment
as used for surface surveys (see Section 5.7.3 ) , with the
additional probe orientation data also recorded. Downhole
EM surveying can be conducted in dry holes and even in
those cased with plastic piping. Magnetic and highly con-
ductive steel casing and drilling rods in the drillhole cause
strong interference to EM systems and prevent the use of
DHEM.
The radius of investigation around the drillhole is deter-
mined by the minimum detectable signal level, which is
determined by the strength of the primary field, the loop
location (coupling) with respect to the target conductor,
receiver sensitivity, background noise and the type of
response measured. Step response systems have greater
radius of investigation for good conductors than impulse
response systems (see Section 5.7.2.5 ). A conductor in the

vicinity of the drillhole, i.e. a , can be detected,

and in favourable conditions good conductors more than
1 km from the drillhole can be detected.
Compared with surface and airborne EM surveys, down-
hole data are collected over a relatively small area with
surveying often confined to just a single drillhole. The
emphasis then is on optimising survey procedures to
ensure the detection and determination of the geometry,
position and orientation of any conductor in the area from
the single downhole survey traverse. Optimally achieving
these objectives requires the acquisition of three-
component measurements of the secondary magnetic

' near-miss '

field
from within several drillholes and with different transmit-
ter loop positions.

5.8.1.1 Variable field coupling
Surveying with transmitter loops in different locations
around the drillhole creates primary magnetic fields with
different orientations and changes the coupling with any
conductors present. Typically five loop positions are used
( Fig. 5.91a ) with the downhole survey repeated for each,
and differences in the shape, amplitude and sign of the
secondary

field are sought. The intention is to ensure that
all potential conductors in the vicinity of the drillhole are
energised. Data may be recorded in multiple drillholes, and
possibly by using just a single transmitter loop ( Figs. 5.91b
and c ). When surveying multiple drillholes using a single
transmitter loop, differences in coupling of the secondary
magnetic fields and the downhole receiver, between the
different drillholes, constrain the location and orientation
of an energised conductor. The different responses
obtained from conductors using these survey procedures,
including when poor or null coupling occurs, provide