Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 290

a) b)

Intermediate volcanics
Clastic sediments
Major faults

Scott lode

Cindy

0 500

Metres

Nancy

Vera

Figure 5.57 Pajingo epithermal system. (a) Image of gradient array resistivity data. Data courtesy of Evolution Mining. (b) Geological map.
Redrawn, with permission, from Hoschke and Sexton ( 2005 ) .

through the conductive layer means that targets in
the underlying region produce weak responses, which are
often obliterated by the background noise. Where the
conductive body is in contact with the overburden,
the current may be channelled into it and a stronger
response produced. Changes in overburden conductivity
and thickness create spurious responses that can resemble
those of potentially economic signi

receiver (see Section 5.2.2.2 ) . The effect is known as elec-
tromagnetic coupling, EM-coupling or inductive coupling
and causes spurious IP-like effects, which may overwhelm
the responses due to electrically polarisable materials in the
subsurface (Wynn and Zonge, 1975 ). Grounded cultural
features such as wire fences and pipelines can also contrib-
ute to coupling. The effect is exacerbated in highly
conductive terrains and in areas of highly conductive
overburden. Resistivity measurements are unaffected.
The removal of EM-coupling, or decoupling the IP
parameters, has been a major area of IP research.
A number of schemes have been devised to calculate the
effect and correct the IP measurements, all with various
limitations. For the case of time domain measurements, it
is often assumed that EM-coupling has little effect in the
late decay times, so the late-time secondary voltages are
preferred for mapping the IP parameters. Techniques that
attempt to correct the full decay-series voltages are
described by Fullagar et al.( 2000 ). In the frequency
domain, correction techniques depend on the changes in
the measured parameters with frequency. Coupling-related
responses vary more rapidly with frequency than normal
IP effects so the coupling component can be estimated and
removed to resolve the IP effect.

cance. Overburden
conductivity and thickness can be obtained from electrical
measurements made on drill samples or determined from
vertical electrical soundings, and are useful in the analysis
of the survey data. Ideally, this information should be
available for computer modelling during survey design to
assist in optimising the survey parameters.
Where the overburden is comparatively resistive, such as
the tills in the glaciated region of the North Hemisphere,
there is less of a problem, but it may still mask economic-
ally significant responses (Reynolds, 1980 ) .

5.6.7.2 Electromagnetic coupling
Measurement of IP parameters is subject to interference
related to changes in current flow in the wires connecting
the current electrodes. Eddy currents are induced in the
conductive ground and into the wires connecting to the