Geoscience Reference
In-Depth Information
high-resistivity features is more accurately determined with electrical methods. EM methods are more sensitive to the
absolute conductivity rather than contrasts in conductivity, so they are better detectors of conductive targets.
Unlike EM methods, electrical methods require contact with the ground and cannot be implemented from the air. Both
types of measurement are routinely made on the ground and downhole.
Electrical conductivity in crystalline rocks is controlled by the interconnectivity of conductive mineral grains, which are
most commonly metal sulphides other than sphalerite, metal oxides and graphite. In rocks with significant porosity it is
the interconnectivity of the pore space that is the main control, with ionic conduction occurring through saline fluids.
Electrical polarisation of rocks is controlled by the presence of disseminated conductive mineral species.
Dielectric properties of rocks are strongly controlled by their water content.
The electrical properties of the near-surface strongly affect electrical and EM measurements. A conductive layer is
common in areas of deep weathering and thick regolith. Permafrost and glacial till may respond as resistive layers.
The self-potential method measures naturally occurring electrical potentials. The origin of the potentials is not well
understood, but conductive mineralisation can be associated with a negative SP anomaly.
Resistivity measurements are usually accompanied by induced polarisation measurements. Establishing the 'depth' of
the source of the response is problematic but, by varying the position of the electrode array and the separation of the
electrodes, lateral and vertical variations in electrical properties can be mapped and used to produce data
pseudosections, volumes and maps.
EM methods rely on the induction of eddy currents in conductive regions of the subsurface. A range of survey
configurations is possible and a variety of systems available, with none being universally applicable, owing to the
complexity of the electrical properties of the subsurface and the different requirements for obtaining responses from
targets with different conductivities, geometries and depths.
Airborne EM surveys are used for reconnaissance exploration, covering large areas quickly but with limited lateral and
vertical resolution.
Downhole EM surveys are used to detect conductors in the vicinity of a drillhole.
Quantitative interpretation of electrical and electromagnetic data differs for the various survey configurations and
systems, and is hindered by non-uniqueness. Sophisticated forward and inverse modelling methods are used and must
take into account survey and system parameters, and the effects of terrain. An exact match between the observed and
the computed responses cannot be expected, owing to the complexity of variations in electrical properties in the
geological environment and the complex interactions between their electromagnetic responses.
Review questions
.....................................................................................................
1.
What are the two main ways in which electric current flows through rocks?
2.
Why is the presence of a conductive overburden so significant for electrical and EM methods?
3.
What are thought to be the main causes of self-potential anomalies?
4.
What factors should be considered when selecting an electrode array for a resistivity/IP survey?
5.
Explain how a pseudosection is created for one of the common electrode arrays. Why does this lead to 'pants-legs'
anomalies?
6.
Explain how the depth of investigation can be varied in electrical and EM methods.
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