Question 5 of 10: Electrical vs. Electromagnetic Methods

Question 5: Electrical vs. Electromagnetic Methods (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Fundamental difference. Electrical (DC resistivity and induced-polarization) methods inject a controlled current directly into the ground through galvanically coupled electrodes and measure a resulting potential difference at other electrodes, so the current path and its coupling into the earth depend on physical, ohmic electrode-ground contact. Electromagnetic (EM) methods instead energize the ground with a time-varying magnetic field from a transmitter loop or wire, which INDUCTIVELY generates eddy currents in the subsurface without requiring any physical electrode contact with the ground at all — the coupling is purely magnetic.

Advantages of electrical methods. DC resistivity gives a direct, quantitative, well-understood measure of bulk resistivity with mature, robust inversion algorithms (1-D Wenner/Schlumberger soundings through modern 2-D/3-D ERT); IP additionally provides a chargeability measurement diagnostic of disseminated sulphide/clay content that most EM systems cannot directly replicate. Example: a Wenner resistivity sounding is the classic, reliable tool for quantitatively determining depth to the water table and aquifer thickness in a straightforward layered sedimentary sequence.

Disadvantages of electrical methods. Good electrode-ground contact is essential and can be difficult or impossible to achieve on paved surfaces, in very dry/resistive sand or gravel, or in frozen ground (permafrost) — all of which give poor galvanic coupling and unreliable or unmeasurable readings; surveys are also comparatively slow and labour-intensive, since electrodes must be physically moved and re-planted for every reading or array expansion, and the method cannot be flown.

Advantages of electromagnetic methods. Because no ground contact is required, EM surveys can be conducted rapidly on foot, from a vehicle, or from the air, giving much faster areal coverage than electrical methods for a given budget; EM also performs well over resistive, poorly-coupling terrain (permafrost, dry sand, paved sites) precisely because it does not need galvanic contact, and it responds particularly strongly and directly to good conductors (sulphide bodies, saline plumes, graphite), which is why EM is the workhorse tool for base-metal conductor detection. Example: an airborne time-domain EM survey rapidly screens a large, remote, permafrost-affected exploration property for base-metal conductors that a ground DC resistivity crew could not have surveyed at all in the same time or terrain.

Disadvantages of electromagnetic methods. The induction physics is inherently more complex than simple DC current flow, so quantitative interpretation (depth, dip, conductance) generally requires numerical/plate modelling rather than a simple closed-form sounding curve; EM is also comparatively insensitive to RESISTIVE targets and to resistive layers lying beneath a conductive layer (the "skin effect" shields deeper structure once the overlying material is conductive enough), a limitation that DC resistivity/IP soundings do not share in the same way; and instrumentation, particularly for time-domain systems, is generally more complex and costly than a basic resistivity meter and electrode set.