Question 6 of 10: Electrical vs. Electromagnetic Methods

Question 6: 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.

Similarities. Both the electrical (DC resistivity) and electromagnetic (EM) methods are ultimately sensitive to the same subsurface physical property — electrical resistivity/conductivity — and both are applied to the same broad target classes: groundwater and aquifer mapping, contamination/leachate plumes, massive-sulphide and other conductive-ore exploration, and geotechnical overburden/bedrock mapping. Both can be configured either as a profiling survey (lateral mapping at roughly fixed depth) or as a sounding (depth investigation at one location), and both ultimately require an inversion step to convert the raw apparent-property measurements into a true resistivity model.

Differences. DC resistivity requires direct galvanic contact with the ground — electrodes are physically driven into the soil and current is injected directly. EM methods instead couple to the ground inductively, through a transmitting loop or coil that generates a time-varying primary magnetic field, inducing eddy currents in the ground with no physical electrode contact required at all. This has major practical consequences: EM surveys are far faster (no electrodes to plant/move) and can be conducted over pavement, permafrost, or very dry/resistive surface material where electrode contact resistance would make DC resistivity difficult or impossible; conversely, DC resistivity's quantitative interpretation (via a well-established, purely geometric factor $k$) is simpler and more robust than EM's coupling-dependent response, and DC handles a resistive near-surface layer more gracefully for reaching depth, whereas EM's depth of investigation is fundamentally limited by the skin-depth relation $\delta\approx503\sqrt{\rho/f}$ — a highly conductive near-surface layer sharply reduces EM's penetration at any given frequency, an issue DC resistivity does not share in the same way.

Advantages/disadvantages. Electrical (DC): well-understood, quantitatively robust inversion and good vertical resolution for layered stratigraphy (e.g. a Schlumberger VES); disadvantages are slow field logistics (planting/moving many electrodes) and poor performance in resistive, frozen, or paved ground where good electrode contact cannot be achieved. Electromagnetic: fast, non-contact reconnaissance mapping (e.g. a walking EM31 conductivity survey covering hundreds of stations per day, or an airborne EM system covering hundreds of line-kilometres per day) and effective over difficult-contact ground; disadvantages are a more complex, less unique response to 3-D bodies, susceptibility to cultural EM noise (power lines, buried pipes, fences generating spurious "conductors"), and skin-depth-limited penetration in highly conductive terrain. Example: a helicopter EM (e.g. a time-domain VTEM) survey for regional massive-sulphide reconnaissance over difficult terrain, followed by detailed ground DC resistivity/IP surveying to quantitatively delineate and prioritize any conductive target the airborne EM identifies — a common, complementary two-stage exploration sequence that plays to each method's strengths.