04-Geol-B10 · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B10-2 Electrical Methods, 2017-May. Closed book; no calculator permitted. All ten questions require an answer in essay format, with diagrams used wherever appropriate. The exam instructs "choose six (6) of the following ten (10) questions, the first six as they appear in the answer book will be marked, each of equal value, about half an hour each".
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed. (electrical properties of rocks ch.5; self-potential ch.6; induced polarization ch.9; resistivity ch.8; electromagnetic methods ch.7; magnetotellurics ch.10); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (resistivity arrays, EM systems, MT surveying, ch.8–9); Simpson & Bahr, Practical Magnetotellurics (MT instrumentation and robust/remote-reference processing, ch.2–6).
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.
Electrical and EM methods are controlled by three related electrical properties of rock and soil: electrical resistivity ρ (or its reciprocal, conductivity σ, in S/m), dielectric permittivity ε (which governs high-frequency EM propagation, e.g. GPR), and chargeability M (the capacity of a material to store and slowly release charge when a current is switched off, exploited by the induced-polarization method). Resistivity in earth materials spans a huge range — roughly 10-3 Ω·m for massive sulphide ore up to 105–108 Ω·m for dry crystalline rock or ice — and in sediments and soils is dominated not by the mineral grains themselves (which are mostly insulators) but by electrolytic conduction through pore water, so it is strongly controlled by porosity, pore-water salinity, saturation and clay content (Archie's law). Chargeability is elevated wherever disseminated metallic-luster minerals (pyrite, chalcopyrite, magnetite, graphite) or clay-mineral membrane polarization is present, which is why IP is the classic tool for disseminated sulphide exploration that resistivity alone under-detects.
In the laboratory, hand samples or core plugs are measured with a four-electrode resistivity cell (a small Wenner-type array pressed onto, or a current/potential electrode pair clamped across, the sample) driven by a low-frequency AC resistivity meter or a spectral-IP (SIP) impedance analyzer that sweeps frequency to recover both resistivity and chargeability/phase. Dielectric permittivity is measured with a time-domain reflectometry (TDR) probe or a network analyzer at radar frequencies. In the field, the same physical properties are measured in situ with a portable four-electrode resistivity meter (e.g. a Wenner or Schlumberger probe pressed into the ground, or a small-scale array laid out on surface, or logged down a borehole), and conductivity can additionally be profiled non-invasively with a portable EM conductivity meter (e.g. an EM31/EM38-type ground-conductivity meter) that requires no galvanic contact at all.
Every one of these instruments has practical weaknesses. Four-electrode contact resistance is sensitive to poor electrode coupling (dry soil, gravel, frozen ground), and AC measurement at a single frequency cannot separate resistivity from IP/dielectric effects without a spectral sweep. More fundamentally, a laboratory measurement on a small core or hand sample is a point measurement at the wrong scale and in the wrong state relative to the subsurface it is meant to represent: coring disturbs in-situ fabric and can induce micro-fracturing that changes bulk resistivity; the sample desaturates (dries) between recovery and testing unless carefully sealed, which can raise its measured resistivity by an order of magnitude relative to its saturated in-situ value; a single core plug cannot capture macroscopic heterogeneity, fracture-controlled anisotropy, or the bulk effect of larger-scale features (fault gouge zones, weathering profiles) that dominate the field-scale response; and near-surface field measurements are themselves biased by seasonal moisture and temperature variation that a single laboratory measurement, taken at one point in time, cannot represent. For all these reasons, laboratory property measurements are best used as a relative guide to contrast between lithologies rather than as an absolute calibration of a field survey.
