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.
Surface tools cannot simply be lowered down a borehole: probes must be slimmed to a diameter that clears the hole (often <10 cm), pressure- and fluid-sealed, and deployed on an armoured, insulated wireline that both lowers the tool and carries the signal to a surface logging truck. Electrode-contact resistivity tools use pad-mounted electrodes pressed against the borehole wall by spring arms/centralizers (or rely on a conductive drilling fluid to couple current into the formation when pads are impractical), whereas EM tools use small transmitter and receiver coils wound on a mandrel, oriented axially or orthogonally, spaced along the tool at fixed offsets tuned to the desired depth of investigation. Deployment procedures also change: measurements are made continuously while the tool is winched up or down the hole (a "log" versus a fixed-station survey), speed must be controlled to preserve depth accuracy and vertical resolution, and casing (where present) must be accounted for because steel casing is highly conductive and can short-circuit or severely attenuate both galvanic and inductive signals.
Three configurations are common. Single-hole logging (resistivity or induction logs run down one hole) gives a continuous, high-resolution vertical profile immediately around that hole, strong for stratigraphic correlation and formation evaluation, but its lateral radius of investigation is small (typically less than a few tool-lengths) so it says little about structure away from the hole. Borehole-to-surface (mise-à-la-masse) methods inject current directly onto a conductive target intersected in the hole and map the resulting potential field at surface, which is excellent for tracing the lateral extent/connectivity of a known conductor (e.g. following a mineralized zone between drill intersections) but requires the target to actually be exposed in the hole and be conductive enough to act as a current electrode. Cross-hole (borehole-to-borehole) tomography, using a transmitter in one hole and receivers in an adjacent hole (electrical resistivity tomography or EM tomography), images the resistivity structure of the entire rock volume between the two holes with much better resolution than either single-hole log, but needs a second borehole (added cost), a good coupling medium/fluid, and inversion software to reconstruct the tomographic image, and its resolution degrades toward the mid-point furthest from both holes.
