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.
Advantages: instrumentation is comparatively simple and often lightweight/rigid-boom (e.g. small hand-carried ground-conductivity meters), giving continuous, real-time apparent-conductivity readings while walking — excellent for fast, high-productivity near-surface conductivity mapping; using several fixed transmitter-receiver coil spacings and/or frequencies gives some crude information about how conductivity changes with depth. Disadvantages: the primary field is present at the receiver at the same time as the (much weaker) secondary field, so the secondary must be extracted from a large primary background (in-phase/quadrature decomposition), which limits dynamic range and depth of investigation compared to time-domain systems, and rigid-boom systems have a fixed, generally shallow-to-moderate depth of investigation set by their coil separation; discrete operating frequencies are also more vulnerable to being swamped by cultural EM noise (power lines, buried metal) at or near those specific frequencies.
Advantages: because the transmitter is switched off before the receiver measures, there is no primary field to separate out, giving much greater dynamic range, deeper depth of investigation for a comparable transmitter moment, and a full decay curve sampled across many time gates that maps resistivity with depth in a single sounding (early gates = shallow, late gates = deep) without changing geometry. Disadvantages: equipment (large transmitter loops, higher power, more complex synchronized recording) is bulkier and more logistically demanding to move and deploy per station; the transmitter turn-off ramp and receiver "dead time" prevent measurement of the earliest time gates, so very shallow structure is poorly resolved; per-station acquisition is generally slower than a continuously walked FDEM profile; and near-surface superparamagnetic soils/laterite can produce a negative apparent-conductivity artifact that has no FDEM counterpart and must be recognized and corrected for.
