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.
All resistivity arrays use two current electrodes (C1, C2 or A, B) to inject current and two potential electrodes (P1, P2 or M, N) to measure the resulting voltage; they differ only in the relative spacing and geometry of these four electrodes, which trades off depth of investigation, resolution, signal strength and field speed.
Four electrodes are equally spaced at interval a (C1–P1–P2–C2). Advantages: the geometric factor is the simplest of any array (2πa), giving the best signal-to-noise ratio for a given current and spacing, straightforward field data reduction, and good vertical resolution for 1-D soundings. Disadvantages: to change depth of investigation ALL FOUR electrodes must be moved and re-planted at every reading, which is slow and labour-intensive; the array is also relatively sensitive to near-surface lateral inhomogeneity beneath the (moving) potential electrodes, which can distort a sounding curve that is meant to reflect only vertical layering.
The potential pair P1P2 is kept close together and fixed while the current pair is progressively opened outward (L ≫ l = P1P2 spacing). Advantages: far fewer electrode moves per sounding (only the outer current electrodes are relocated; the potential pair may be left in place for several readings), so soundings are faster to run and the fixed potential pair reduces sensitivity to lateral near-surface noise between readings. Disadvantages: the potential difference measured becomes very small at large AB/2, demanding a more sensitive voltmeter or higher injected current, and the potential electrodes must occasionally be moved when the signal becomes too weak, which reintroduces a lateral-consistency assumption at that point in the sounding.
A closely spaced current dipole (C1C2, separation a) and a closely spaced potential dipole (P1P2, also separation a) are kept apart by a variable separation na (n = 1, 2, 3…). Advantages: because both dipoles can be moved independently along a line (or grid), the array is very efficient for building 2-D pseudosections and 3-D volumes for lateral mapping, and its bipolar sensitivity pattern gives good resolution of steeply dipping structures (dykes, faults, veins). Disadvantages: the geometric factor grows rapidly with n, so the measured voltage becomes very weak at large separations (poor signal-to-noise, requiring higher transmitted power), and the array is comparatively more susceptible to telluric and instrument noise than Wenner at equivalent depth.
