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.
A magnetotelluric (MT) station passively records the Earth's own naturally occurring, time-varying electromagnetic field (driven by ionospheric/magnetospheric currents at long periods and worldwide lightning/Schumann resonance activity at short periods) and does not transmit any signal of its own. The instrument suite comprises: (1) two orthogonal electric-field dipoles, typically 50–100 m lines of wire with non-polarizing Ag–AgCl (or Pb–PbCl2) porous-pot electrodes buried at each end, measuring the horizontal electric field Ex and Ey; (2) three orthogonal magnetic-field sensors — induction coil magnetometers (sensitive at higher frequencies) or fluxgate magnetometers (better at very low frequencies), buried and levelled to record Hx, Hy and often the vertical Hz; (3) a data logger with a precise GPS-disciplined clock (critical for later cross-correlating multiple stations and for remote-reference processing) and sufficient dynamic range/sampling rate to capture the very broad frequency band MT exploits (roughly 10-4 Hz to 104 Hz, spanning near-surface to lower-crust/upper-mantle depths); and (4) a battery/solar power supply for unattended deployment, often for many hours to days per station.
Raw time-series from each station are converted to frequency-domain apparent resistivity and phase curves (via the magnetotelluric impedance tensor Z, where E = Z·H) using robust spectral/cross-power processing codes (e.g. Egbert-style robust remote-reference processing, or vendor packages such as those bundled with Phoenix Geophysics MTU systems), ideally using a simultaneously recording remote reference station away from local cultural noise to statistically reject uncorrelated noise from the impedance estimate. The processed sounding curves (apparent resistivity and phase vs. period, plus tensor invariants such as skew and the phase-tensor ellipse that diagnose 1-D/2-D/3-D dimensionality) are then inverted with 1-D, 2-D (e.g. non-linear conjugate-gradient or Occam-style codes) or 3-D (e.g. ModEM) inversion software to recover a subsurface resistivity model, commonly visualized and interpreted in packages such as WinGLink or similar MT-specific modelling/interpretation suites.
