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.
The IP waveform is a symmetric bipolar square wave: current is switched fully on for a period, then fully off for an equal (or specified) period, then switched on in reverse polarity, and off again — the polarity reversal cancels electrode self-potential and telluric drift. During each off-time, the slowly decaying secondary voltage caused by polarizable minerals/clays discharging is measured (chargeability is the time-integral of this decay curve normalized by the on-time primary voltage). The TDEM waveform is also bipolar but uses a rapid turn-off ramp (ideally a step, in practice a fast linear or exponential ramp of a few to tens of microseconds) rather than a square edge: the changing current during turn-off induces eddy currents in the ground by Faraday's law, and it is the resulting secondary magnetic field's decay (dB/dt, measured in a receiver coil across many discrete time gates spanning early to late time) that is recorded once the primary field has vanished — not a voltage decay from chemical polarization.
Because the underlying physics of TDEM depends on how fast the primary field is removed, different manufacturers' transmitters use different turn-off ramp shapes and durations (e.g. some systems favour a very fast linear ramp for the best near-surface/shallow resolution, while others use a longer or differently shaped ramp that trades some shallow resolution for lower transmitter self-noise or larger moment at depth), and receiver time-gate spacing (logarithmic gate widths, earliest usable gate) also differs between systems. This matters because the turn-off ramp length sets the earliest reliable time gate, and the earliest time gate sets the shallowest depth the system can resolve — a slower ramp smears/loses the very-early-time response that carries information about shallow, thin or resistive-cover targets, and it also means that raw sounding curves from two different TDEM systems are not directly comparable without deconvolving each system's own transmitter waveform first.
