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.
Self-potential (SP) is a passive method measuring naturally occurring DC electric potentials at surface, generated here by streaming potential — the electrokinetic voltage created when groundwater flows through a porous/fractured medium, dragging the mobile part of the electrical double layer on grain surfaces along with it. An earth-fill dam or levee with an active internal seepage path (through a poorly compacted zone, a crack, or piping along a conduit) produces exactly this condition: water flowing from the reservoir side, through the embankment, and exiting on the downstream face or toe drives a measurable SP anomaly, making SP a fast, non-invasive first screen for suspected seepage before committing to piezometers or a dye trace.
A gradient (roving dipole) or fixed-base array is appropriate: one non-polarizing porous-pot electrode (Cu–CuSO4) is planted as a fixed base station well away from the suspected seepage zone, and a second, roving electrode is moved along profile lines on the downstream face and crest, with potential measured relative to the fixed base at closely spaced stations (metre-scale). Profiles are laid out perpendicular to and along the dam crest/toe to bracket the seepage exit point in both directions, and repeat readings/loop closures are used to correct for slow electrode drift.
Streaming-potential theory predicts a negative SP anomaly centred over the point where seepage water exits the embankment (relative to the far-field base station), because the streaming current flows in the direction of water flow and the resulting external return current makes the downstream exit point electrically negative relative to background. The data are interpreted by contouring the anomaly map to locate its minimum (the inferred exit/discharge point), by correlating anomaly amplitude and gradient qualitatively with flow rate and near-surface flow-path geometry, and, where warranted, by fitting the profile to a simple point- or line-current-source model to estimate the approximate depth of the flow path — always cross-checked against piezometric or visual (wet-spot, seep) evidence rather than relied on alone, since SP amplitude also depends on subsurface resistivity structure, not flow rate alone.
