18-Geol-A7 Applied Geophysics · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; approved Casio or Sharp calculator permitted. The paper offers a choice of six of the following ten questions, each worth 16.66% of the total mark, and every question requires an essay-format answer — this is a genuinely all-essay sitting with no numeric data, formula sheet, or figure supplied. All ten questions are answered below so the set stands as a complete study resource.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (density/rock physics, seismic refraction, magnetotellurics, resistivity, induced polarization, magnetics, data enhancement, well logging, EM systems, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, array geometry, data display; Simpson & Bahr, Practical Magnetotellurics — MT acquisition/processing (Q3); Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field forward/inverse modelling (Q6, Q10); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q8).
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.
Chargeability describes a rock or soil's capacity to store and slowly release electrical charge when an applied current is switched off, rather than the resistivity property (which describes how easily current flows while it is on) — it is the basis of the induced polarization (IP) method.
Membrane polarization. In fine-grained, clay-bearing material, the negatively-charged clay-mineral surfaces attract a diffuse cloud of cations in the adjacent pore fluid; when current flows through a narrow pore throat, this ion cloud partially blocks (acts as a semi-permeable "membrane" to) the flow of anions relative to cations, building up an ionic concentration gradient across the constriction. When current is switched off, this gradient dissipates diffusively, releasing a slowly-decaying voltage — the IP effect. Membrane polarization dominates in clay-rich, fine-grained, disseminated-mineral-free material.
Electrode polarization. At the surface of an electronically-conductive mineral grain (metallic sulphides, graphite, some oxides) sitting in an ionically-conductive pore fluid, applied current must locally change carrier type from ionic to electronic and back, requiring an electrochemical reaction at the mineral/fluid interface. This reaction has its own reaction kinetics/overpotential, so charge briefly accumulates at the grain boundary while current flows and then discharges when current stops. Electrode polarization is typically a much stronger effect than membrane polarization and is the basis for using IP to directly target disseminated sulphide mineralization, which is often too low-grade or too finely disseminated to produce a strong resistivity anomaly on its own.
Time-domain measurement. Using a standard four-electrode array and a switched-current transmitter/receiver, the decaying secondary voltage $V_s(t)$ is measured over one or more time windows after current shut-off, referenced against the steady-state primary voltage $V_p$ just before shut-off. The quantity reported is chargeability $M$, conventionally computed as the time-integral of the decay curve normalized by $V_p$: $M=\frac{1}{V_p}\int_{t_1}^{t_2}V_s(t)\,dt$, with units of milliseconds (ms) — the equipment is essentially the same transmitter/receiver used for resistivity, with the receiver additionally sampling several time gates after each current pulse.
Frequency-domain measurement. Apparent resistivity is measured at two (or more) different AC frequencies (typically a low frequency ∼0.1–1 Hz and a high frequency ∼1–10 Hz), since polarizable material shows higher apparent resistivity at low frequency than at high frequency (at higher frequency the polarization has less time to build up and impede the current). The reported quantity is the (dimensionless) percent frequency effect, $PFE=100\times\dfrac{\rho_{a,low}-\rho_{a,high}}{\rho_{a,high}}$ (a related quantity, the metal factor $MF=2\pi\times10^{5}(\rho_{a,low}-\rho_{a,high})/(\rho_{a,low}\rho_{a,high})$, normalizes the dispersion by resistivity and so is not dimensionless); since PFE is a ratio it carries no physical unit and is simply expressed as a percentage computed directly from the two measured apparent-resistivity values.
Case history. IP chargeability surveying is the standard exploration tool for porphyry copper deposits, where disseminated, low-grade chalcopyrite/pyrite mineralization is too fine-grained and too low in bulk metal content to produce a strong resistivity contrast, but the sheer abundance of small, electronically-conductive sulphide grains produces a strong, broad chargeability high that closely outlines the mineralized envelope — IP chargeability maps are routinely used to define porphyry drill targets that resistivity or magnetics alone would miss or under-represent.
