18-Geol-A7 Applied Geophysics · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 04-Geol-A7 Applied Geophysics. Three-hour, open-book exam; any non-communicating calculator permitted. The NOTES state that SIX questions constitute a complete paper, but eight numbered questions are printed on the exam — all eight, and every lettered/numbered sub-part, are solved below. Two figures (the CMP gather of Q6 and the reversed-refraction time-distance plot of Q8) carry real numeric data that is only given graphically on the printed page; both were read from the printed figures, calibrated against each figure's own printed axes, and the reading tolerance is given in a check callout beside each calculation.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method in this paper (gravity, magnetics, seismic refraction and reflection, electrical resistivity, induced polarization, electromagnetics); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design and interpretation context; Blakely, Potential Theory in Gravity and Magnetic Applications — the dipole/sphere anomaly shapes used in Q2–Q3.
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.
(i) Two polarization (charge-storage) mechanisms. Electrode (metallic-mineral) polarization occurs where an electronically-conducting mineral grain (disseminated sulphides such as pyrite, chalcopyrite, galena, or graphite) blocks a pore channel that otherwise carries current ionically through the pore fluid. Current arriving at the grain must change from ionic to electronic conduction; ions of one sign pile up (accumulate) at the near face of the grain while the opposite-sign ions are depleted, building a local electrochemical double layer across the grain — a genuine charge-separation "capacitor" at the grain scale. When the driving current is switched off, these accumulated ions slowly diffuse back to equilibrium, discharging a measurable decaying voltage. Geologically this is the classic disseminated-sulphide signature (porphyry Cu-Mo deposits, massive sulphide halos, graphitic shear zones), and it is the dominant, strong IP effect exploration geophysicists specifically target when looking for disseminated ore. Membrane (electrolytic) polarization instead requires NO metallic mineral at all: it occurs in clay-rich zones, where the negatively-charged clay mineral surfaces attract a cloud of mobile cations (the electrical double layer of ordinary electrochemistry) that partially obstructs the pore throat. An applied current preferentially pushes cations through and anions are impeded (or vice versa depending on the local charge selectivity), building up local ion concentration gradients on either side of the constriction that likewise relax and discharge a small voltage when current is removed. Geologically this is associated with clay alteration, shale/clay-rich sediments, and weathered zones; it produces a much WEAKER IP response than electrode polarization, but its utility is that a distinct membrane-polarization anomaly can map clay alteration haloes (itself an indirect exploration guide, e.g. around a porphyry system) or must be recognized and distinguished from a genuine sulphide response so exploration effort is not wasted following a clay signature.
(ii) Time-domain and frequency-domain IP measurement. In the time-domain method, a DC current is applied for a fixed period and then abruptly switched OFF; the (much larger) primary voltage collapses almost instantly, but the polarized ground continues to discharge a small, slowly-decaying secondary voltage $V(t)$ for a second or more afterward. Chargeability $M$ quantifies this decay, most commonly as the (dimensionless, though often reported in mV/V or ms) integral of the decay voltage over a specified time window normalized by the primary voltage, $M=\dfrac{1}{V_p}\displaystyle\int_{t_1}^{t_2}V(t)\,dt$ (units of milliseconds when defined this way, since $V(t)/V_p$ is dimensionless and the integral is over time) — a larger, more sustained decay voltage (relative to the primary) means a larger chargeability and a stronger polarizable response. In the frequency-domain method, the SAME apparent resistivity is measured twice, using a low frequency (near-DC, $\rho_{aDC}$) and a much higher frequency ($\rho_{a,\text{high freq}}$; practical pairs are about 0.05–0.1 Hz for the low frequency and 1–10 Hz for the high one); a polarizable ground shows measurably HIGHER apparent resistivity at low frequency than at high frequency, because at low frequency the polarization "capacitor" has time to charge fully and oppose the current, whereas at high frequency the charge has no time to build up and the current passes more easily. This is quantified by the Frequency Effect, $FE=\dfrac{\rho_{aDC}-\rho_{a,\text{high freq}}}{\rho_{a,\text{high freq}}}$ (dimensionless, often reported as a Percent Frequency Effect, $PFE=100\times FE$, in %), and the related Metal Factor, $MF=\dfrac{2\pi\times10^5\times FE}{\rho_{aDC}}$ (in SI-ish mho/m units, the $2\pi\times10^5$ scaling factor being conventional so MF values come out as convenient round numbers), which normalizes the frequency effect by the DC resistivity itself so that a highly conductive, weakly-polarizable rock and a weakly-conductive, strongly-polarizable rock (which could otherwise give similar raw FE values) are separated — MF is specifically designed to emphasize metallic-mineral (electrode-polarization) responses over background conductivity variations.
