18-Geol-A7 Applied Geophysics · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 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 in the source. All ten questions are answered below so the set stands as a complete study resource for choose-N-of-M exams.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (magnetics, seismic reflection, radiometrics, downhole resistivity, EM/IP, filtering, well logging, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, array geometry, data display; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field filtering and forward/inverse modelling (Q7, 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.
Gamma-ray spectrometry passively measures the natural gamma radiation emitted by potassium-40 (K), and by the bismuth-214 and thallium-208 daughters of the uranium-238 and thorium-232 decay series (U and Th, reported as "equivalent" eU/eTh since the parent isotopes are not measured directly), from the top roughly 30–45 cm of rock or soil — gamma photons attenuate rapidly with depth, so the method is exclusively a surface/near-surface tool.
Application 1 — regional lithological and alteration mapping. Different rock types carry characteristic K/U/Th signatures (felsic rocks generally K- and Th-rich, mafic rocks depleted in all three, evaporites K-rich), and hydrothermal alteration commonly produces a diagnostic radioelement halo (e.g. potassic alteration around a porphyry copper deposit elevates K, while propylitic/argillic alteration can deplete it) — making the method a fast, low-cost reconnaissance mapping and vectoring tool in porphyry and epithermal exploration. Survey specification: fixed-wing or helicopter airborne platform, 100–250 m line spacing for exploration-scale reconnaissance, flown as low as safely practicable (60–100 m AGL, since count rate falls off rapidly with height) with tie lines every 5–10 flight lines for levelling.
Application 2 — uranium exploration. Direct detection of anomalous eU, cross-checked against the eU/eTh and eU/K ratios (a high eU/eTh ratio helps distinguish primary uranium mineralization from simple Th-rich lithology). Survey specification: the same airborne reconnaissance platform for regional targeting, followed by detailed ground follow-up (hand-held or cart-mounted spectrometer, walking traverses at 10–25 m station spacing or continuous logging) over any airborne anomaly to confirm and tightly locate the source.
Application 3 — geotechnical/environmental overburden and contamination mapping. Distinguishing organic/peat soils (low K/U/Th) from mineral till or bedrock-derived fill, and establishing a pre-development radiological baseline (or monitoring for anomalous radioactive contamination, e.g. from mine tailings or NORM — naturally occurring radioactive material). Survey specification: ground-based, hand-held or vehicle-mounted spectrometer on a tight grid (5–20 m spacing) appropriate to the small footprint of most engineering sites.
Processing and interpretation. Raw counts in the K, U and Th energy windows must first be energy-calibrated and corrected for spectral overlap ("stripping" — Compton-scattered high-energy Th-series photons contaminate the lower-energy U and K windows, and U-series photons contaminate the K window, and the stripping ratios are removed using calibration-pad-derived coefficients), then corrected for aircraft/detector height (radon-in-air background and cosmic-ray background are subtracted, and count rate is height-normalized since, for a broad ground source, it falls off approximately exponentially with height through atmospheric attenuation), and finally converted from counts-per-second to ground concentration units (%K, ppm eU, ppm eTh) using calibration-pad sensitivity factors. The calibrated grids are then interpreted individually and as ratio maps (K/Th, U/Th, U/K) and ternary RGB (K-red, U-green, Th-blue) composite images, which are far more diagnostic of lithology and alteration than any single channel alone, since ratios cancel out common systematic effects (e.g. moisture content) that affect all three channels similarly.
