18-Geol-A7 Applied Geophysics · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 18-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 (physical properties, gravity, magnetics, electrical/EM methods, seismic refraction/reflection, radiometrics, well logging, magnetotellurics); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design, array geometry, data acquisition, processing and display; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field survey design and reduction (Q2); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q7).
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 measures natural gamma radiation from the decay of potassium-40 and the uranium-238 and thorium-232 decay series, resolved into separate K, U (via the 1.76 MeV Bi-214 line) and Th (via the 2.62 MeV Tl-208 line) energy channels, and is flown airborne, driven on the ground, or carried on foot.
Strengths. It is fast and inexpensive to acquire large areas of coverage, particularly airborne, where thousands of line-kilometres can be flown per day; it is entirely passive and non-invasive (no source deployed, no ground contact required for airborne surveys); it directly maps lithology and alteration, since K, U and Th abundances and ratios are diagnostic of rock type (e.g. potassic alteration halos around porphyry deposits show up as strong K highs, and Th/K or U/K ratio maps discriminate rock units that look identical on other geophysical data); it is one of the very few methods that directly measures a compositional (geochemical) rather than purely physical property, so its results correlate well with surface geological mapping; and it has important applications well beyond mineral exploration, including uranium exploration, environmental/radioactive-contamination monitoring, and soil/regolith mapping for agricultural and engineering purposes.
Weaknesses. Gamma rays are heavily attenuated by rock and soil, so the method senses only the top few tens of centimetres of material — it has essentially zero depth penetration and is completely blind to anything below a thin surface cover (even a metre of soil, vegetation litter, snow or standing water can mask the underlying bedrock signature entirely); readings are strongly affected by moisture content (wet ground attenuates and scatters gamma rays, so surveys are avoided after rain and results must be moisture-corrected); results require careful calibration against known-activity calibration pads and stripping-ratio corrections to separate the overlapping K/U/Th spectral windows, and airborne data need cosmic-ray, aircraft-background and radon-in-air corrections; and the uranium channel in particular is vulnerable to radioactive disequilibrium, so an apparent uranium reading (equivalent uranium, eU) can significantly over- or under-represent the true present-day uranium content if the decay chain has been recently disturbed by weathering or groundwater leaching.
