18-Geol-A7 Applied Geophysics · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2016 — 04-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; no 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 an all-essay paper with no numeric data, formula sheet or figure supplied. All ten questions are answered below.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (survey design, seismic reflection, well logging, gamma-ray spectrometry, electrical/EM methods, EM systems, data enhancement, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, data display, case-history context; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field forward/inverse modelling theory (Q9); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging context (Q3); Freeze & Cherry, Groundwater — hydrogeophysics context (Q10).
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 the natural radioactivity from potassium-40 and the uranium and thorium decay series, resolved into separate K, eU (equivalent uranium) and eTh (equivalent thorium) energy windows, and because the signal originates only in the top few tens of centimetres of rock or soil, it is fundamentally a surface/near-surface mapping tool rather than a depth-sounding one.
Application 1 — regional geological/lithological mapping and uranium exploration. K, U and Th abundances vary systematically with rock type (felsic igneous rocks are generally K- and Th-enriched relative to mafic/ultramafic rocks; certain alteration assemblages and pegmatites are U-enriched), so a spectrometer survey maps lithological boundaries and alteration halos directly, especially under thin cover where outcrop mapping is difficult, and is the primary reconnaissance tool for uranium exploration. Survey specification: airborne (fixed-wing for large regional reconnaissance, helicopter for rugged terrain), line spacing 100–250 m, terrain clearance 60–120 m (a compromise between spatial resolution and safe/efficient flying), a large-volume NaI crystal detector pack (hundreds of litres) to achieve adequate counting statistics at typical aircraft speed.
Application 2 — engineering/environmental soil and regolith characterization. The K/Th ratio and total count are diagnostic of soil clay content and weathering degree (clay minerals concentrate K and Th relative to quartz-rich sand), which is directly relevant to geotechnical siting (identifying expansive-clay-prone ground) and to radon-hazard screening (elevated eU indicates elevated radon-source potential in underlying soil/rock, relevant to residential foundation design). Survey specification: ground carborne or hand-carried survey with much tighter station spacing (5–25 m) than an airborne reconnaissance survey, since the engineering decision (foundation design, radon mitigation) is made at the scale of an individual site or lot rather than a regional block.
Processing. Raw count rates require: a dead-time correction for detector saturation at high count rate; Compton-scatter (spectral) stripping, because each of the K/U/Th energy windows receives contamination from higher-energy photons of the other two isotopes, corrected using empirically determined stripping ratios; background subtraction (cosmic and aircraft/equipment background, measured over water or a lead-shielded test); and, for airborne data, an altitude (attenuation) correction, since gamma flux falls off roughly exponentially with source-to-detector distance through air. Ground surveys additionally need a moisture correction, since soil water attenuates the gamma flux and can significantly suppress readings after rainfall.
Interpretation. The corrected K%, eU ppm and eTh ppm grids are typically displayed as a ternary (red-green-blue) K-U-Th image, in which each rock type or alteration zone produces a characteristic colour, and as individual element and ratio maps (U/Th, Th/K) used to distinguish primary lithological signal from secondary alteration or weathering effects.
