18-Geol-A7 Applied Geophysics · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — Applied Geophysics (18-Geol-A7), undated filing. Three-hour, closed-book exam; an approved calculator is 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 with diagrams as appropriate — this is a genuinely all-essay sitting with no numeric data table or figure supplied. 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 (gravity, magnetics, electrical/EM, seismic reflection/refraction, well logging, gamma-ray spectrometry); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, instrumentation, data reduction and case-history context; Blakely, Potential Theory in Gravity and Magnetic Applications — gravity/magnetic instrumentation and correction theory (Q2, Q5); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging context (Q3, Q9).
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.
Physical property. Gamma-ray spectrometry senses the natural gamma radiation emitted by the radioactive decay of potassium-40 (K-40, 1.46 MeV gamma line), and the decay-series daughters of uranium-238 (via bismuth-214, 1.76 MeV line, used to infer "equivalent uranium," eU) and thorium-232 (via thallium-208, 2.62 MeV line, "equivalent thorium," eTh). The three characteristic energy windows let a spectrometer estimate the concentration of each of the three radioelements (K in %, eU and eTh in ppm) separately, rather than only a bulk total count.
Limitations. Gamma rays are strongly attenuated by matter, so the method senses only the TOP few tens of centimetres of rock or soil (an effective sampling depth of roughly 30–45 cm in typical rock) — any cover of soil, vegetation, snow or water essentially masks whatever lies beneath, and even modest overburden thickness renders the underlying bedrock signal undetectable. The signal is also weak and requires a slow, low, careful survey to accumulate adequate counting statistics, and radon gas (a mobile decay product in the uranium series) can migrate and locally disturb the apparent uranium channel independent of the source rock's true U content.
Best times and places. Data are best collected in dry conditions (moisture strongly attenuates the signal and is itself a confounding variable), with no snow cover, over exposed or thinly covered ground, and avoiding periods/locations of high radon emanation (e.g. immediately after heavy rain, when radon can be flushed unpredictably) or where recent precipitation has left the ground surface wet.
Correction and calibration procedures. Raw count rates must be corrected for: (i) COSMIC background (a fixed high-energy contribution measured at altitude with the detector shielded, subtracted from every reading), (ii) AIRCRAFT/INSTRUMENT background (radioactivity of the aircraft or sensor housing itself, measured over water or a known-zero test area), (iii) RADON background in airborne surveys (measured via an upward-looking detector that senses only atmospheric radon, then subtracted from the downward-looking channel), (iv) ALTITUDE (height-attenuation) correction, since count rate falls off with increasing sensor-to-ground distance, and (v) COMPTON SCATTERING "stripping" between the three energy windows, since higher-energy gamma rays scatter down into lower-energy windows and contaminate them, corrected using stripping ratios determined on calibration pads. Finally, the whole system is CALIBRATED against a set of concrete calibration pads (or a designated calibration range) of independently known K, eU and eTh concentration, which converts corrected count rates into true radioelement concentrations.
