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.
A single target type — a porphyry copper system is used here — is pursued with a deliberately different survey at each successive stage of a project, each one narrower in area, finer in resolution, and more expensive per unit area than the one before it, because the questions being asked narrow at every stage.
Stage 1 — Regional reconnaissance (grassroots exploration, 10s–100s of km²). An airborne magnetic and radiometric survey flown on widely spaced lines (e.g. 200–400 m line spacing, 60–80 m terrain clearance) maps regional structure, alteration-related magnetite destruction (a magnetic low core within a magnetic high halo is a classic porphyry signature) and K-alteration (elevated potassic gamma-ray response) over the whole permit area at low cost per km². Interpretation at this stage is qualitative/pattern-based: analysts look for the characteristic donut-shaped magnetic low and coincident K-anomaly that flags a porphyry centre worth following up, without attempting quantitative depth or geometry estimates.
Stage 2 — Property-scale target definition (1–10 km²). Once a prospective centre is identified, a much tighter ground induced-polarization/resistivity survey (e.g. 25–50 m station spacing, 100 m line spacing, dipole-dipole array) is run over the specific anomaly, because IP chargeability directly detects the disseminated sulfide mineralization (pyrite ± chalcopyrite) typical of a porphyry's mineralized envelope, which magnetics alone cannot distinguish from barren magnetite destruction. Interpretation becomes quantitative: 2-D/3-D IP-resistivity inversion produces a chargeability/resistivity model used to rank and prioritize specific drill targets within the property.
Stage 3 — Resource delineation and borehole geophysics (10s–100s of m around individual drill holes). Once drilling begins, downhole logging (gamma, IP, resistivity) and cross-hole methods (Q3) refine the 3-D geometry and grade continuity of the mineralized zone directly around and between holes, at metre-scale resolution far finer than any surface method, feeding directly into resource estimation and mine planning.
The common thread is that each stage answers a progressively narrower question — "where might something be?" (regional), "what specifically is it, and where exactly?" (property-scale), "what is its precise 3-D geometry and grade?" (borehole) — and the survey design (platform, spacing, method choice) at each stage is deliberately matched to the scale of the question being asked, not simply scaled down uniformly from the previous stage.
