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.
Every geophysical method exploits a contrast between a target's physical property and that of its host, so a method is only useful where that specific contrast genuinely exists. The table below summarizes the diagnostic property, a representative crustal background value, and a typical anomalous value for the target class most often sought with each method.
| Method | Property | Typical background | Typical anomalous value |
|---|---|---|---|
| Gravity | Density $\rho$ | continental crust $\approx 2.67\text{ g/cm}^3$ | massive sulfide $\approx 4.0$–$4.8\text{ g/cm}^3$; salt/karst void $\approx 2.2\text{ g/cm}^3$ or less |
| Magnetics | Magnetic susceptibility $\kappa$ (+ remanence) | sedimentary rock $\kappa \approx 0$–$0.001$ SI | magnetite-rich iron ore/mafic intrusion $\kappa \approx 0.1$–$1$ SI |
| Electrical resistivity | Resistivity $\rho_e$ | sedimentary sequence $\approx 10$–$1000\ \Omega\cdot\text{m}$ | massive sulfide $<1\ \Omega\cdot\text{m}$; saline groundwater $<10\ \Omega\cdot\text{m}$ |
| Induced polarization | Chargeability $M$ | background $<10\text{ mV/V}$ | disseminated sulfide $\approx 10$–$50+\text{ mV/V}$ |
| Electromagnetics | Conductivity $\sigma$ (=$1/\rho_e$) | resistive bedrock $<10\text{ mS/m}$ | massive sulfide/graphite/clay-rich plume $>1000\text{ mS/m}$ |
| Seismic (refl./refr.) | Velocity $V_p,V_s$ (via elastic moduli, density) | dry unconsolidated soil $\approx 300$–$800\text{ m/s}$ | competent bedrock $\approx 3000$–$6000\text{ m/s}$; water table jump to $\approx 1500\text{ m/s}$ |
| Gamma-ray spectrometry | K, eU, eTh radioelement abundance | average crust K$\approx1$–$2.5\%$, eU$\approx1$–$3\text{ ppm}$, eTh$\approx6$–$12\text{ ppm}$ | K-alteration zone or U mineralization several times background |
| GPR | Dielectric permittivity $\varepsilon_r$ | dry sand/concrete $\varepsilon_r\approx3$–$6$ | water-saturated material or void $\varepsilon_r\approx25$–$80$ (water) |
Two patterns recur across every row of this table. First, the "anomalous" value is only meaningful relative to its own local background — a resistivity of $50\ \Omega\cdot\text{m}$ is a strong conductive anomaly in a $1000\ \Omega\cdot\text{m}$ granite terrain but unremarkable in a $10\ \Omega\cdot\text{m}$ marine clay sequence, so a survey must first establish the local background before an anomaly can be recognized as such. Second, several methods share the same underlying property family (electrical resistivity and EM conductivity are reciprocals of one another; gravity and magnetics both sense a bulk rock property that varies with mineralogy) which is precisely why joint interpretation of two or more methods so often resolves an ambiguity that no single method can resolve alone.
