Question 2 of 8: Method Selection for Five Applications

Question 2: Method Selection for Five Applications (15 marks)

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) Acid-rock-drainage screening of proposed highway rock cuts. (i) Induced polarization (time-domain chargeability), possibly paired with resistivity. (ii) IP responds to the volume percentage of polarizable sulphide grains essentially independently of whether they form an interconnected (massive) or disseminated, disconnected network — exactly the ARD-generating textures — whereas resistivity alone only clearly flags massive, interconnected sulphide. (iii) IP is slow and relatively expensive per line-km for a linear highway corridor, is sensitive to cultural noise (buried metal, rebar, pipelines — a real risk near existing roads), and cannot distinguish acid-generating sulphides (pyrite, pyrrhotite) from non-acid-generating ones (sphalerite, galena) or from graphite, which gives an equally strong chargeability response.

(b) Reconnaissance for iron ore over a large (50×50 km) area. (i) Airborne magnetics. (ii) Magnetite/hematite-rich iron formations produce some of the largest magnetic-susceptibility contrasts of any rock type, giving a very strong, easily mapped signal, and airborne acquisition covers 2500 km² at reconnaissance line spacing far faster and cheaper per km² than any ground method. (iii) Magnetics maps susceptibility, not grade or tonnage directly, so ore delineation still needs ground follow-up (drilling, possibly gravity for tonnage); strong remanent magnetization in iron formations (see Q1b) can distort anomaly shape/position; and man-made ferrous cultural noise requires careful flight-line/altitude planning and filtering.

(c) Imaging stratigraphy in a 0–40 m sand/gravel deposit. (i) Ground-penetrating radar (GPR), with shallow seismic reflection as a fallback in wetter/more conductive settings. (ii) Dry-to-moist, clean sand and gravel is resistive and has very low electromagnetic attenuation, so GPR achieves centimetre-to-decimetre vertical resolution of bedding, channel fills and the water table over the 0–40 m depth range of interest — far finer resolution than seismic refraction can offer at this depth. (iii) GPR penetration and resolution both degrade sharply if clay or saline pore water is present (high electrical conductivity attenuates the radar signal), and the method requires careful velocity calibration (a coring/logging tie) to convert two-way travel time to true depth.

(d) Structure of a folded/faulted salt layer, 500–1500 m deep, between clastic formations. (i) Seismic reflection. (ii) At these depths seismic reflection is essentially the only method with adequate resolution and penetration; salt has a strong, sharp acoustic-impedance contrast with the enclosing clastics (fast $V_p$, low density) giving a clean top-of-salt reflector, and a multi-fold 2-D or 3-D survey directly images the fold/fault geometry needed for structural mapping. (iii) Salt's very high velocity and its tendency to form complex overhangs/diapirs badly distorts ray paths beneath and around it ("velocity pull-up," poor imaging of sub-salt and steeply-dipping salt flanks), requiring depth migration with an accurate salt-velocity model, and the survey is comparatively expensive and slow to acquire/process versus the potential-field methods.

(e) Fresh groundwater in sand channels buried under clay-rich sediments. (i) Electrical resistivity (vertical electrical sounding and/or resistivity imaging/ERT). (ii) Fresh-water-saturated sand is markedly more resistive than the enclosing clay-rich sediments (clay's high cation-exchange-capacity surface conduction and any residual saline pore water make it strongly conductive), so a buried sand channel shows up as a well-defined resistive body/lens against a conductive host — precisely the geometry VES/ERT was developed to resolve. (iii) Resolution and depth of investigation both degrade with the thickness/conductivity of the overlying clay (a highly conductive overburden "shields" deeper structure), and — as developed in Question 5(b) — the principle of equivalence can leave the sand channel's own resistivity and thickness poorly resolved even once its presence is confirmed.