18-Geol-A7 Applied Geophysics · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 18-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; approved Casio or Sharp 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 a genuinely all-essay sitting with no numeric data, formula sheet, or figure supplied in the source. 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 (physical properties, gravity, magnetics, electrical/EM methods, seismic refraction/reflection, radiometrics, well logging, magnetotellurics); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design, array geometry, data acquisition, processing and display; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field survey design and reduction (Q2); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q7).
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.
Acquisition. A line of geophones is laid out at regular spacing along the survey line, with source (shot) points at both ends and, for longer spreads, at intermediate positions as well, so that every geophone position is recorded from at least two opposing shot directions ("forward" and "reverse" shooting) — essential for detecting a dipping refractor, which produces a different apparent velocity when shot updip versus downdip. Geophone spacing is chosen relative to the target depth (closer spacing for shallower, higher-resolution targets); maximum source–receiver offset must be large enough (typically 3–5× the depth of interest) that the head wave has overtaken the direct wave (i.e. offsets well beyond the crossover distance) at the far end of the spread.
Processing. First-arrival ("first break") times are picked at every geophone for every shot; these are plotted as travel-time-versus-offset curves for each shot. Straight-line segments are fitted to the direct-wave and head-wave branches, and their reciprocal slopes give apparent velocities for each layer; because the true refractor dip and true layer velocities require reconciling both forward- and reverse-shot travel-time curves together, a reciprocity check (the total travel time from one end shot to the other must be identical to that from the other end shot back) validates the picks, and a method such as the generalized reciprocal method (GRM) or the delay-time method is applied to handle an irregular (non-planar) refractor and produce a proper depth section rather than a single flat-layer estimate.
Interpretation. The resulting velocity–depth model is interpreted geologically: a low-velocity layer over a higher-velocity layer is read as soil/overburden over bedrock (or weathered over unweathered rock, or unsaturated over saturated ground, since water saturation raises $V_p$ substantially); the depth-to-refractor profile along the line becomes a depth-to-bedrock or depth-to-water-table section, and the refractor velocity itself is often used, via published velocity–rippability charts, to estimate whether the rock can be ripped by mechanical excavation or requires blasting.
Case example. A geotechnical site investigation for a proposed highway cut or building foundation, where boreholes are sparse and expensive: a refraction line run along the alignment between a handful of boreholes gives a continuous depth-to-bedrock profile across the whole line rather than only at the discrete borehole locations, at a small fraction of the cost of closely spaced drilling, while the borehole logs independently calibrate the refractor velocity to the actual rock type encountered — a combination that is both faster and cheaper than drilling alone, and more geologically reliable than refraction alone.
