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.
Type 1 — Wireline (downhole) logging. Purpose. A logging suite (natural gamma, resistivity/induction, sonic, density-neutron) is run down an existing borehole to build a continuous, high-resolution physical-property profile of the rock adjacent to the hole for lithological correlation, formation evaluation (porosity, saturation) or geotechnical rock-quality assessment. Specifications. The tool string is lowered on a wireline at a controlled logging speed (typically a few metres per minute, slow enough that the tool's sample rate resolves thin beds) and each sensor records continuously versus depth, referenced to a calibrated depth-wheel measurement at the surface. Corrections. Borehole-diameter (caliper) correction where the hole is washed out or under-gauge, mud-type correction for the resistivity/induction tool (an induction tool is unreliable in highly conductive water-based mud; a laterolog is unreliable in resistive oil-based mud), and a temperature/pressure correction for logs run at depth. Processing/interpretation. Logs from multiple tools are depth-matched and cross-plotted (e.g. a neutron-density crossplot for lithology and porosity, a resistivity-porosity crossplot via Archie's equation for water saturation) and correlated bed-by-bed between adjacent boreholes to build a stratigraphic/structural cross-section.
Type 2 — Cross-hole seismic tomography. Purpose. A seismic source is fired in one borehole while a string of receivers records in one or more adjacent boreholes, imaging the velocity structure of the rock volume BETWEEN the holes — used, for example, to detect a weak or fractured zone, a void, or a contaminant plume between two geotechnical boreholes at a dam or tunnel site, at a resolution far finer than any surface method can achieve at that depth. Specifications. Source and receiver depths are stepped systematically through the full interval of interest in both holes so that many ray paths cross the target volume from different angles (a dense ray-path fan is essential for a well-constrained tomographic inversion); receiver spacing and source-receiver offset are chosen relative to the target's expected size, following the same Nyquist-type sampling logic used in surface surveys. Corrections. Borehole deviation surveys (the holes are rarely perfectly vertical) to obtain the true 3-D position of every source and receiver, and a static time correction for the fluid/casing the wave must cross near each borehole wall. Processing/interpretation. First-arrival travel times for every source-receiver ray path are picked and inverted (typically via a damped least-squares or SIRT-type tomographic algorithm) for a 2-D (or 3-D, with additional hole pairs) velocity model of the inter-borehole volume, which is then interpreted directly in terms of rock quality, fracturing or saturation using the same velocity-property relationships that govern surface seismic refraction/reflection.
