18-Geol-A7 Applied Geophysics · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-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. All ten questions are answered below so the set stands as a complete study resource.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (density/rock physics, seismic refraction, magnetotellurics, resistivity, induced polarization, magnetics, data enhancement, well logging, EM systems, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, array geometry, data display; Simpson & Bahr, Practical Magnetotellurics — MT acquisition/processing (Q3); Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field forward/inverse modelling (Q6, Q10); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q8).
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.
Well logs measure physical properties of the rock immediately surrounding a borehole as a continuous depth profile, giving far higher vertical resolution than any surface geophysical method, at the cost of only sampling the rock directly along the well path.
Gamma-ray log. A scintillation or Geiger–Müller detector passively records natural gamma radiation from the formation (chiefly from K, U and Th isotopes concentrated in clay minerals) as the tool is lowered/raised through the hole; no source is required, so the tool can run inside cased or uncased holes and even through drill pipe. It is sensitive to clay/shale content, since shales are far more radioactive than clean sands, carbonates or evaporites. Application: distinguishing shale (high gamma-ray count) from clean reservoir sandstone (low count) to pick the top/base of a reservoir sand and correlate stratigraphy between wells — the single most universally run log in petroleum and groundwater well completion.
Resistivity (induction or laterolog) log. An induction log transmits an AC current through a transmitter coil, inducing eddy currents in the formation that are sensed by a receiver coil, giving a measurement proportional to formation conductivity (best in resistive, oil-based mud or air-filled holes); a laterolog instead forces a focused current directly into the formation through electrodes and measures the resulting potential (best in conductive, water-based mud). Both are sensitive to pore-fluid resistivity, porosity, and especially fluid type (via Archie's law, since hydrocarbon and fresh water are far more resistive than saline formation water). Application: identifying a hydrocarbon-bearing zone (anomalously high resistivity relative to the same porosity/lithology zone filled with saline formation water) — the primary reservoir-fluid-typing log in oil and gas exploration.
Sonic (acoustic) log. A piezoelectric transmitter emits an acoustic pulse; one or more receivers at fixed spacing up-hole record the first-arrival travel time of the compressional (and often shear) wave through the formation, giving interval transit time $\Delta t$ (μs/ft or μs/m), the reciprocal of P-wave velocity. It is sensitive to porosity (via the Wyllie time-average relation, $\Delta t=\Delta t_{ma}(1-\phi)+\Delta t_{fl}\phi$) and to lithology/cementation. Application: computing a porosity log independent of the resistivity/fluid-type logs, and generating a synthetic seismogram for tying well data to surface seismic reflection surveys — the sonic-derived acoustic impedance log is the direct link between borehole and surface seismic interpretation.
