18-Geol-A7 Applied Geophysics · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 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 (electrical/EM methods, seismic refraction/reflection, radiometrics, magnetics, gravity, well logging); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design, array geometry, data acquisition and processing; Blakely, Potential Theory in Gravity and Magnetic Applications — magnetic-mineral behaviour and gravity reduction (Q5, Q7); 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.
Natural gamma-ray log. Measures the natural gamma radioactivity of the formation immediately surrounding the borehole, using a scintillation (or gamma-ray) detector lowered down the hole; radioactivity arises mainly from potassium, uranium and thorium hosted in clay minerals, so the log is sensitive to clay/shale content — shale reads high, clean sandstone or limestone reads low. It works through casing (gamma rays penetrate steel casing and cement) and requires no borehole fluid, unlike most electrical tools. Application: distinguishing shale (impermeable, low reservoir quality) from clean sand or carbonate reservoir intervals, and correlating stratigraphy between wells.
Electrical resistivity log. Measures the formation's electrical resistivity by injecting current from electrodes on a sonde and measuring the resulting potential at other electrodes at fixed spacing (a normal or lateral device), or, in a laterolog, focusing the current electrically to control depth of investigation and minimize borehole-fluid effects; an induction tool instead uses coils to induce current loops in the formation, and is used in non-conductive (oil-based or air-filled) boreholes. Resistivity is highly sensitive to pore-fluid content and salinity (via Archie's law) and to porosity, and is lowered by clay/shale (surface conduction), but otherwise responds to lithology mainly through its control on porosity — a high-porosity, saline-water-saturated sand reads very low resistivity, while the same sand saturated with hydrocarbon (a poor conductor) reads high. Application: distinguishing a hydrocarbon-bearing reservoir zone from a water-bearing one, and estimating water saturation for reserve calculations.
Sonic (acoustic) log. Measures the travel time of an acoustic pulse, transmitted from a source on the sonde and received at two or more receivers at fixed spacing further up the tool, expressed as interval transit time $\Delta t$ (the reciprocal of P-wave velocity). It is sensitive primarily to porosity (a more porous, less consolidated rock has a lower velocity/higher $\Delta t$) and to lithology/degree of cementation, but is comparatively insensitive to pore-fluid type compared with resistivity. Application: computing a porosity log (via a time-average or empirical transform) for reservoir evaluation, and generating a synthetic seismogram to tie well control directly to a surface seismic-reflection survey.
