18-Geol-A7 Applied Geophysics · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2016 — 04-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; no 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 an all-essay paper with no numeric data, formula sheet or figure supplied. All ten questions are answered below.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (survey design, seismic reflection, well logging, gamma-ray spectrometry, electrical/EM methods, EM systems, data enhancement, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, data display, case-history context; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field forward/inverse modelling theory (Q9); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging context (Q3); Freeze & Cherry, Groundwater — hydrogeophysics context (Q10).
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 radiation emitted by potassium-40 and the uranium and thorium decay series in the formation. How: a passive scintillation (NaI) or Geiger-counter detector records photon count rate as the tool moves through the hole. Property contrast: shale and clay minerals concentrate K, U and Th far more than clean sand, carbonate or evaporite, so the log responds primarily to clay/shale content. Example application: correlating stratigraphy between two nearby boreholes drilled for a groundwater or geotechnical investigation, where the gamma log reliably identifies and traces a distinctive clay aquitard layer between holes even through casing, when no core samples are available for direct comparison.
Electrical resistivity log (induction or laterolog). Measures: the bulk electrical resistivity of the formation immediately around the borehole. How: an induction tool induces AC eddy currents from a transmitter coil and measures the secondary field at a receiver coil (works best in resistive, e.g. oil-based, mud); a laterolog instead injects focused current directly into the formation through electrodes (works best in conductive, water-based mud). Property contrast: resistivity is controlled mainly by pore-water salinity and saturation — a hydrocarbon- or freshwater-saturated interval is far more resistive than the same rock saturated with saline formation water. Example application: in a groundwater investigation, distinguishing a freshwater sand aquifer (high resistivity) from an underlying brackish or saline-intruded zone (low resistivity), which is essential for correctly siting a water-supply well screen above the salt/fresh interface.
Sonic (acoustic) log. Measures: the compressional-wave travel time (interval transit time, µs/ft or µs/m) through the formation. How: a transmitter emits an acoustic pulse and two or more receivers at fixed spacing time its first arrival. Property contrast: travel time responds to porosity and degree of consolidation/cementation (via the elastic moduli discussed in Q2) — more porous or less consolidated rock has slower velocity and longer transit time. Example application: in a geotechnical borehole investigation for a tunnel alignment, distinguishing competent, low-porosity rock (fast, short transit time, good for tunnelling) from a weathered, fractured or higher-porosity interval (slow, long transit time, requiring additional ground support), and integrating the sonic log with a surface seismic reflection survey via a synthetic seismogram to extend that rock-quality assessment away from the borehole itself.
