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.
1. Helicopter time-domain EM (e.g. VTEM-style system). A large, rigid transmitter loop is towed beneath the helicopter on a rigid frame ("bird"), with the receiver coil mounted concentrically at the loop's centre; the transmitter is pulsed on and off and the decaying secondary-field response is recorded during the off-time, at a series of time gates from early (shallow) to late (deep). The whole bird is flown at low, constant height (~30–40 m) along parallel survey lines spaced tens to a few hundred metres apart. Target and rationale: a large, moderately-to-strongly conductive massive sulphide body at depths up to several hundred metres beneath cover, in rugged or swampy terrain inaccessible to ground crews — the large transmitter moment and multi-time-gate late-time response give both good depth penetration and rapid reconnaissance-scale coverage that a ground system simply cannot match over difficult terrain or a large claim block.
2. Ground frequency-domain horizontal-loop EM (Slingram-type, e.g. Geonics EM-31/EM-34). A small transmitter coil and receiver coil are held at a fixed separation (coplanar or coaxial) and carried by two operators (or on a rigid boom for the smallest instruments), continuously energized at one or more fixed frequencies; in-phase and quadrature components of the secondary field are read as the pair is walked along closely spaced traverse lines. Target and rationale: shallow, near-surface conductivity mapping — soil/groundwater contamination plumes, shallow clay/overburden thickness, or UXO/utility detection — where the target is within a few tens of metres of surface; the small, portable, continuously-reading ground system achieves far tighter station spacing and shallow resolution, at far lower cost, than an airborne system designed for deep, large-target reconnaissance.
3. Borehole (downhole) EM. A large transmitter loop is laid out on the surface around the collar of a drill hole (or, in the "in-hole" variant, a small transmitter is lowered into the hole), while a small three-component receiver probe is lowered down the hole on a wireline and logged at successive depth stations, recording the secondary field's amplitude and direction at each depth. Target and rationale: a conductor lying close to, but not intersected by, an existing drill hole that has already been collared for another purpose — downhole EM is specifically used as a follow-up tool after drilling has "just missed" a conductive target identified by surface EM or another method, because the receiver's proximity inside the hole gives it far higher sensitivity and resolution to nearby off-hole conductors than any surface-based system could achieve at that same depth.
