18-Geol-A7 Applied Geophysics · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 04-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; no calculator permitted. The NOTES state that SIX questions constitute a complete paper (the first six as they appear in the answer book), but the printed paper offers a choice of six of the following nine questions, and every question requires an essay-format answer with no numeric data, formula sheet or figure supplied — this is an all-essay paper. All nine questions are answered below.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (gravity, magnetics, seismic reflection/refraction, resistivity, IP, EM, radiometrics, well logging); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, data display and case-history context; Blakely, Potential Theory in Gravity and Magnetic Applications — magnetic anomaly shape and reduction-to-pole theory (Q6); Simpson & Bahr, Practical Magnetotellurics (Q3); Selley & Sonnenberg, Elements of Petroleum Geology (Q4, Q8 well-logging context).
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.
Similarities – wave propagation fundamentals. Both methods generate elastic (P- and/or S-) waves at a surface source and record the arrivals with an array of geophones; both obey the same wave equation and Snell's law, $\sin\theta_1/V_1=\sin\theta_2/V_2$, at every velocity interface; both require the near-surface (weathering layer) to be characterized, since its low, variable velocity distorts travel times for deeper targets in both methods; and both are non-invasive, repeatable and yield a velocity model that is diagnostic of lithology, saturation and rock quality.
Differences – acquisition. A reflection survey uses closely spaced sources and receivers (metres to tens of metres) with SHORT-to-moderate offsets, because the target is imaged directly beneath the source-receiver midpoint at near-normal incidence; each subsurface point is illuminated by many different source-receiver pairs (the common-midpoint principle), which is deliberately exploited for redundancy and noise cancellation. A refraction survey instead uses WIDE offsets, often several times the target depth, because a head wave only overtakes the direct wave (becomes the first arrival) beyond its crossover distance; refraction lines are commonly shot in both directions (a "reversed" or "forward-reverse" profile) so that dip on the refracting interface can be resolved, which reflection profiling does not require in the same way.
Differences – processing. Reflection processing centres on preserving and enhancing WAVEFORM: static corrections for near-surface delays, normal-moveout (NMO) correction using the stacking velocity, common-midpoint (CMP) stacking to boost signal-to-noise, deconvolution to sharpen the wavelet, and migration to collapse diffractions and place dipping/faulted reflectors in their true position. Refraction processing works instead from TRAVEL TIMES ONLY (first-break picks): intercept-time and delay-time methods, or the more robust generalized reciprocal method (GRM), invert the picked times directly for layer velocities and depths, and the raw waveform shape after the first break is rarely used.
Differences – interpretation. Reflection surveys resolve detailed STRUCTURAL geometry — bedding, faults, unconformities — at essentially any depth the source energy can reach, and resolution is largely independent of depth (governed by frequency/wavelength), but the method gives comparatively coarse absolute velocity information (stacking velocity is an approximation) and needs a reasonably well-behaved (non-chaotic) velocity structure to migrate correctly. Refraction surveys give a direct, robust VELOCITY-DEPTH model that is easy to relate to rock quality/rippability/water table, but resolution degrades sharply with depth (each layer must be thick enough, and each velocity must be faster than the layer above, or the layer is "hidden" — the velocity-inversion and thin-layer blind zones), and refraction alone struggles to resolve structure that is not a simple layered sequence.
