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.
Petroleum exploration and development is arguably the industry that has driven the most investment in, and refinement of, applied geophysics, because the target — a trap containing hydrocarbon in porous, permeable rock, typically kilometres below surface — cannot be seen or directly sampled cheaply, and the cost of drilling a dry well is enormous. Geophysics is used at every stage from regional reconnaissance to reservoir monitoring.
Gravity and magnetics are the cheapest, fastest reconnaissance tools and are typically flown first over a new basin: gravity maps regional basement depth and structure (a sedimentary basin is a broad low relative to denser crystalline basement) and directly detects salt bodies (salt is markedly less dense than surrounding clastic sediments, producing a strong gravity low useful for locating salt-related traps); magnetics maps the depth to magnetic basement (since sedimentary cover is essentially non-magnetic) and helps define basin geometry and major structural trends (faults, basement highs) that control where traps might form.
Seismic reflection is the dominant tool for actually finding and delineating a prospect: 2-D and, since the 1990s, 3-D surveys directly image stratigraphic and structural traps (anticlines, fault traps, stratigraphic pinch-outs, salt-flank traps, reef buildups) with the resolution needed to plan a well; amplitude-versus-offset (AVO) analysis of the reflection data exploits the fact that a gas- or oil-saturated reservoir has a markedly different elastic response with offset/incidence angle than a brine-saturated one of otherwise identical lithology, giving a direct (though not infallible) hydrocarbon indicator; time-lapse ("4-D") repeat 3-D surveys over a producing field track the moving fluid contacts during production and guide infill drilling.
Well logging (see also Q8) is the essential calibration link between the surface seismic image and the actual rock/fluid properties: sonic and density logs from a well are combined into a synthetic seismogram that ties specific seismic reflections to specific formation tops, resistivity and porosity logs quantify hydrocarbon saturation at the well, and vertical seismic profiling (a source at surface, receivers in the borehole) refines the velocity model used to convert seismic time sections to depth.
Electromagnetic methods, particularly marine controlled-source EM (CSEM), have become an important complement to seismic AVO in offshore exploration: a resistive hydrocarbon-saturated reservoir produces a detectable EM anomaly that is largely independent of the seismic amplitude response, so a coincident seismic amplitude anomaly AND a CSEM resistivity anomaly together substantially reduce the risk of drilling a "false" seismic bright spot that is not actually hydrocarbon-charged.
The overarching importance of geophysics in this industry is RISK REDUCTION: each method contributes an independent line of evidence (structure from gravity/magnetics/seismic, direct hydrocarbon indication from AVO/CSEM, calibration from well logs), and integrating them before committing to an expensive well is what makes deep-water and frontier-basin exploration economically viable at all.
