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18-Geol-A7 Applied Geophysics · December 2018

Question 7 of 10: Gravity Surveying — Instrumentation, Data Reduction, and a Case History

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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).

Check: page 1's NOTES list is numbered 1–5 with a genuine duplicate — two distinct instructions are both numbered "5." (5. Each question should take about half an hour. / 5. All questions require an answer in essay format…).

Question 7: Gravity Surveying — Instrumentation, Data Reduction, and a Case History (16.66% of paper)

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.

Instrument. Applied-geophysics gravity surveys are almost universally acquired with a relative spring gravimeter (e.g. a Scintrex CG-5/CG-6 or the classic LaCoste & Romberg meter). Inside, a small test mass is suspended from a "zero-length" spring; small differences in gravitational pull from one station to the next displace the mass, and a precision feedback/nulling mechanism measures the force needed to restore it to a reference position, converting that force into a gravity-difference reading with microgal-level precision. Because it measures only relative differences between stations, the survey must always be tied back to at least one absolute or previously-established base station.

Data reduction and processing. Raw readings must pass through a sequence of corrections before they can be interpreted geologically. Drift correction removes the gravimeter's own slow instrumental drift, using repeat readings at a base station over the survey day. Tidal correction removes the small, predictable gravitational pull of the sun and moon, computed from the survey's time and location. Latitude correction removes the systematic increase of normal (theoretical) gravity from the equator to the poles, computed from the International Gravity Formula. Free-air correction accounts for the change in gravity with elevation above the reference datum (essentially $1/r^2$ with distance from the Earth's centre), independent of what material lies between the station and the datum. Bouguer (slab) correction then accounts for the gravitational attraction of the rock actually present between the station and the datum, modelled as an infinite horizontal slab of an assumed reduction density. Terrain correction refines this further by accounting for the real, non-slab topography (nearby hills and valleys) surrounding the station. Applying all of these in sequence to the raw observed gravity yields the Bouguer Anomaly, the quantity actually interpreted for subsurface density variation.

Case history. Gravity is a standard tool for regional and detailed exploration where the target has a density contrast with its host: salt-dome delineation in hydrocarbon exploration (salt is markedly less dense than the surrounding sediment, producing a distinctive negative Bouguer anomaly used for decades to locate salt-associated hydrocarbon traps), kimberlite-pipe exploration for diamonds (a kimberlite pipe's brecciated infill is typically less dense than the host craton rock, again giving a negative anomaly), and, geotechnically, the detection of subsurface voids or karst/sinkhole cavities beneath a proposed foundation or roadway, where the missing rock mass of a void produces a small but detectable negative "micro-gravity" anomaly at the surface directly above it.