04-Geol-B10 · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B10-1 Gravity and Magnetic Fields, 2017-Dec. Closed book; no calculator permitted. All ten questions require an answer in essay format, with diagrams used wherever appropriate. The exam instructs "choose six (6) of the following ten (10) questions, the first six as they appear in the answer book will be marked, each of equal value".
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed. (physical properties ch.2 & 5; gravimeters, gravity reduction and terrain correction ch.2; magnetometers and magnetic surveying ch.4–5; anomaly interpretation throughout); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (survey design, diurnal correction, case-history applications ch.6 & 7); Blakely, Potential Theory in Gravity and Magnetic Applications (potential-field theory, Fourier-domain filters, reduction-to-pole, non-uniqueness ch.2, 5, 9 & 12).
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.
A representative case history is a Phase II environmental site assessment at a former fuel-distribution/service-station property in advance of redevelopment, where historical records suggested one or more steel underground storage tanks (USTs) and associated buried piping remained in the ground at unknown locations, posing both a contamination risk and a geotechnical hazard (a shallow void or collapse risk if a tank had corroded and partially collapsed).
Steel tanks and piping have very high magnetic susceptibility and produce strong, easily detectable dipolar anomalies even when small and shallow, so a magnetic (or magnetic-gradiometer) survey is fast, low-cost, and directly sensitive to the ferrous target itself, unlike ground-penetrating radar, which performs poorly in the clay-rich or saturated soils typical of many such sites (attenuating the radar signal before it can reach and return from a buried tank), and unlike electrical/EM methods, which respond more broadly to any conductive feature (buried utilities, soil moisture, contamination plumes) and so are less specific to a steel target. Magnetics was therefore chosen as the primary, rapid reconnaissance tool, with excavation reserved for confirmation of anomalies found.
A fluxgate gradiometer (measuring the vertical gradient of the total field between two vertically separated sensors) was walked on a tight grid, roughly 0.5–1 m line and station spacing, over the suspected tank-yard area, with station positions logged by RTK GPS. Using the VERTICAL GRADIENT rather than the total field suppresses the smooth regional/diurnal field automatically (it largely cancels between the two closely spaced sensors) and enhances the response of shallow, compact, near-surface sources such as tanks, at the cost of reduced sensitivity to deeper or more diffuse targets.
The gridded gradient data were levelled, interpolated onto a regular grid, and contoured; tight, high-amplitude dipolar (or, for the vertical gradient, simpler bipolar) anomalies with a compact, roughly circular or elongated footprint consistent with a tank's dimensions were flagged as candidate targets, distinguished from the more diffuse, linear anomalies typical of buried pipe runs or fence lines. Approximate depth to each candidate source was estimated from the anomaly's half-width (a rule-of-thumb relationship between anomaly width and source depth) to help plan excavation depth, and the strongest, most tank-like anomalies were prioritized for confirmatory test-pitting.
Magnetics cannot by itself DISTINGUISH a tank from other compact ferrous debris (rebar fragments, scrap metal, an old fence post, a manhole cover), so the survey generated a number of false-positive candidates that required excavation to rule out, adding cost and time; nearby infrastructure (a chain-link fence, rebar in an adjacent building foundation, overhead powerlines) also produced strong local anomalies that partly masked or were confused with genuine targets. If repeated, the survey could be improved by adding a complementary EM61-type time-domain metal detector (which responds more specifically to compact conductive metal and less to weakly magnetic soil/fill) or GPR for independent confirmation, using finer station spacing in the areas of highest anomaly density to better resolve closely spaced targets, and applying Euler deconvolution to the gradient data to obtain semi-automated depth and location estimates for every candidate anomaly before prioritizing excavation.
