04-Geol-B10 · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B10-1 Gravity and Magnetic Fields, 2016-May. 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 and gravity reduction ch.2; magnetometers and magnetic surveying ch.4–5; forward/inverse modelling throughout); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (survey design, data processing and interpretation workflow ch.6 & 7); Blakely, Potential Theory in Gravity and Magnetic Applications (potential-field theory, Fourier-domain filters, reduction-to-pole ch.2, 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.
Regional interpretation follows a staged, increasingly quantitative workflow. (1) Compilation and QC: assemble all available gravity/magnetic surveys covering the region, check line levelling and datum consistency between surveys flown/measured at different times, and remove the IGRF (magnetics) or apply standard gravity reductions (free-air, Bouguer, terrain) so all data are on a common, comparable basis. (2) Gridding: interpolate the levelled point/line data onto a regular grid (cell size roughly a quarter to half the station spacing) using an appropriate method (minimum curvature is standard) so wavenumber-domain filters can be applied. (3) Regional–residual separation: separate the long-wavelength regional field (deep crustal/basement structure) from the shorter-wavelength residual (nearer-surface targets) using upward continuation or a trend surface, since the two are usually interpreted separately. (4) Qualitative interpretation: map anomaly domains, trends and lineaments, and correlate them with mapped geology, structural trends and known mineralization/basin margins to build a first-pass geological framework. (5) Semi-quantitative interpretation: apply enhancement filters (Question 7) and automated depth/location estimators such as Euler deconvolution (which solves for source depth and position from the field and its gradients using an assumed structural index) to rapidly scan the whole grid for anomaly sources and their approximate depths without needing a full model. (6) Quantitative forward/inverse modelling: build detailed 2D, 2.75D or 3D models of selected key anomalies or the whole region (see Question 10) constrained by everything known independently. (7) Integration and iteration: fold the modelling results back into the regional geological interpretation, refine the model as new constraints arrive, and use the result to rank/target areas for follow-up.
Surface geological mapping and structural data (bedding, fold axes, fault traces) provide the geometric framework a model should honour; drill-hole logs and downhole density/susceptibility logs give direct, local ground-truth on the physical properties and depths used in modelling; seismic reflection/refraction sections constrain basement depth and major structural boundaries independently of potential-field data; other geophysical layers (electromagnetic, radiometric, resistivity) help discriminate conductor type and near-surface geology; and regional geochronology/geochemistry helps interpret what a given magnetic or density domain actually represents geologically.
Simple forward modelling of individual, well-isolated anomalies is appropriate early, at the reconnaissance stage, to test whether a proposed geological hypothesis (e.g. "is this anomaly consistent with a dipping mafic dyke at this depth?") is even physically plausible, using whatever independent constraints (outcrop, previous drilling) already exist to fix as many parameters as possible. Full regional 3D inversion is more appropriate later, once enough independent constraints (drilling, seismic, detailed mapping) exist to meaningfully regularize the otherwise non-unique inverse problem (Question 10) and once the target has been narrowed down enough that the computational cost of a large 3D inversion is justified — typically at the advanced-exploration or resource-definition stage rather than during initial regional reconnaissance.
