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.
Planning begins with the exploration target: its expected depth, size and susceptibility contrast set the required line spacing (typically ≤ 1–2 times the target depth so at least two or three lines cross the anomaly) and station spacing along line (fine enough to define the anomaly's half-width, commonly 5–25 m for near-surface targets). Lines are oriented perpendicular to the expected strike of the target/geological trend so anomalies are crossed, not run along. A base/tie-line network is laid out at right angles to the main lines to allow levelling of line-to-line mismatches. The site is checked for magnetic "cultural" noise — fences, buildings, vehicles, buried pipes, power lines, even the surveyor's own steel-toed boots or field gear — and stations/lines are routed to avoid or at least document proximity to these sources. A base station location is chosen in a magnetically quiet area accessible throughout the day.
Execution follows the planned grid, reading total field (or gradient) at each station with the sensor held at a fixed height above ground, recording station coordinates/elevation, and logging time at each reading (essential for the temporal correction below). Field crews watch for magnetic storms (rapid, large, irregular field changes) that invalidate a linear diurnal assumption and may need to suspend surveying.
The Earth's field varies with time of day (solar-quiet diurnal variation, a smooth curve of order tens of nT over a day) and, less predictably, during magnetic storms. With only one instrument, the standard strategy is the loop (repeat-station) method: the crew periodically returns to reoccupy a previously read station (or a fixed base station) every 15–30 minutes as they proceed along the survey lines, closing short loops rather than working continuously outward. The assumption made is that the diurnal field varies smoothly and can be treated as linear in time between successive ties at the same point (valid on a quiet day; not valid through a magnetic storm, which the loop closures themselves will reveal as a repeat reading that is anomalously far from a straight-line trend).
Reduction proceeds in the following order: (1) plot each tie station's repeated readings against time and fit a linear (or smooth) drift curve between consecutive ties; (2) interpolate that curve to every station's own observation time and subtract it from every raw reading, removing the diurnal/instrument-drift component; (3) subtract the International Geomagnetic Reference Field (IGRF) evaluated for the survey date and location, removing the regional/core-field contribution and leaving the local anomaly of interest; (4) level any residual line-to-line mismatch using the tie-line network (least-squares or manual levelling); (5) grid and contour the levelled, corrected anomaly for interpretation. No elevation/terrain correction analogous to gravity's is normally needed for magnetics, since the magnetic field of the ground itself (as opposed to buried anomalous bodies) is negligible.
