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.
Planning starts from the exploration target's expected depth, size and susceptibility contrast, which set the required line spacing (typically no more than one to two times target depth, so at least two or three lines cross an anomaly) and along-line station spacing (fine enough to resolve the anomaly's half-width). Lines are oriented perpendicular to the expected geological strike so anomalies are crossed rather than followed. A magnetically quiet base-station site, accessible throughout the survey, is selected, and a reconnaissance is made for magnetic "cultural" noise sources — fences, buildings, vehicles, buried pipelines, powerlines, even the crew's own steel gear — which are avoided or at least logged for later editing.
Execution follows the planned grid, reading total field (or vertical gradient) at each station with the sensor held at a fixed, repeatable height, logging station position, elevation and, critically, the exact time of each reading so temporal corrections can later be applied. If a second, continuously recording base-station magnetometer is available, it logs the field at a fixed point throughout the survey day; if not, the field crew periodically reoccupies ("loops" back to) a previously read station every 15–30 minutes.
The Earth's magnetic field measured at any fixed point is not constant in time. The dominant, smooth daily variation is the solar-quiet (Sq) diurnal variation, of order tens of nT over a day, caused by solar-UV-driven ionospheric current systems in the E-region of the ionosphere that change strength and geometry as the Sun-facing side of the Earth rotates through the day. Superimposed on this are magnetic storms and substorms, larger and far less predictable disturbances (up to hundreds or thousands of nT) caused by enhanced solar wind and coronal mass ejections compressing and disturbing the Earth's magnetosphere, driving strong ring-current and auroral-electrojet currents. Over years, the field also drifts through secular variation, a slow change in the main core-generated field.
Diurnal variation and storms are monitored and removed using either a fixed base-station magnetometer recording continuously throughout the survey (the preferred method: the base record is simply subtracted, time-synchronized, from every roving-station reading) or, with a single instrument, the loop (repeat-station) method, in which periodic reoccupations of the same point let the field crew fit a drift curve (assumed linear between successive ties on a quiet day) and interpolate it to every station's own observation time for subtraction. Secular variation, together with the smooth main-field/regional trend, is removed separately by subtracting the International Geomagnetic Reference Field (IGRF) evaluated for the survey date and location, which isolates the local, near-surface anomaly of geological interest. Any residual line-to-line mismatch after these corrections is removed by levelling against the tie-line network. Crews also watch for magnetic storms in progress (rapid, large, irregular changes inconsistent with a linear diurnal trend) and suspend surveying, since the linear-interpolation assumption underlying both the base-station and loop methods breaks down during a storm.
