05-Geol-B10 · December 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-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 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, uniqueness/equivalent sources ch.5, Fourier-domain filters ch.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.
Timescale: years to decades to centuries (and, over millions of years, complete polarity reversals). Physics: convective motion of the electrically conducting molten-iron outer core, sustained by the geodynamo, slowly redistributes the electric currents that generate the main field, so the field's spherical-harmonic (Gauss) coefficients drift with time. Impact on surveys: it changes the reference main field (IGRF/DGRF, Question 4) that must be subtracted from every survey, so a model evaluated for the wrong epoch mis-corrects the data, particularly important when comparing or merging surveys flown years apart. Mitigation: always subtract the IGRF/DGRF evaluated at the actual survey date, and re-tie repeat/4D surveys to the current-epoch model rather than reusing an old one.
Timescale: a smooth cycle over roughly 24 hours, amplitude of order tens of nT. Physics: solar UV heating drives tidal winds in the electrically conductive ionospheric E-region; those winds moving through the main field generate ionospheric dynamo currents whose external field is felt at the surface, strongest near local noon and near the equator, and modulated by season and the solar (11-year sunspot) cycle. Impact on surveys: superimposes a slow, smoothly time-varying drift on every station reading across a survey day, which if uncorrected looks like a broad spurious regional trend across the grid. Mitigation: continuous base-station recording (or, with a single instrument, the repeat-station/loop method of Question 3) with the interpolated diurnal curve subtracted from every reading.
Timescale: storms last hours to a day or two, sub-storms tens of minutes, and micropulsations seconds; all are irregular rather than smooth. Physics: enhanced solar wind (often from a coronal mass ejection) compresses and couples energy into the magnetosphere, injecting energetic particles into the ring current and driving intense, spatially non-uniform auroral electrojet currents at high latitude — a fundamentally different, far more violent process than the quiet-day ionospheric dynamo. Impact on surveys: storm-time field changes can be large, rapid and non-linear, invalidating the linear-drift assumption used for diurnal correction and potentially swamping genuine local anomalies; a repeat-station loop closed across a storm shows a residual that is not a straight line, betraying the problem. Mitigation: monitor space-weather indices (Kp, Dst) before and during the survey, suspend or reject data acquired during active/storm periods, and where possible use a continuously recording base magnetometer so the actual (non-linear) storm-time curve, not a linear approximation, is what gets removed.
