18-Geol-B10 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-B10-1 Gravity and Magnetics Fields, 2019-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, drift and tidal correction ch.2; magnetometers, gradiometers and magnetic surveying ch.4–5; anomaly enhancement and interpretation throughout); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (survey design, temporal-variation correction, case-history applications ch.6 & 7); Blakely, Potential Theory in Gravity and Magnetic Applications (potential-field theory, derivative and Fourier-domain filters, regional-residual separation 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.
| Variation | Typical time scale | Physical cause |
|---|---|---|
| Micropulsations | Fractions of a second to several minutes | Resonant ULF (ultra-low-frequency) hydromagnetic waves in the magnetosphere, driven by solar-wind/magnetosphere interaction; amplitudes typically a few nT, larger and more frequent during geomagnetic disturbance |
| Diurnal (solar-quiet, Sq) variation and magnetic storms | Hours to about a day (diurnal); storms last hours to a few days | Solar-UV-driven ionospheric current systems in the E-region that strengthen/shift through the day (Sq); enhanced solar wind and coronal mass ejections compressing the magnetosphere and driving ring-current/auroral-electrojet currents (storms) |
| Secular variation | Years to centuries (and, over millions of years, full polarity reversals) | Slow changes in the pattern of fluid convection/dynamo action in the Earth's liquid outer core, which generates the main field |
All three variation types add a TIME-varying component to a magnetic reading that is indistinguishable, at the moment of measurement, from a real spatial (geological) change, unless it is separately identified and removed. Diurnal variation and storms are the most disruptive for a single survey day, since the field can drift by tens to hundreds of nT over the hours it takes to complete a grid, comparable to or larger than many geological target anomalies. Micropulsations add short-period noise/scatter to individual readings. Secular variation matters less within a single survey but is significant when COMPARING surveys flown or walked years apart over the same ground (time-lapse monitoring), or when reducing to a common reference field.
Diurnal variation and storms are removed using a continuously-recording BASE-STATION magnetometer (subtracted, time-synchronized, from every station reading) or, with a single roving instrument, by periodically re-occupying a tie station and linearly interpolating the drift between ties — and by suspending surveying altogether during an active magnetic storm, since the underlying linear-interpolation assumption breaks down when the field is changing rapidly and irregularly. Micropulsation noise is reduced by stacking/averaging multiple readings at each station and, for gradiometer surveys, is substantially self-cancelling because both sensors see nearly the same pulsation signal simultaneously (Question 5). Secular variation is corrected for by referencing readings to the International Geomagnetic Reference Field (IGRF) evaluated at the correct epoch (survey date), which is exactly why every quoted magnetic reduction or declination value must state the date it applies to.
