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.
Induced magnetization Mi = κH is proportional to, and aligned with, the ambient inducing field — the present-day Earth's field — and exists only while that field is applied; it disappears (to first order) if the field were removed. Remanent magnetization is magnetization the rock retains independently of the present field, acquired at some point in the rock's history: thermoremanent magnetization locked in as the rock cooled through its Curie temperature in whatever field existed at that time, detrital remanent magnetization acquired as magnetic grains aligned themselves while settling during sediment deposition, or chemical remanent magnetization acquired during a later mineral-growth/alteration event. Because it was frozen in at a past time, remanent magnetization can point in a direction quite different from today's local field — reflecting an older field direction, a different paleolatitude, or even a reversed polarity.
The relative importance of the two is expressed by the Koenigsberger ratio Q = Mr/Mi, the ratio of remanent to induced magnetization intensity. Q < 1 means induced magnetization dominates and the total magnetization is reasonably well approximated by susceptibility × the present field alone; Q > 1 (common in many volcanic and mafic/ultramafic rocks, sometimes Q ≫ 1) means remanence dominates, and the body's TOTAL magnetization vector M = Mi + Mr can differ substantially in both magnitude and direction from what susceptibility alone would predict.
Standard forward and inverse modelling of magnetic anomalies most often assumes purely induced magnetization aligned with the present Earth field, using measured or estimated susceptibility as the only input — a reasonable simplification when Q is small, but a significant source of error when it is not, because a remanence-dominated body's anomaly can be offset from, or even opposite in polarity to, what the same body's susceptibility alone would produce. When remanence is known or suspected to be significant, rigorous interpretation must use the full magnetization vector, with the remanent component's direction and intensity determined independently — by paleomagnetic measurement on oriented drill core or hand samples — or else carried as additional unknowns in the inversion, which increases the number of free parameters and worsens the non-uniqueness already inherent in potential-field inversion (Question 10). In practice, many expedient surveys still assume Q ≈ 0 because remanence is difficult to measure in situ, which is a recognised and sometimes serious source of misinterpretation, particularly for iron-oxide, sulphide and volcanic-terrane targets.
