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.
The physical property that is usually most important for the magnetic method is magnetic susceptibility, κ (dimensionless in SI), which measures how strongly a material becomes magnetized by INDUCTION in the Earth's ambient field. Susceptibility is controlled almost entirely by the abundance of ferrimagnetic accessory minerals — principally magnetite and, to a lesser degree, pyrrhotite — in a rock or buried object, so it is the property surveyed for whenever a target is expected to differ from its host in magnetite content (an intrusion, a fault-hosted alteration zone, a buried steel object).
In the field or lab, susceptibility is most commonly measured with a hand-held susceptibility meter (e.g. a Kappameter/Kappabridge-type instrument such as the KT-9 or a Bartington MS2 with a surface-scan sensor). The sensor coil generates a small, low-intensity oscillating magnetic field and measures the change in the coil's inductance (or the secondary field) produced when the sample or outcrop surface is placed against it; that change is directly proportional to the sample's bulk susceptibility, and the instrument reads κ (in SI or cgs units) essentially instantaneously with no sample preparation.
The technique is fast, non-destructive, portable and needs no external field beyond the instrument's own small excitation coil, so hundreds of outcrop or core readings can be logged in a single field day, directly informing survey design and rock-property compilations. Its principal weaknesses are: (1) the reading only samples a small volume immediately at the sensor face, so it may not represent the bulk susceptibility of a heterogeneous unit at the anomaly's true measurement scale; (2) a fresh, unweathered surface is needed, since surface oxidation/weathering commonly destroys magnetite and lowers the apparent susceptibility relative to the unweathered rock at depth; and (3) the meter measures only the INDUCED response and is blind to any REMANENT magnetization the sample may carry, which can be a large, even dominant, part of a rock's total magnetization (see below).
| Material | Type | Typical susceptibility κ (SI) |
|---|---|---|
| Limestone / quartz sandstone | Rock (sedimentary) | 0 to 3×10-4 |
| Basalt / gabbro | Rock (mafic igneous) | 1×10-3 to 1×10-1 |
| Buried steel drum / pipeline (ferrous) | Buried material | κ effectively very large (100–102, saturates the meter) |
| Unexploded ordnance (UXO) / cast-iron debris | Buried material | κ very large, similar order to ferrous scrap |
Beyond induced susceptibility, the OTHER physical property that is sometimes important — and can dominate — is remanent magnetization (natural remanent magnetization, NRM): magnetization a rock retains independently of, and not necessarily aligned with, the present-day inducing field, acquired when the rock cooled through the Curie temperature (thermoremanence, e.g. basalts) or during deposition/diagenesis. The relative importance of remanent versus induced magnetization is expressed by the Koenigsberger ratio Q = (remanent intensity)/(induced intensity); when Q > 1 the anomaly's shape and even sign are controlled by the rock's magnetic history rather than by the present field direction, which is why a full magnetic characterization of a rock unit ideally measures both κ and NRM, not susceptibility alone.
