Question 1 of 10: Magnetic Susceptibility and Remanence

Question 1: Magnetic Susceptibility and Remanence (16.66% of paper)

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.

Magnetic susceptibility ($\kappa$) is the constant of proportionality between an applied magnetic field and the magnetization it induces in a material:

$$\kappa=\dfrac{M}{H}$$

where $M$ is the induced magnetization (A/m) and $H$ is the applied field strength (A/m); in SI units $\kappa$ is dimensionless, and the older CGS/emu convention differs by a factor of $4\pi$ (SI values are always $4\pi$ times the equivalent CGS values), which is a frequent source of unit-conversion error when comparing older and modern published tables.

Typical values. Materials fall into three broad classes. Diamagnetic minerals (quartz, calcite, feldspar) have very small, negative susceptibility ($\kappa\approx-1\times10^{-5}$ SI) — they weakly oppose the applied field. Paramagnetic minerals (most silicates, biotite, pyroxene, amphibole) have small, positive susceptibility ($\kappa\approx10^{-4}$–$10^{-3}$ SI). Ferromagnetic (strictly ferrimagnetic) minerals, chiefly magnetite and to a lesser extent pyrrhotite, dominate the magnetic response of most rocks even at low modal abundance, with $\kappa$ ranging from about $10^{-3}$ up to several SI units for magnetite-rich material. For whole rocks, this translates to: sedimentary rocks (limestone, sandstone, shale) essentially non-magnetic, $\kappa\approx0$–$10^{-3}$ SI, controlled almost entirely by their trace magnetite/heavy-mineral content; granite intermediate, $\kappa\approx10^{-4}$–$10^{-2}$ SI; mafic igneous rocks (basalt, gabbro, diabase) strongly magnetic, $\kappa\approx10^{-2}$–$10^{-1}$ SI or higher, because they crystallize with abundant primary magnetite; and massive magnetite/banded-iron-formation ore can exceed 1 SI. For geotechnical/engineering studies, this contrast is what makes magnetics useful for mapping buried mafic dykes, iron/steel infrastructure (utilities, drums, USTs, unexploded ordnance — all essentially pure ferromagnetic targets with $\kappa$ orders of magnitude above any natural rock), and for distinguishing fill/waste (often containing scrap metal, $\kappa$ elevated and erratic) from undisturbed native soil.

Magnetic remanence. Remanent magnetization ($J_r$, also written $M_r$) is the permanent magnetization a rock retains in the absence of any applied field, locked in at the time the rock formed — thermoremanent magnetization (TRM) as an igneous rock cools through the Curie temperature of its magnetic minerals in the presence of the Earth's field, detrital remanent magnetization (DRM) as magnetic grains settle and align in sediment, or chemical remanent magnetization (CRM) as new magnetic minerals grow during diagenesis/alteration. Unlike induced magnetization, which is proportional to and parallel with the present-day field, remanence can point in a completely different direction (recording the field direction at the time of formation, which may differ from today's due to polar wander or rock rotation/faulting) and its magnitude is independent of the present field strength.

Measurement and quantification. Susceptibility is measured directly and rapidly with a hand-held susceptibility meter (an AC induction-bridge instrument) on outcrop, core or drill cuttings, or derived from a downhole susceptibility log. Remanence requires an oriented sample (its in-situ orientation must be recorded before removal) measured on a spinner magnetometer or a cryogenic SQUID magnetometer inside a magnetically shielded room, to isolate the sample's own permanent field from the ambient (much larger) Earth's field. The two properties are compared via the Koenigsberger ratio

$$Q_n=\dfrac{J_r}{\kappa H}$$

the ratio of remanent to induced magnetization in the present-day field $H$. $Q_n<1$ means the total (induced + remanent) magnetization is dominated by induced magnetization, so a magnetic anomaly's shape closely follows what a susceptibility-only model would predict; $Q_n>1$ (common in strongly-TRM'd basalts and some massive sulphides) means remanence dominates, and the anomaly's amplitude, sign and offset from the causative body can differ substantially from a susceptibility-only forward model — a critical distinction for correctly interpreting a magnetic anomaly over remanence-dominated terrain.