Question 1 of 10: Physical Properties in Gravity and Magnetic Methods

Question 1: Physical Properties in Gravity and Magnetic Methods (Choose 6 of 10 – equal value)

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.

Gravity method – density

The controlling property for the gravity method is bulk density, ρ (kg/m³ or g/cm³), because the measured quantity — the vertical component of the gravitational attraction of a subsurface mass — is directly proportional to the density of the rock relative to its surroundings. Typical values span a wide range: unconsolidated sediments and soil 1.6–2.0 g/cm³, sedimentary rocks (sandstone, shale, limestone) 1.8–2.7 g/cm³, felsic crystalline rocks (granite) 2.5–2.7 g/cm³, mafic/ultramafic rocks (gabbro, peridotite) 2.7–3.3 g/cm³, and massive sulphide or iron-oxide ore bodies 3.5–5.2 g/cm³. Water is 1.0 g/cm³ and an air-filled void (karst cavity, mine working) is effectively 0, which is why gravity is the tool of choice for detecting cavities and for monitoring reservoir/aquifer mass changes.

Magnetic method – susceptibility and remanence

The controlling properties for the magnetic method are magnetic susceptibility, κ (dimensionless, SI), which measures how strongly a rock is magnetized by the Earth's field (induced magnetization), and remanent magnetization, which is magnetization the rock retains independently of the present field (acquired when the rock cooled through the Curie point or was deposited/altered). Susceptibility is controlled almost entirely by accessory magnetite/pyrrhotite content: quartz, limestone and salt are essentially non-magnetic to weakly diamagnetic (κ ≈ 0 to -10^-5 SI), most sedimentary and felsic igneous rocks are weakly paramagnetic (κ ≈ 10^-4–10^-3 SI), mafic/ultramafic rocks are moderately to strongly magnetic (κ ≈ 10^-3–10^-1 SI), and magnetite-rich ore such as banded iron formation or magnetite skarn can exceed κ = 1 SI. The relative importance of remanence versus induced magnetization is expressed by the Koenigsberger ratio Q = (remanent intensity)/(induced intensity); igneous rocks with strong thermoremanence (e.g. basalt, some iron ores) can have Q > 1, meaning the anomaly's direction is controlled by the rock's magnetic history rather than by the present Earth field — a critical complication for interpretation (see Question 6).

Why physical property contrast, not absolute value, is what matters

In both methods the field measured at surface is generated by the difference between a body's property and that of its surroundings, not by the body's absolute property value. A dense body embedded in equally dense host rock produces zero gravity anomaly no matter how dense it is in absolute terms; a magnetite-rich unit surrounded by equally magnetic host rock is invisible to a magnetometer. It is the density contrast Δρ and the susceptibility contrast Δκ between target and host that scale the anomaly amplitude (together with the body's volume and depth), which is why exploration programs always begin by characterizing the property contrast expected between the target (ore, cavity, intrusion, fault zone) and the host rock from outcrop samples, drill core or published property compilations before a survey is designed — a small contrast requires closer station spacing and a more sensitive instrument to resolve than a large one.