Question 1 of 10: Physical Properties Sensed by Geophysical Methods — Range and Implications

Question 1: Physical Properties Sensed by Geophysical Methods — Range and Implications (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.

Each geophysical method is sensitive to one physical property of earth materials: gravity to bulk density $\rho$; magnetics to magnetic susceptibility $\chi$ and remanent magnetization; the electrical and electromagnetic methods to electrical resistivity $\rho_e$ (or its reciprocal, conductivity $\sigma$); the seismic methods to elastic-wave velocity ($V_p$, $V_s$) and acoustic impedance; the radiometric method to natural gamma radioactivity (K, U, Th concentration); and induced polarization to chargeability, an electrochemical relaxation property of disseminated conductive/metallic grains.

Density. Air-filled voids/karst are the low extreme, $\rho\approx0$–$0.001\,\text{g/cm}^3$; unconsolidated sand/soil is $\rho\approx1.6$–$2.0\,\text{g/cm}^3$; typical crustal rock (granite, sandstone, limestone) spans $\rho\approx2.2$–$2.9\,\text{g/cm}^3$; massive sulphide ore and magnetite skarn reach the high extreme, $\rho\approx4.5$–$5.2\,\text{g/cm}^3$. Excluding the void end-member, density for ordinary rock varies only by a factor of roughly 2–3.

Electrical resistivity. Native metal, graphite and massive sulphide ore are extremely conductive, $\rho_e\approx10^{-6}$–$10^{-4}\,\Omega\cdot\text{m}$; saline groundwater and clay/shale are moderately conductive, $\rho_e\approx1$–$100\,\Omega\cdot\text{m}$; while dry, unfractured crystalline rock, quartzite and permafrost are extremely resistive, $\rho_e\approx10^{4}$–$10^{8}\,\Omega\cdot\text{m}$ or more. This is a span of roughly twelve to fourteen orders of magnitude — by far the broadest-ranging physical property used in geophysics.

Seismic velocity, magnetic susceptibility and radioactivity, for comparison. P-wave velocity runs from $V_p\approx340\,\text{m/s}$ in air and $\approx1500\,\text{m/s}$ in saturated soil/water up to $V_p\approx6000$–$7000\,\text{m/s}$ in dense unweathered crystalline rock — roughly one to two orders of magnitude, intermediate between density and resistivity. Magnetic susceptibility is bimodal and also very broad: most rock-forming silicates carry almost no magnetite and sit near $\chi\approx0$, while magnetite-rich rock (banded iron formation, some mafic intrusions) reaches $\chi\approx1$ SI or more — again several orders of magnitude, driven almost entirely by accessory-mineral (magnetite) content rather than the bulk rock type.

Broad vs. narrow, and the implication. Electrical resistivity has by far the broadest range of the properties above; density has the narrowest range among common rock types. A broad-range property demands acquisition equipment with a very wide dynamic range and multiple gain settings or automatic ranging (a DC resistivity meter and an EM receiver must resolve signals that differ by many decades from one survey to the next, and sometimes within one survey), and it gives strong, easily detected contrasts between a target and its host — a small volume fraction of sulphide or clay can dominate the bulk resistivity reading. The cost is interpretive non-uniqueness: because so many different combinations of porosity, saturation, clay content and mineralization can produce the same apparent resistivity, resistivity/EM inversions are notoriously equivalent (many earth models fit the same data) and are best constrained with independent control. A narrow-range property such as density instead needs a very precise, low-noise instrument (a gravimeter resolving microgals against a background of ~2.67 g/cm³ average crustal density) to detect what is often only a few tenths of a g/cm³ of contrast, but once measured, that small contrast maps more directly and unambiguously onto a specific rock type or structure, since fewer geological causes can produce it.