Question 1 of 9: Physical Properties in Applied Geophysics

Question 1: Physical Properties in Applied Geophysics (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.

Every branch of applied geophysics measures a field or a travel time at the surface (or in a borehole) that responds to ONE underlying physical property of the rock mass below, and the six properties below cover essentially the whole discipline.

Density (gravity method) ranges from about 1.0 g/cm³ for water and unconsolidated peat, through roughly 1.6–2.0 g/cm³ for dry sand and gravel, 2.5–2.7 g/cm³ for typical sedimentary and granitic rocks, up to 2.8–3.3 g/cm³ for mafic/ultramafic rocks, and 4–5 g/cm³ or higher for massive sulphides and some ore concentrations — the gravity method exploits a density CONTRAST, e.g. a buried void or salt dome (low density) against country rock, or a massive sulphide lens (high density) against volcanic host.

Magnetic susceptibility (and remanent magnetization; magnetics method) spans an enormous range — from essentially zero (a few ×10⁻⁵ SI) for quartz, limestone and salt, through 10⁻⁰–10⁻² SI for most sedimentary and felsic igneous rocks, to 10⁻²–1 SI for magnetite-rich mafic/ultramafic rocks and iron ore — magnetite content dominates the signal, so magnetics is exceptionally good at mapping mafic intrusions, banded iron formation, and pyrrhotite-bearing sulphide zones against a non-magnetic host.

Electrical resistivity (resistivity, induced polarization and most EM methods) is the widest-ranging property of all, from a fraction of an ohm-metre for massive sulphides and graphite, through 1–100 Ω·m for clay, saline groundwater and shale, 100–10,000 Ω·m for sand, gravel and most sedimentary rock, up to 10⁴–10⁵ Ω·m for dry, unfractured crystalline rock — resistivity is controlled far more by pore-water content and salinity than by the mineral matrix itself, which is why it is the workhorse property for groundwater, contamination and permafrost/ice studies as well as sulphide exploration.

Elastic moduli, expressed through seismic velocity (reflection and refraction methods), range from about 330 m/s in air and 1450 m/s in water, through 300–1000 m/s in dry unconsolidated soil, 1500–2500 m/s in saturated soil and weathered rock, 2000–4000 m/s in sedimentary rock, up to 5000–6500 m/s in crystalline igneous/metamorphic rock — velocity increases with confining pressure, cementation and the degree of water saturation, so a seismic velocity contrast maps the water table, the depth to bedrock, and internal stratigraphy.

Dielectric permittivity (ground-penetrating radar) ranges from about 1 for air and 3–5 for dry sand/rock up to 80 for water, so GPR is exceptionally sensitive to moisture content and shallow structure, but attenuates rapidly in conductive (clay-rich, saline) ground.

Natural radioactivity (radiometrics, from K, U and Th decay) is a surface-only property (gamma rays travel at most a few tens of centimetres through rock) used mainly for lithological mapping, alteration-halo detection and uranium exploration.

Physical property contrast is the entire basis of applied geophysics: a method has NO signal at all across a boundary where the property in question is unchanged on both sides, no matter how important that boundary is geologically. This is why (a) a survey must be designed around the property that actually differs across the target of interest — a coal seam might have almost no density contrast with its shale roof but a strong velocity contrast, making seismic the right tool and gravity the wrong one — and (b) integrating several INDEPENDENT properties (e.g. a body that is simultaneously conductive, magnetic and dense) gives far more confident discrimination between a real target and a look-alike than any single method alone, since it is unlikely that an unrelated feature would coincidentally share all three contrasts.