Question 1 of 8: Geophysical Terms

Question 1: Geophysical Terms (12 marks)

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 exam asks for any four; all six are explained below so the set is a complete reference.

(a) Shear wave (S-wave). A body wave in which particle motion is transverse to the direction of propagation, so the medium is sheared rather than compressed: $V_s=\sqrt{\mu/\rho}$, where $\mu$ is the shear modulus and $\rho$ the density. Because fluids and gases cannot sustain shear ($\mu=0$), S-waves do not propagate through liquids or air — this is how seismology first proved the Earth's outer core is liquid, and in engineering surveys the $V_s$-to-$V_p$ ratio (or Poisson's ratio derived from it) is diagnostic of saturation and rock competency.

(b) Remanent magnetization. The permanent, "frozen-in" magnetization a rock retains after the inducing field that created it is removed, acquired when magnetic minerals (chiefly magnetite) cool through their Curie temperature in the ambient geomagnetic field (thermoremanent magnetization) or are deposited/chemically altered in its presence. It adds vectorially to the induced magnetization and can be comparable to or larger than it (high Koenigsberger ratio $Q$), so a magnetic anomaly's shape and even its sign can differ from what induced magnetization alone would predict — a key complication in interpreting magnetic surveys over strongly magnetic (e.g. mafic/ultramafic) rocks.

(c) Acoustic impedance. The product $Z=\rho V$ of density and seismic (P-wave) velocity for a rock layer. It is impedance CONTRASTS across an interface, expressed as the normal-incidence reflection coefficient $R=(Z_2-Z_1)/(Z_2+Z_1)$, that generate seismic reflections; the significance in geophysics is that a reflection survey directly images impedance boundaries (not necessarily lithologic or stratigraphic ones), so a bright, high-amplitude reflector marks a large $Z$ contrast (e.g. a gas sand or a coal seam), not necessarily a major geological unconformity.

(d) Bouguer Anomaly gravity (map or profile). The observed gravity value corrected for elevation (free-air correction), the attraction of the rock slab between the station and a datum (Bouguer correction, using an assumed reduction density), and terrain, then referenced to a theoretical (latitude) gravity value: $g_B = g_{obs}+\text{FAC}-\text{BC}\pm\text{TC}-g_{theoretical}$. What remains is attributable to subsurface density contrasts alone, which is what makes the Bouguer anomaly — not raw or free-air gravity — the map used for geological interpretation (regional structure, ore bodies, cavities such as the tunnel in Question 7).

(e) Induced polarization (IP). A time- or frequency-dependent electrical effect in which the ground, after a current is switched off (or as frequency is varied), behaves as a leaky capacitor: a decaying "over-voltage" is measured (time domain, as chargeability $M$) or an apparent-resistivity change with frequency is measured (frequency domain, as percent frequency effect). It arises from electrochemical polarization at mineral grain/electrolyte interfaces and is strongly diagnostic of disseminated sulphide or graphite content even when the sulphides are too dilute to lower the bulk resistivity noticeably — the standard method for disseminated sulphide/porphyry exploration.

(f) Self-potential (SP). A naturally occurring, passively-measured DC electrical potential (tens to hundreds of mV) generated without any injected current, from electrochemical (mineralization, redox gradients across a sulphide body), electrokinetic (streaming potential from groundwater flow through porous media), or thermoelectric sources. Its significance is that it is a free, single-electrode-pair reconnaissance tool — classically used to detect the tops of massive sulphide/graphite bodies (strong negative anomalies) and, increasingly, to map seepage paths through dams and levees from the electrokinetic effect.