Introduction
Geophysics probes the subsurface indirectly, using measurable physical properties, rather than through direct observation of exposed rock — an essential complement to surface-based geological mapping, especially for resource exploration (Chapters 8 and 10) and engineering applications (Chapter 9).
1. Seismic methods
Seismic methods generate artificial seismic waves (via a controlled source, such as a small explosive charge or a mechanical vibrator) and measure how they travel through and reflect or refract off subsurface rock layers, using an array of surface-based receivers (geophones). The travel times and patterns of the returning waves are used to infer subsurface layer depths, geometry, and, to some extent, rock type — the same fundamental wave behaviour (reflection, refraction) introduced physically in Physics Chapter III, applied here at a much larger geological scale.
2. Gravity methods
Gravity methods measure minute local variations in the Earth’s gravitational field using a sensitive instrument (a gravimeter); denser subsurface rock or ore bodies produce a slightly stronger local gravitational pull (a positive gravity anomaly), while less dense material (such as a salt dome, an underground cavity, or certain sedimentary basins) produces a slightly weaker pull (a negative anomaly) — mapping these anomalies across an area reveals variations in subsurface density and, indirectly, subsurface structure and composition.
3. Magnetic methods
Magnetic methods measure local variations in the Earth’s magnetic field, which are affected by the magnetic properties of subsurface rock (particularly its content of magnetic minerals like magnetite) — rocks with a comparatively high magnetic mineral content produce a positive magnetic anomaly. This method is especially useful and rapid for locating certain igneous intrusions and specific ore deposits directly associated with magnetic minerals.
4. Electrical and resistivity methods
Electrical/resistivity methods measure how subsurface material resists (or conducts) an applied or naturally occurring electric current — resistivity varies significantly with rock type, porosity, and especially the presence and salinity of pore fluids (water-saturated or saline material generally conducts more readily, showing lower resistivity, than dry or highly resistive rock) — making these methods particularly valuable and widely used for groundwater exploration and for characterising near-surface engineering site conditions.
5. Interpretation of geophysical anomalies and subsurface modelling
A crucial and recurring limitation across all geophysical methods is non-uniqueness: a given observed surface anomaly (gravity, magnetic, electrical, or seismic) can, in principle, be produced by more than one different subsurface configuration — different combinations of depth, size, shape, and physical-property contrast can sometimes match the same observed surface data equally well. Reliable subsurface interpretation therefore requires combining multiple independent geophysical methods together, and, wherever available, constraining the resulting models with actual direct geological knowledge (from surface mapping, or from borehole/well data) rather than relying on any single geophysical method’s results in isolation.
Key Takeaways
- Seismic methods use artificial wave generation and reflection/refraction travel times to infer subsurface layer depth and geometry.
- Gravity and magnetic methods are passive techniques measuring naturally occurring field variations caused by subsurface density or magnetic-mineral content contrasts respectively.
- Electrical/resistivity methods are especially sensitive to pore-fluid presence and salinity, making them particularly valuable for groundwater and engineering-site investigation.
- Every geophysical method suffers from fundamental interpretive non-uniqueness — the same surface anomaly can correspond to more than one subsurface configuration.
- Reliable subsurface interpretation requires combining multiple geophysical methods with independent direct geological data (surface mapping, boreholes) rather than trusting a single method in isolation.