Geophysics applies principles of physics and quantitative mathematics to investigate Earth and its physical properties, structures and processes. It encompasses the study of gravity, magnetism, seismic waves, electrical and electromagnetic properties, heat flow, fluid behaviour, Earth's rotation, deformation and other measurable physical phenomena. Geophysics can investigate scales ranging from microscopic material properties to the global structure of Earth's interior and can address both fundamental scientific questions and practical problems.
Because direct sampling of Earth's deep interior is extremely limited, geophysical measurements provide essential indirect evidence about regions that cannot be reached by drilling or excavation. Seismic waves, gravitational fields, magnetic fields and electrical responses can reveal variations in density, elasticity, temperature, composition, fluid content and structure. Geophysics therefore complements direct geological observation by converting measurable physical signals into models of the hidden Earth.
Seismology is one of the most important areas of Geophysics and studies earthquakes, seismic waves and the elastic behaviour of Earth materials. Primary waves travel through solids and fluids, whereas secondary waves cannot propagate through fluids, providing evidence for the structure of Earth's interior. Surface waves travel along or near Earth's surface and contribute strongly to observed ground motion during earthquakes. Seismic wave velocities depend on properties such as density, elastic moduli, temperature, pressure, fracture state and composition.
Reflection and refraction methods use controlled or natural seismic sources to investigate subsurface structures, making them important in both research and applied exploration. Earthquake seismology can locate seismic sources and characterise faulting, while global seismic observations have revealed the broad structure of the crust, mantle and core. Seismic methods therefore connect fundamental physics with geological interpretation and hazard assessment.
Gravity and magnetic methods provide other major geophysical approaches. Variations in the gravitational field arise from differences in mass distribution and density, allowing gravity surveys to identify broad geological structures, sedimentary basins, intrusive bodies and crustal variations. Magnetic surveys detect spatial changes in Earth's magnetic field caused partly by magnetic minerals in rocks, especially minerals containing iron. Magnetic anomalies can reveal buried structures, igneous bodies, faults and patterns associated with seafloor spreading.
Earth's magnetic field has also changed polarity through geological time, and the resulting magnetic signatures preserved in rocks provide valuable evidence for plate motion and chronological correlation. Interpretation of gravity and magnetic data is inherently non unique because different subsurface configurations can generate similar observations. Geophysicists therefore integrate these datasets with geological information and other constraints to reduce ambiguity.
Electrical and electromagnetic geophysics investigates how geological materials respond to electric currents and electromagnetic fields. Electrical resistivity depends on factors including mineral composition, porosity, water saturation, salinity, temperature and connectivity of conducting pathways. Resistivity surveys can therefore help identify groundwater, cavities, contaminated zones, geological boundaries and changes in lithology. Electromagnetic methods can investigate conductivity variations over different depths and spatial scales and are used in mineral exploration, environmental studies, engineering investigations and hydrogeology. Induced polarisation methods examine electrical charge storage behaviour and can be particularly useful in certain mineral exploration contexts. The interpretation of electrical data requires understanding that measured responses can be influenced by several variables simultaneously, so geological context remains important.
Geophysics is fundamental to understanding Earth's interior because physical properties change systematically with pressure, temperature, composition and phase. Seismic observations have demonstrated that Earth contains major internal boundaries, including the transitions between crust, mantle and core. The outer core behaves as a fluid, while the inner core is solid under the prevailing pressure conditions, and the behaviour of seismic waves across these regions provides evidence for this structure.
Geophysical observations also reveal mantle heterogeneity, subducting slabs, thermal anomalies and other deep features that cannot be sampled directly. Geodesy and measurements of Earth's shape and deformation contribute further information about tectonic motion, crustal loading, sea level and mass redistribution. Together these methods make geophysics indispensable for investigating Earth's deep structure and dynamic behaviour.
Applied Geophysics uses physical measurements to solve practical geological and engineering problems. Seismic reflection is widely used to image subsurface structures in sedimentary basins, while seismic refraction can help characterise near surface layers and engineering conditions. Gravity and magnetic surveys can identify geological structures relevant to mineral and energy exploration. Electrical and electromagnetic techniques can investigate groundwater, contamination and subsurface voids. Ground penetrating radar provides high resolution information near the surface in suitable materials.
Geophysical monitoring is also used to detect changes associated with volcanic activity, reservoir processes, fluid injection, groundwater extraction and deformation. Applied interpretations must consider survey resolution, noise, non uniqueness and geological assumptions, because a geophysical model is an inference from measurements rather than a direct photograph of the subsurface.
Quantitative modelling is central to modern Geophysics. Forward modelling calculates expected physical measurements from an assumed Earth model, while inverse methods estimate plausible subsurface properties from observed data. Inverse problems can be underdetermined because many models may explain the same measurements, making regularisation, prior information, uncertainty analysis and independent constraints important. Computational methods allow geophysicists to process large datasets, simulate wave propagation, model heat transfer, calculate gravitational and magnetic fields and estimate deformation.
Modern instruments also generate continuous observations from seismic networks, satellites, observatories and monitoring systems. Machine learning and automated data processing can assist pattern recognition and classification, but physical constraints and geological validation remain essential. The reliability of a geophysical interpretation therefore depends not only on measurement quality but also on model assumptions and the degree to which independent evidence constrains the solution.
Geophysics differs from Geology primarily in methodological emphasis rather than subject matter. Geology often begins with direct observations of rocks, structures, fossils and field relationships, while geophysics frequently begins with physical measurements from which subsurface properties are inferred. The two approaches are complementary because geophysical anomalies require geological interpretation and geological models can be tested or refined using geophysical evidence. Geophysics has applications in earthquake science, tectonics, volcanology, hydrology, engineering, resource exploration, environmental assessment and planetary science.
The same physical principles can also be applied beyond Earth to investigate the interiors and surfaces of planets, moons and other bodies. Its broader significance lies in making otherwise inaccessible structures measurable through their physical effects, thereby extending scientific observation from exposed rocks and shallow samples to the deep and dynamic interior of planetary systems.
