Geophysics

Geophysics is a branch of natural science devoted to the study of the physical properties and processes of the Earth and its surrounding space environment. It employs quantitative methods to analyse the planet’s internal structure, surface features, and dynamic systems. Traditionally, the field focused on the solid Earth—its shape, gravitational and magnetic fields, internal composition, and tectonic activity. Contemporary geophysics, however, encompasses a wider scope that includes the water cycle, glaciology, atmospheric dynamics, ionospheric and magnetospheric phenomena, and comparative studies of the Moon and other planets.
The discipline plays a crucial role in advancing fundamental understanding of planetary behaviour while addressing societal needs such as resource exploration, earthquake hazard assessment, and environmental monitoring. Geophysical surveys support the identification of mineral and petroleum deposits, groundwater resources, archaeological features, ice thickness, and soil structure, and help to evaluate sites requiring environmental remediation.

Historical development

Although geophysics emerged as a formal scientific field in the nineteenth century, many of its principles trace back to antiquity. Early humans used lodestones as primitive magnetic compasses, and by classical times the relationship between celestial motions and tides was recognised. In 132 AD the first seismoscope was devised, signalling a long-standing interest in Earth vibrations. Isaac Newton later applied classical mechanics to explain tidal forces and precession, spurring further advances.
By the twentieth century, geophysical instrumentation matured to enable remote investigation of Earth’s deep interior and oceans. Techniques for measuring gravity, magnetism, electricity, and seismic waves yielded crucial insights and contributed to the development of plate tectonics theory. Modern geophysics continues to integrate new technologies to illuminate Earth processes from the core to the upper atmosphere.

Gravity

The behaviour of gravity underpins numerous geophysical phenomena. Tidal forces generated by the Moon and Sun produce two high tides and two low tides each lunar day. Increasing pressure with depth causes rocks to become denser, influencing seismic wave propagation and geodynamic processes.
Precise measurements of gravitational acceleration and potential—using gravimetry and analyses of gravity anomalies—provide information on tectonic plate movement, crustal thickness, and subsurface density contrasts. The geoid, a geopotential surface approximating global mean sea level, is fundamental for defining Earth’s shape and understanding mass distribution within the planet.

Vibrations and seismology

Seismic waves are elastic vibrations generated by earthquakes, explosions, or other sources, and they propagate through Earth’s interior or along its surface. Seismographs measure both travelling waves and global oscillations known as normal modes. By analysing wave arrival times at different locations, geophysicists can locate earthquake epicentres and reconstruct subsurface structure.
Reflection seismology, which records echoes of seismic waves at interfaces where rock properties change, is widely used in hydrocarbon exploration and geological mapping. Seismic refraction methods reveal variations in density and composition. Earthquake studies also inform hazard assessment and engineering design, with different earthquake types—such as intraplate or deep-focus events—requiring tailored understanding of their mechanisms.

Electricity and atmospheric conductivity

Electrical phenomena are pervasive throughout the Earth system. Even in fair weather a downward electric field averaging about 120 volts per metre exists near the surface. Ionisation from cosmic rays produces a net positive charge in the atmosphere, while thunderstorms maintain a global electric circuit through currents flowing between the ionosphere and Earth’s surface.
Geophysical methods exploit natural and induced electrical fields. Spontaneous potentials arise from natural disturbances, and telluric currents in the crust and oceans—driven by electromagnetic induction or fluid motion in the magnetic field—reveal variations in subsurface resistivity. Techniques such as induced polarization and electrical resistivity tomography enable imaging of geological structures, aquifers, and areas affected by contamination.

Electromagnetic waves

Electromagnetic phenomena occur in the ionosphere, magnetosphere, and deep within Earth’s core. High-energy particles trapped in the Van Allen belts produce dawn chorus emissions, while whistler waves originate from lightning strikes. Electromagnetic signals may also accompany earthquakes.
In the fluid outer core, electric currents generate magnetic fields by electromagnetic induction. Magnetohydrodynamic waves such as Alfvén waves propagate within the magnetosphere and possibly in the core, although slower modes—such as magnetic Rossby waves—are more strongly linked to long-term changes in the geomagnetic field.
EM survey methods include transient electromagnetics, magnetotellurics, nuclear magnetic resonance, and seabed logging, each capable of probing conductivity and structure at varying depths.

Magnetism

Earth’s magnetic field, sustained by convective motions in the outer core, shields the planet from solar wind and enables navigation. Interactions between solar particles and the upper atmosphere produce the aurora. The geomagnetic field resembles a tilted dipole but undergoes continuous secular variation. Over geological timescales, polarity reversals occur irregularly, averaging intervals of several hundred thousand years. The brief Laschamp reversal, around 41,000 years ago, is a recent example.
Magnetic signatures preserved in volcanic rocks and marine sediments record these reversals, providing the basis for magnetostratigraphy. Linear magnetic anomaly stripes on the ocean floor revealed seafloor spreading and helped establish plate tectonics as a unifying theory in Earth science.

Applications and contemporary significance

Geophysics integrates physics, mathematics, geology, and computational modelling to address key questions about Earth’s structure and behaviour. Its applications include:

• Resource exploration, especially for petroleum, gas, and minerals.
• Natural hazard mitigation, involving earthquakes, tsunamis, landslides, and volcanic activity.
• Environmental assessment, such as groundwater mapping and contamination studies.
• Planetary science, where geophysical principles illuminate the interiors and magnetic fields of other planets and moons.