Geophysics: Physics of the Earth and Its Systems

Geophysics applies the principles of physics to the study of Earth's structure, composition, and dynamic processes — spanning the planet's interior, surface, oceans, atmosphere, and electromagnetic environment. The discipline underpins natural resource exploration, seismic hazard assessment, climate monitoring, and planetary defense planning. As a reference domain, geophysics sits at the intersection of applied physics, earth sciences, and engineering, with professional practice governed by credentialing bodies and regulatory frameworks across federal and state jurisdictions.

Definition and scope

Geophysics is the quantitative investigation of Earth using physical methods, including seismology, gravity measurement, magnetometry, heat-flow analysis, and electrical resistivity surveys. The American Geosciences Institute defines geophysics as a subdiscipline of the earth sciences that uses physics-based observations to infer subsurface and atmospheric properties that cannot be directly sampled (American Geosciences Institute).

The scope of geophysics spans four primary domains:

1. Solid Earth geophysics — seismology, geodesy, geomagnetism, and geothermal analysis of the lithosphere, mantle, and core.
2. Exploration geophysics — subsurface imaging for petroleum, mineral, and groundwater resources using reflection seismics, gravity, and electromagnetic methods.
3. Environmental and engineering geophysics — near-surface investigation for contamination mapping, foundation assessment, and archaeological prospection.
4. Atmospheric and space geophysics — ionospheric physics, magnetospheric dynamics, and Earth–Sun interaction, overlapping with plasma physics as described at Plasma Physics.

The Society of Exploration Geophysicists (SEG), founded in 1930, and the American Geophysical Union (AGU), founded in 1919, are the principal professional organizations establishing publication standards and technical nomenclature for the field (SEG; AGU).

How it works

Geophysics operates by measuring physical fields at or above Earth's surface and inverting those measurements to infer subsurface or atmospheric structure. The core measurement types and their physical basis are:

Seismic methods exploit the propagation of elastic waves through rock. Compressional (P-wave) velocities in consolidated crustal rock typically range from 4 to 7 km/s, while shear (S-wave) velocities range from 2 to 4 km/s (USGS Earthquake Hazards Program). Reflection seismology records travel times of waves bounced off subsurface interfaces; refraction seismology uses first-arrival times to map velocity gradients with depth. These principles connect directly to wave mechanics and interference and fluid mechanics and dynamics.

Gravity methods measure spatial variations in gravitational acceleration, expressed in milligals (1 mGal = 10⁻⁵ m/s²). The Bouguer anomaly — the residual after removing theoretical gravity, elevation, and terrain effects — isolates density contrasts in the subsurface. Dense mafic intrusions produce positive anomalies; sedimentary basins, which are less dense, produce negative anomalies. The underlying physics is treated in detail at Gravity and Gravitational Fields.

Magnetic methods map variations in Earth's magnetic field caused by variations in rock magnetization. The geomagnetic field, generated by convection in the liquid outer core (the geodynamo), has a surface intensity ranging from approximately 25,000 to 65,000 nanoteslas depending on latitude (NOAA National Centers for Environmental Information). Magnetic susceptibility contrasts between lithologies produce anomalies detectable by airborne or ground magnetometers. The physics of the geomagnetic field is continuous with Magnetic Fields and Magnetism.

Electromagnetic methods pass controlled alternating currents into the ground and measure the resulting electromagnetic response to infer resistivity. Conductive ore bodies, saline groundwater, and clay-rich sediments produce characteristically low resistivities; resistive crystalline basement or hydrocarbons produce high resistivities.

Heat flow measurements at drill sites relate surface thermal gradients to the flux of heat from Earth's interior. Average global continental heat flow is approximately 65 milliwatts per square meter (mW/m²), while oceanic heat flow averages around 100 mW/m² (International Heat Flow Commission). Elevated heat flow marks volcanic regions and rifts; suppressed heat flow marks ancient cratons.

The scientific methodology underlying all geophysical measurement cycles is consistent with the framework described at How Science Works: Conceptual Overview.

Common scenarios

Geophysics encounters specific operational contexts that determine which method suite is appropriate:

• Hydrocarbon exploration: 3D reflection seismic surveys over sedimentary basins, combined with well-log calibration, remain the primary tool for imaging reservoir-scale structures at depths of 1 to 8 km. The US Geological Survey's National Oil and Gas Assessment uses geophysical data to estimate undiscovered technically recoverable resources (USGS Energy Resources Program).
• Earthquake hazard assessment: The USGS National Seismic Hazard Model incorporates seismic velocity models derived from regional seismic tomography to produce probabilistic ground-motion maps used in building codes.
• Groundwater mapping: Electrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys delineate aquifer geometry and salinity variation in regions where drilling costs are prohibitive.
• Volcano monitoring: Networks of seismometers at volcanically active sites detect fluid migration and magma movement through characteristic low-frequency seismic signatures. The USGS Volcano Hazards Program operates 5 volcano observatories in the US (USGS Volcano Hazards Program).
• Archaeological prospection: Ground-penetrating radar (GPR) and magnetometry locate buried structures without excavation, a non-destructive application well-established in environmental and engineering practice.

Decision boundaries

Distinguishing between geophysical method classes requires matching physical contrasts to geological targets:

The boundary between exploration geophysics and environmental/engineering geophysics is primarily one of depth and resolution: exploration problems target structures at hundreds to thousands of meters, while engineering problems often require sub-meter resolution in the upper 10 to 50 meters. Method selection also depends on logistical constraints — airborne magnetic surveys can cover 10,000 km² in a single campaign, whereas ERT surveys acquire data meter by meter along ground profiles.

Geophysics contrasts with geology proper in that it produces indirect, model-dependent inferences rather than direct observations of rock. The non-uniqueness problem — multiple subsurface models can fit the same surface data equally well — is a fundamental epistemological constraint that practitioners manage through integration of multiple method types, well-log calibration, and Bayesian inversion frameworks. This non-uniqueness is rooted in the mathematical structure of inverse problems and connects to broader topics in Statistical Mechanics and computational physics.

For professionals navigating the broader landscape of physics subdisciplines, the Physics Authority index provides structured access to related fields including Nuclear Physics, Astrophysics and Cosmology, and Solid-State and Condensed Matter Physics.

References

• American Geophysical Union (AGU)
• Society of Exploration Geophysicists (SEG)
• USGS Earthquake Hazards Program
• USGS National Seismic Hazard Maps
• USGS Energy Resources Program — National Oil and Gas Assessment
• USGS Volcano Hazards Program
• NOAA National Centers for Environmental Information — Geomagnetism
• International Heat Flow Commission (IHFC)
• American Geosciences Institute