What Is Geophysics?

Geophysics is a natural science concerned with the physical properties and processes of the Earth, studied through quantitative measurement and mathematical analysis. It applies the principles of physics, including mechanics, electromagnetism, thermodynamics, and fluid dynamics, to problems of Earth structure, interior dynamics, surface processes, and planetary evolution. The discipline spans scales from borehole measurements of centimeter-scale rock properties to global models of mantle convection and geomagnetic field generation.

The scope of geophysics is broad enough to overlap substantially with geology, oceanography, meteorology, and astronomy. Its boundary with geochemistry is especially close: both disciplines address Earth's composition and interior, but geophysics approaches these questions through physical measurement and wave propagation rather than elemental analysis. Where geoengineering applies human interventions to Earth systems, geophysics provides the observational and modeling foundation that makes such interventions assessable.

Earth's Interior and Seismology

Seismology, the study of elastic wave propagation through the Earth, has produced most of what is known about Earth's internal structure. Analysis of P-waves and S-waves generated by earthquakes and recorded by global seismometer networks has revealed the crust, mantle, outer core, and inner core, each with distinct seismic velocities and physical states. The U.S. Geological Survey earthquake hazards program coordinates real-time monitoring of seismic activity and maintains research programs on crustal structure. Earth's outer core, confirmed as liquid through the absence of S-wave transmission, generates the geomagnetic field through convective dynamo action, making seismology and geomagnetism closely linked sub-disciplines.

Gravity, Geomagnetism, and Solid Earth Dynamics

Gravity surveys measure the spatial variation of Earth's gravitational acceleration, revealing density anomalies in the crust and mantle that constrain tectonic processes, isostatic adjustment, and subsurface structure. Satellite gravity missions, including GRACE and GRACE-FO, extend these measurements globally and detect temporal changes caused by ice sheet melting, groundwater depletion, and post-glacial rebound. Geomagnetic measurements record spatial and temporal variations in the main field and its secular drift, informing both dynamo theory and paleomagnetism. The USGS geophysical survey methods documentation outlines how gravity, magnetic, and electrical methods are deployed in both research and applied settings.

Oceans and Atmospheric Geophysics

Marine geophysics applies seismic, gravity, magnetic, and bathymetric measurements to the ocean floor, where spreading ridges, subduction zones, and oceanic plateaus record the history of plate tectonics. Ocean bottom seismometers and deep-tow instruments extend land-based methods into submarine settings. Atmospheric geophysics covers the thermal and electrical structure of the atmosphere, including ionospheric physics, magnetospheric dynamics, and the propagation of atmospheric waves. The interaction between the solid Earth and its fluid envelopes, including atmospheric loading on the crust and oceanic tidal forcing of the mantle, creates a coupling that geophysics addresses across its sub-disciplines. Research in these areas appears regularly in the Geophysical Journal International, one of the field's principal publication venues.

Applications

Geophysics has applications in a range of fields, including:

• Petroleum and natural gas exploration, imaging reservoir geometry with seismic reflection surveys
• Mineral resource discovery, locating ore deposits through gravity and magnetic anomalies
• Earthquake hazard assessment, mapping active faults and seismic velocity structure
• Groundwater and environmental investigations, characterizing aquifer geometry and contamination
• Climate monitoring, using satellite gravity and GPS data to track ice mass change and sea-level rise
• Natural disaster preparedness, monitoring volcanic unrest through seismicity, ground deformation, and gas emissions