Geophysics is the study of Earth’s physical properties and processes using measurements of things like gravity, magnetism, seismic waves, and electrical conductivity. Rather than digging into the ground directly, geophysicists use instruments at the surface (or in the air, or in space) to build pictures of what lies beneath. The field spans everything from mapping earthquake faults to finding oil reserves to studying the interior of Mars.
How Geophysics Works
The core idea is straightforward: different rocks, fluids, and structures underground have different physical properties. Dense rock bends gravity slightly differently than lighter sediment. Iron-rich minerals distort the local magnetic field. Sound waves travel faster through solid bedrock than through loose gravel. By measuring these variations from the surface, geophysicists can infer what’s happening below without ever breaking ground.
The main physical properties geophysicists measure include seismic wave velocity (how fast vibrations travel through rock), magnetic susceptibility (how strongly a material responds to a magnetic field), electrical resistivity (how easily current flows through the ground), density variations that affect local gravity, and natural radioactivity. Each property tells a different story about subsurface geology, and combining multiple measurements creates a far more detailed picture than any single method alone.
Major Branches
Geophysics splits into several overlapping subdisciplines. Seismology studies seismic waves, both from earthquakes and from artificial sources like controlled explosions or hammer strikes. Geomagnetism focuses on Earth’s magnetic field, its origin in the planet’s molten iron core, and the way rocks record ancient magnetic orientations. Geodesy measures Earth’s gravitational field and the precise shape of its surface, tracking changes as subtle as the slow rebound of land that was once pressed down by ice-age glaciers.
A practical distinction separates global geophysics from exploration geophysics. Global geophysics asks big questions: how do tectonic plates move, what drives volcanic eruptions, how is the planet’s interior structured? Exploration geophysics applies the same physics at a smaller scale to find resources like oil, gas, minerals, or groundwater. Environmental geophysics, a newer branch, uses near-surface techniques to investigate contaminated sites, map buried infrastructure, and monitor groundwater systems.
Seismic Methods
Seismic surveys are the workhorses of geophysics. A controlled energy source, sometimes as simple as a sledgehammer hitting a metal plate, sends vibrations into the ground. Those vibrations travel as compressional waves (which move through solids, liquids, and gases) and shear waves (which only pass through solid material). Sensors called geophones record the returning signals, and software reconstructs a cross-section of the underground.
Two main approaches exist. Seismic refraction measures how long it takes for the first wave arrivals to reach detectors at increasing distances from the source. It works best for mapping shallow geology, typically at depths less than about 65 feet, and is commonly used to find the depth to bedrock or the water table. Seismic reflection, by contrast, records waves that bounce off boundaries between rock layers. It dominates in deeper exploration, from petroleum prospecting thousands of feet down to shallow environmental work at depths between 65 and 100 feet. Oil and mining companies have relied on reflection surveys for decades to map underground structures before drilling.
Gravity and Magnetic Surveys
Tiny variations in Earth’s gravitational pull reveal differences in rock density underground. A buried body of dense ore, for example, produces a slightly stronger gravitational pull than the surrounding rock. Gravity surveys measure these minute differences with extremely sensitive instruments, helping geologists locate mineral deposits, map the thickness of sedimentary basins, and study the deep structure of mountain ranges and tectonic boundaries.
Magnetic surveys work on a similar principle but measure distortions in Earth’s magnetic field caused by magnetically susceptible rocks and minerals beneath the surface. These surveys are often conducted from aircraft, covering large areas quickly. The U.S. Geological Survey routinely flies airborne magnetic and radiometric surveys to map geology across broad regions, revealing features that would take years to find through surface fieldwork alone.
Monitoring Natural Hazards
Geophysics is central to earthquake and volcano monitoring. Modern volcano observatories deploy a wide array of instruments: broadband seismometers that detect ground vibrations across a range of frequencies, electronic tiltmeters that sense when a volcano’s surface bulges as magma rises, strainmeters that measure rock deformation, GPS networks that track ground movement down to millimeters, and even microphones tuned to detect infrasound (low-frequency sound waves produced by eruptions that are too deep-pitched for human ears). Satellite-based radar can map surface deformation across entire volcanic systems from orbit.
For earthquakes, networks of seismometers provide the data behind every earthquake alert you see on the news. The same instruments reveal the deep structure of fault zones, helping scientists estimate where stress is building and which faults pose the greatest risk. This information feeds directly into building codes and emergency planning.
Energy and Resource Exploration
The oil and gas industry has historically been the largest employer of exploration geophysicists. Seismic reflection surveys, processed into detailed 3D models of underground rock layers, guide decisions about where to drill. But geophysics also plays a growing role in renewable energy.
Geothermal energy exploration relies heavily on a technique called magnetotellurics, which measures natural electromagnetic signals to map electrical resistivity deep underground. Hot, fluid-filled rock conducts electricity differently than cold, dry rock, so resistivity maps can outline geothermal reservoirs. In one large-scale survey on the Tibetan Plateau, researchers deployed over 300 measurement sites across a 100-by-100-kilometer area, then used the data to build a 3D resistivity model extending to about 50 kilometers depth. That kind of imaging helps identify where to drill geothermal wells with far less guesswork.
Environmental and Engineering Uses
Near-surface geophysics has become essential for environmental work. According to the U.S. Environmental Protection Agency, geophysical surveys are used to locate underground storage tanks, map buried utilities and wells, characterize landfills, track contaminant plumes in groundwater, and monitor the progress of cleanup efforts at polluted sites. These methods are nondestructive, meaning you can survey a site without disturbing the contamination or the structures you’re trying to find.
Groundwater investigations are another major application. Geophysical methods can map aquifer boundaries, monitor saltwater intrusion in coastal areas, and track how water moves between surface streams and underground reservoirs. For civil engineering projects, surveys help identify unstable ground, locate buried voids or sinkholes, and assess whether bedrock is shallow enough to affect construction.
Geophysics Beyond Earth
The same principles apply to other planets. NASA’s InSight lander, which operated on Mars from 2018 to 2022, carried a seismometer that recorded hundreds of “marsquakes.” Analysis of those seismic signals revealed the tectonic setting around a region called Cerberus Fossae, where the faulting pattern turned out to be predominantly “normal faulting,” meaning the crust is being pulled apart rather than compressed. These were the first focal mechanisms ever determined for quakes on another planet, giving scientists direct evidence of active tectonics on Mars. Similar geophysical tools have been proposed for future missions to the Moon, Europa, and other bodies in the solar system.
Careers in Geophysics
Geophysicists typically hold at least a bachelor’s degree in geophysics, geology, physics, or a related field, though many positions in research and exploration require a master’s or doctorate. The work can be office-based (processing and interpreting data) or highly field-oriented, involving travel to remote locations for surveys.
The U.S. Bureau of Labor Statistics reports a median annual wage of $99,240 for geoscientists as of May 2024. Employment is projected to grow about 3 percent from 2024 to 2034, roughly matching the average for all occupations. Jobs exist across the energy sector, environmental consulting, government agencies like the USGS and EPA, mining companies, and academic research institutions. The shift toward renewable energy and the growing need for environmental site assessments have broadened the field beyond its traditional oil-and-gas base.