Geophysics: Using Physics to Study Earth's Interior and Surface

Geophysics applies the principles of physics — gravity, magnetism, heat flow, elasticity, and electromagnetism — to understand structures and processes that no drill has ever reached and no eye has ever seen. It sits at the intersection of earth science and physical measurement, making it one of the most instrumentally rigorous disciplines in the geosciences. This page covers how geophysics is defined, how its core methods work, where it gets applied, and how practitioners decide which technique is appropriate for a given problem.

Definition and scope

The deepest borehole ever drilled — the Kola Superdeep Borehole in Russia, which reached 12.26 kilometers before being abandoned in 1989 (USGS) — barely scratched the planet's surface. Earth's radius is roughly 6,371 kilometers. Geophysics exists precisely because direct sampling of the interior is, in practical terms, impossible at meaningful depth.

Geophysics is formally defined as the study of Earth using quantitative physical methods. According to the Society of Exploration Geophysicists, the field encompasses solid-earth physics, atmospheric science, hydrosphere studies, and near-surface investigations, though in common usage it refers most specifically to subsurface investigations using seismic, gravitational, magnetic, electrical, and thermal measurements.

The field divides broadly into two branches:

1. Global geophysics — concerned with large-scale Earth structure: the crust-mantle boundary (the Mohorovičić discontinuity, or "Moho"), the liquid outer core, the solid inner core, and Earth's geomagnetic field.
2. Applied (exploration) geophysics — focused on near-surface and crustal investigations for resource extraction, earthquake hazard assessment, engineering site characterization, and environmental monitoring.

Both branches draw on the same physical principles but differ dramatically in scale. Global geophysics works with data from networks of hundreds of seismometers spanning continents. Applied geophysics might deploy a dozen sensors across a construction site.

Geophysics connects directly to plate tectonics, seismology, and the rock cycle — disciplines that depend on geophysical data for their foundational models.

How it works

Every geophysical method exploits a measurable physical contrast between materials. Rock types differ in density, magnetic susceptibility, electrical resistivity, and seismic wave velocity. Instruments detect those contrasts from the surface — or from satellites and aircraft — and inversion algorithms convert the measurements into subsurface models.

The four dominant methods work as follows:

1. Seismic methods — Mechanical waves are generated (by explosives, air guns, or vibrating trucks) and timed as they refract or reflect off subsurface interfaces. Velocity contrasts reveal lithology changes and structural geometry. The USGS Earthquake Hazards Program uses passive seismic networks that record natural earthquakes for the same purpose.

2. Gravity surveys — A gravimeter measures minute variations in gravitational acceleration across a survey area. Dense rock pulls the meter slightly harder; low-density sediment or voids pull less. The difference, measured in milligals (1 milligal = 10⁻⁵ m/s²), maps subsurface density variations without any energy source.

3. Magnetic surveys — Magnetometers record variations in Earth's magnetic field caused by iron-bearing minerals. Surveys flown by aircraft at 100–300 meters altitude can map geological structures across hundreds of square kilometers in days — a resolution impossible with ground methods at that scale.

4. Electrical resistivity tomography (ERT) — Current is injected into the ground through electrodes; voltage is measured at other electrodes. Resistivity contrast distinguishes clay from sand, water-saturated from dry sediment, or contaminated groundwater from clean aquifer.

Seismic methods deliver the highest spatial resolution of any remote subsurface technique, which is why groundwater and aquifer systems investigations and oil exploration both rely on them as primary tools. The trade-off: they are also the most logistically intensive and expensive to deploy.

For a broader look at how measurement and inference function across scientific disciplines, how science works as a conceptual overview provides useful framing for understanding why geophysical models are probabilistic rather than deterministic.

Common scenarios

Geophysics appears in contexts that range from the dramatic to the quietly utilitarian.

• Earthquake hazard mapping — Seismic reflection surveys delineate fault geometry at depth, informing the probabilistic hazard maps published by the USGS National Seismic Hazard Model.
• Mineral and hydrocarbon exploration — Reflection seismology and airborne magnetics guide decisions worth billions of dollars before a single drill bit touches rock.
• Volcanic monitoring — Seismic tomography images magma bodies beneath volcanoes, helping scientists estimate eruption potential (USGS Volcano Hazards Program).
• Engineering site assessment — A ground-penetrating radar survey across a proposed bridge foundation can locate buried voids, old landfill boundaries, or unconsolidated sediment layers in hours.
• Archaeological prospection — Resistivity and ground-penetrating radar surveys locate buried structures without excavation, a practice now standard in European and North American field archaeology.

Decision boundaries

Choosing a geophysical method is a problem of matching physical contrast to instrument sensitivity and survey geometry. The decision tree is not arbitrary.

Seismic vs. gravity: Seismic is preferred when structural resolution matters — fault locations, layer thicknesses, basin geometry. Gravity is preferred when the target is a broad density anomaly, such as a buried granite intrusion, that doesn't require meter-scale precision.

Active vs. passive methods: Active methods (seismic reflection, ERT) inject energy and offer controlled acquisition but require permits, equipment, and sometimes vibration restrictions near infrastructure. Passive methods (magnetometry, gravity, ambient seismic noise) measure naturally occurring fields, reducing logistical burden at the cost of some resolution control.

Depth vs. resolution trade-off: In virtually every geophysical method, deeper investigation requires longer wavelengths or larger electrode spacings — which reduces spatial resolution. A survey optimized to image 500 meters depth cannot simultaneously resolve features 2 meters across. This is physics, not engineering limitation.

The earth science tools and technologies overview expands on instrumentation families used across geoscience disciplines, including those shared with geophysics. For context on how geophysics fits within the broader structure of earth science, the key dimensions and scopes of earth science page maps the full disciplinary landscape. The home reference index provides navigation across the full range of earth science topics covered on this site.