What Are Geophysical Measurements?

Geophysical measurements are quantitative observations of the physical properties of the Earth, gathered through non-invasive or minimally invasive techniques to characterize subsurface geology, atmospheric conditions, and soil composition without direct sampling. These measurements exploit the contrasts in density, magnetic susceptibility, electrical resistivity, elastic wave velocity, and radioactivity that distinguish different rock types, soil layers, and buried structures. The field draws from classical physics, applied mathematics, and instrumentation engineering, and its methods underpin natural resource exploration, environmental monitoring, and hazard assessment.

Geophysical measurements are organized around the physical phenomenon being observed. Passive methods record naturally occurring signals, such as Earth's gravity field or ambient seismic noise, while active methods introduce a controlled stimulus and record the response, as in reflection seismology or ground-penetrating radar. Measurements may be collected at the surface, in boreholes, from aircraft, or from satellites, with each acquisition geometry offering different trade-offs in resolution and depth of investigation.

Seismic and Subsurface Methods

Seismic measurements record the propagation of elastic waves through the subsurface and are the principal tool for imaging geological structure at depth. In reflection surveys, a controlled source generates compressional or shear waves that reflect off impedance contrasts between rock layers; hydrophone or geophone arrays then record the returning wavefield. The U.S. Geological Survey uses seismic instrumentation extensively for both crustal imaging and earthquake monitoring. Refraction surveys complement reflection work by measuring wave travel times to determine velocity profiles, while borehole seismology provides high-resolution measurements at depth.

Gravity, Magnetic, and Electrical Methods

Gravity surveys measure lateral variations in Earth's gravitational acceleration caused by density contrasts in the crust. A gravimeter capable of resolving differences of a few microgals can detect subsurface voids, ore bodies, or sedimentary basins. Magnetic surveys map variations in the geomagnetic field arising from differences in rock magnetization, with airborne magnetometry widely used in mineral and petroleum exploration. As documented by the USGS geophysical methods program, electrical and electromagnetic methods measure resistivity and induced polarization to identify conductive targets such as sulfide ore deposits, contaminated groundwater, and buried infrastructure.

Atmospheric and Soil Measurements

Atmospheric geophysical measurements include radiosonde profiling, radar wind profiling, and lidar backscatter, all of which characterize temperature, humidity, and wind structure in the troposphere and stratosphere. Soil measurements use electrical resistivity tomography, time-domain reflectometry, and ground-penetrating radar to map soil moisture, stratigraphy, and buried objects, including unexploded ordnance or archaeological features. The integration of these surface and near-surface datasets with deeper subsurface imagery relies on geophysical signal processing to correct for noise, geometric distortions, and instrument response, as described in the SEG's Geophysical Signal Analysis reference.

Applications

Geophysical measurements have applications in a range of fields, including:

• Petroleum and natural gas exploration, using seismic reflection surveys to map reservoir geometry
• Mineral exploration, using airborne magnetic and gravity surveys to locate ore deposits
• Environmental site characterization, detecting contaminant plumes and buried waste
• Earthquake hazard assessment, measuring crustal structure and seismicity rates
• Archaeological prospection, imaging buried structures with ground-penetrating radar
• Hydrological investigations, mapping aquifer extent and depth with electrical resistivity methods