Earth systems and geophysics

Topsoil

Topsoil, also known as surface soil, is the uppermost and most fertile soil layer, holding most organic matter and plant roots, and the part most lost to erosion.

Transient electromagnetic method

The transient electromagnetic method (TEM), also called time-domain electromagnetic or TDEM surveying, maps subsurface conductivity from the decay of induced electromagnetic fields after transmitter turn-off.

Transverse Mercator projection

The Transverse Mercator projection, also called the Gauss-Krüger projection, is a conformal map projection whose scale is true along a central meridian, used in UTM and the British National Grid.

Travel-time tomography

Travel-time tomography is an imaging method that inverts seismic wave arrival times to reconstruct the Earth's internal velocity structure, adapted from medical imaging in the 1970s.

Traveltime tomography

Traveltime tomography is an inverse method that reconstructs a medium's internal velocity structure from measured wave travel times, imaging the Earth's subsurface from oil fields to the entire mantle.

Tundra

Tundra, also called the tundra biome, is a treeless biome of the Arctic and high mountains where permafrost, permanently frozen subsoil, underlies a short growing season.

Tunguska event

The Tunguska event, also called the Tunguska blast, was a powerful airburst over central Siberia on June 30, 1908, flattening about 2,150 square kilometers of forest without a crater.

Ultisols

Ultisols are one of the twelve orders of USDA Soil Taxonomy, strongly weathered acidic forest soils of humid temperate and tropical regions with base saturation below 35 percent.

UNAVCO

UNAVCO was a Boulder, Colorado non-profit consortium, founded in 1984 to share GPS equipment and archive geodetic data for Earth science research, merging with IRIS in 2023.

Universal Soil Loss Equation

The Universal Soil Loss Equation (USLE) is an empirical model that estimates long-term average annual soil loss from sheet and rill erosion using six factors for rainfall, soil, topography, cover, and support practices.

Upper mantle

The upper mantle is the outermost part of Earth's mantle, extending from the Moho down to the 660-km discontinuity and hosting the lithosphere and asthenosphere that carry tectonic plates.

Upward continuation

Upward continuation is a potential-field technique that recalculates gravity or magnetic data at a higher elevation, acting as a low-pass filter that suppresses shallow-source noise.

USArray

USArray is a continental-scale seismic observatory and component of NSF's EarthScope program, funded in 2003 to record earthquakes and image the structure beneath North America with dense seismograph networks.

USDA Soil Taxonomy

USDA Soil Taxonomy is a hierarchical soil classification system developed by the United States Department of Agriculture under Guy Smith, published in 1975 and used worldwide.

Velocity analysis (seismology)

Velocity analysis is a seismic processing method estimating subsurface velocities from how reflection arrival times vary with source–receiver offset, producing a velocity spectrum of semblance values.

Velocity model building

Velocity model building is the process of constructing a 3D model of seismic wave speeds in the subsurface, iteratively updated so depth migration images reflectors at their true positions.

Vertical derivative (potential-field method)

In use since 1936, the vertical derivative is a transformation of gravity and magnetic data that narrows anomalies, helping interpreters locate the rock bodies that cause them.

Vertical electrical sounding

Vertical electrical sounding (VES) is a surface geophysical method that measures subsurface resistivity at increasing electrode spacings, producing a one-dimensional resistivity-versus-depth profile used mainly for groundwater prospecting.

Vertical seismic profile

A vertical seismic profile is a borehole geophysical method that records seismic arrivals from surface sources using receivers placed at depth in a well, providing higher-resolution velocity and imaging information than surface seismic surveys.

Vertical seismic profiling

Vertical seismic profiling (VSP) is a borehole seismic-reflection method using receivers clamped in a well, recording both downgoing and upgoing wavefields for depth calibration and imaging.

Vertisols

Vertisols are a soil order in USDA Soil Taxonomy, clayey soils rich in expanding clay that shrink when dry and swell when wet, opening deep cracks in most years.

Vibroseis

Vibroseis is a seismic surveying method in which a truck-mounted hydraulic vibrator radiates a frequency-modulated signal into the ground, replacing explosives; it was developed by Conoco in the 1950s.

Vine–Matthews–Morley hypothesis

The Vine–Matthews–Morley hypothesis (VMM), proposed in 1963 by Frederick Vine, Drummond Matthews, and Lawrence Morley, explains the ocean floor's striped magnetic anomalies as records of Earth's magnetic field reversals.

Viridien

Viridien, known as CGG (Compagnie Générale de Géophysique) until its 2024 rebrand, is a French geoscience, seismic imaging, and Earth data company headquartered in Paris, with about 3,500 employees worldwide.

Wavefield reconstruction

Wavefield reconstruction is a geophysical method that recovers a complete seismic or acoustic wavefield from sparse, irregularly sampled data, a prerequisite for full waveform inversion and least-squares migration.

Wavefield separation

Wavefield separation is a signal-processing method in seismology that decomposes recorded seismic data into components such as upgoing and downgoing waves or P- and S-waves, feeding migration and full-waveform inversion.

Waveform tomography

Waveform tomography, known as full waveform inversion (FWI), is an iterative seismic method that estimates subsurface velocity structure by minimizing the difference between recorded and synthetic waveforms.

Well tie

A well tie, or seismic-well tie, is the alignment of a synthetic seismogram computed from well logs with the actual seismic trace at the borehole, linking depth to time.

Wet sieving

Wet sieving is a particle size analysis method that washes a sample through stacked sieves with water, separating particles into size ranges down to about 20 µm.