Earth systems and geophysics

Soil catena

A soil catena is a sequence of related soils along a hillslope, from hilltop to valley floor, that share parent material and age but differ because of drainage.

Soil classification is the systematic grouping of soils into named categories based on measurable properties, dominated by USDA Soil Taxonomy with 12 orders and the international World Reference Base.

Soil contamination

Soil contamination, also known as soil pollution, is the presence of chemicals in soil above natural background levels, posing risks to human health, ecosystems, food, and water.

Soil horizon

A soil horizon is a layer of soil roughly parallel to the surface that differs from adjacent layers in color, texture, and structure, forming the soil profile.

Soil incubation experiments

A soil incubation experiment is a laboratory method in which soil is held under controlled temperature and moisture while carbon or nitrogen mineralization is measured.

Soil pH

Soil pH, also known as soil acidity or soil reaction, measures how acid or alkaline a soil is and controls which nutrients plants can take up.

Soil science

Soil science is the study of soils as a natural resource, covering how they form, their classification and mapping, and their physical, chemical, and biological properties.

Soil sealing

Soil sealing is the covering of ground by impermeable materials such as asphalt or concrete, considered a major and nearly irreversible cause of soil degradation in the European Union.

Soil texture

Soil texture, also called soil texture classification, is the relative proportion by weight of sand, silt, and clay in a soil, read on the USDA textural triangle.

Soil type

A soil type is a class of soil defined by properties such as texture, color, and structure, classified in systems like USDA Soil Taxonomy and the WRB.

Sonic logging

Sonic logging is a borehole geophysical method that measures the interval transit time of acoustic waves through the rock around a well, used to estimate porosity and elastic moduli.

Southern California Seismic Network

The Southern California Seismic Network, operated jointly by Caltech and the U.S. Geological Survey, monitors earthquakes across southern California, keeping a continuous catalog of over 1,000,000 events since 1932.

Spectral decomposition (seismic)

Spectral decomposition in seismology is a family of time-frequency analysis methods that breaks a seismic trace into its frequency components, revealing bed thickness variations, discontinuities, and fluid effects.

Spherical Earth

Spherical Earth, also called the sphericity of the Earth, is the model of Earth's figure as a sphere, argued from antiquity and refined into an oblate spheroid.

Spodosol

A Spodosol is a soil order in USDA soil taxonomy, defined by a spodic horizon of accumulated organic matter and aluminum, known as Podzol in the World Reference Base.

Square array (electrical resistivity)

The square array is a four-electrode resistivity surveying configuration introduced by Habberjam and Watkins in 1967, used chiefly to detect the directional fabric of fractured bedrock.

Sulfur cycle

The sulfur cycle is the set of pathways by which sulfur moves among Earth's crust, oceans, atmosphere, and living matter, moving over 50 million tons yearly.

Surface conductivity (geophysics)

Surface conductivity in geophysics is an extra electrical conductivity of porous rock or soil that flows along charged mineral surfaces rather than through pore water, and ignoring it biases resistivity interpretation in clays and shaly sands.

Surface wave tomography

Surface wave tomography is a seismological imaging method that converts measurements of how seismic surface waves disperse into 3D shear-wave velocity models of the crust and upper mantle.

Taiga

The taiga, also called the boreal forest, is the belt of conifer forest across northern North America and Eurasia south of the tundra, holding 27 percent of the world's forest area.

Technosols

Technosols are soils dominated by human-made material, with 20 percent or more artifacts, one of the 32 Reference Soil Groups of the World Reference Base.

Telluric current

A telluric current, also called an Earth current, is a natural electric current flowing through the Earth's crust, oceans, or mantle, driven by magnetic-field variations from the Sun and lightning.

Tensiometer (soil science)

A tensiometer is a field instrument that measures the matric potential of soil water through a water-saturated porous ceramic cup, used chiefly to schedule irrigation in medium to fine textured soils.

Tensor tomography

Tensor tomography is the reconstruction of a tensor field, such as anisotropic elastic or transport properties, from its integrals along ray paths, generalizing computed tomography's X-ray transform.

TerraSAR-X

TerraSAR-X is a German Earth-observation satellite carrying an X-band synthetic aperture radar, launched in 2007 to image Earth in all weather; it later flew with its twin TanDEM-X.

Terrestrial heat flow

Terrestrial heat flow is the rate at which heat escapes from the Earth's interior through its surface, measured in milliwatts per square meter and compiled in the Global Heat Flow Database.

Test pit (soil survey)

A test pit, or soil pit, is an excavation dug during a soil survey so the soil profile can be observed in place and sampled by horizon.

Thermo-hydro-mechanical coupling

Thermo-hydro-mechanical (THM) coupling is a modeling framework that solves heat transfer, fluid flow, and mechanical deformation together in porous media, originating in nuclear waste disposal.

Time reversal imaging

Time reversal imaging is a wavefield method that reverses recorded signals and propagates them back through a medium model, refocusing energy on the original source or scatterer.