Earth systems and geophysics

Reverse time migration

Reverse time migration (RTM) is a seismic imaging method that propagates recorded wavefields backward in time using the full two-way wave equation, introduced in the early 1980s.

Revised Universal Soil Loss Equation

The Revised Universal Soil Loss Equation (RUSLE) is an empirical model estimating annual soil loss from water erosion, released in 1992 as a computerized USLE.

Satellite clock estimation determines GNSS satellite clock offsets and drifts from code and carrier-phase observations at ground stations, underpinning precise point positioning and real-time navigation.

Satellite laser ranging

Satellite laser ranging (SLR) is a geodetic technique that measures round-trip laser pulse times to satellites carrying retroreflectors, deriving orbits, station positions, and Earth's gravity field.

Schlumberger array

The Schlumberger array is a four-electrode configuration for measuring the electrical resistivity of the ground, with closely spaced potential electrodes, and the most common configuration for vertical electrical sounding.

Schumann resonances

Schumann resonances, also called Earth-ionosphere cavity resonances, are electromagnetic oscillations near 8, 14, 20, 27, and 34 Hz excited by global lightning, predicted in 1952.

SECOR (satellite)

SECOR (Sequential Collation of Range), also designated EGRS, was a United States Army geodetic satellite program of 1964 to 1969 that fixed remote positions by electronic trilateration.

Seismic attribute analysis

Seismic attribute analysis is a geophysics method that extracts quantitative measures from seismic reflection data to characterize subsurface structure, stratigraphy, and fluid content, revealing faults without hand picking.

Seismic attributes

A seismic attribute is a quantity extracted or derived from seismic data, such as amplitude or frequency, analyzed to enhance subtle information and improve geological interpretation.

Seismic facies analysis

Seismic facies analysis is a geophysical interpretation method that classifies groups of seismic reflections into mappable facies units, inferring depositional environments, lithofacies, and reservoir properties of the subsurface.

Seismic interferometry

Seismic interferometry, also known as noise interferometry, is a geophysical method that estimates the Green's function between two receivers by cross-correlating their recordings, introduced to seismology by Jon Claerbout.

Seismic inversion

Seismic inversion is a geophysical method that converts recorded seismic reflection amplitudes, constrained by well logs, into estimates of subsurface properties like velocity, density, and acoustic impedance.

Seismic microzonation

Seismic microzonation is a geotechnical and seismological method that divides a city or municipality into zones of similar seismic behavior, refining national hazard maps with local soil conditions.

Seismic migration

Seismic migration is a geophysical imaging method that repositions recorded seismic reflection events to their true subsurface locations, producing an image of geological structures in the Earth's interior.

Seismic moment

Seismic moment (symbol M0) is the physical quantity seismologists use to measure earthquake size: the product of rock rigidity, fault rupture area, and average slip.

Seismic network

A seismic network is a coordinated set of seismometers at distributed locations whose recordings are combined at a data center to detect, locate, and characterize earthquakes.

Seismic phase picking

Seismic phase picking is the task of determining the arrival times of P and S waves on seismograms, a prerequisite for earthquake location and early warning.

Seismic refraction

Seismic refraction is a geophysical surveying method that infers subsurface layer velocities, depths, and dips from refracted wave travel times; it was the first seismic technique used in oil exploration.

Seismic refraction tomography

Seismic refraction tomography (SRT) is a geophysical imaging method that inverts first-arrival seismic travel times into a grid model of subsurface P-wave velocity, used from engineering site characterization to crustal-scale experiments.

Seismic survey

A seismic survey is a geophysical exploration method that generates seismic waves and records reflected returns, imaging subsurface rock layers for oil, gas, and mineral exploration.

Seismic survey vessel

A seismic survey vessel is a specialized ship that tows acoustic sources and hydrophone streamers to image rock layers beneath the seabed, mainly for offshore oil and gas exploration.

Seismic tomography

Seismic tomography is an imaging method that reconstructs three-dimensional maps of Earth's interior from seismic wave travel times recorded by seismometer networks, adapted from medical imaging in the 1970s.

Seismology (σεισμολογία)

Seismology, the study of earthquakes and the Earth's structure through seismic waves, is the principal means of imaging the Earth's interior and underpins earthquake monitoring, hazard assessment, early warning, and nuclear test ban verification.

ShakeAlert

ShakeAlert is the USGS earthquake early warning system for the West Coast, detecting quakes that have already started and alerting people in California, Oregon, and Washington since 2019.

Shear wave splitting

Shear wave splitting is a seismological technique in which anisotropic rock splits a passing shear wave into two waves of different speed and polarization, revealing mantle deformation.

Small Baseline Subset

Small Baseline Subset (SBAS) is a differential InSAR time-series method, reported in 2002, that measures ground deformation from stacks of short-baseline SAR interferograms, producing velocity maps and displacement time series.

Snowball Earth

Snowball Earth, also known as the Snowball Earth hypothesis, is the idea that Earth's surface was almost entirely frozen from poles to tropics several times in the distant past.

Soil

Soil is the unconsolidated mineral and organic material on Earth's surface that serves as the natural medium for land plants, formed over hundreds to thousands of years.

Soil acidification

Soil acidification is the process by which soil pH declines over time, affecting roughly 30 to 40 percent of agricultural land globally, with liming as the main remedy.