Earth systems and geophysics

Geodesy

Geodesy, also called Erdmessung or Geodetic Science, is the science of measuring the Earth's shape, orientation in space, and gravity field, and how they change.

Geoid

The geoid is the equipotential surface of Earth's gravity field that best fits global mean sea level; it is irregular, undulating above and below a reference ellipsoid.

Geomagnetic reversal

A geomagnetic reversal, also called a magnetic pole reversal, is a change of Earth's magnetic field to the opposite polarity; the last occurred about 780,000 years ago.

Geomagnetic secular variation

Geomagnetic secular variation is the slow change in Earth's core-generated main magnetic field over years to centuries, at rates up to about 200 nanotesla per year.

Geomagnetically induced current

A geomagnetically induced current (GIC) is an electric current driven through long conductors such as power lines by geomagnetic storms; it caused the 1989 Québec blackout.

Geophysical imaging

Geophysical imaging is a family of earth-science methods that reconstructs models of subsurface structures and properties such as seismic velocity, resistivity, and density from field measurements.

A geophysical survey maps buried features and geological structure by remotely measuring physical properties of the subsurface, with magnetometry, resistivity tomography, and ground-penetrating radar the most used techniques.

Geophysics

Geophysics, also called Earth physics, is the natural science applying physics to Earth's structure and dynamics, using gravity, magnetic, electrical, and seismic measurements to infer subsurface properties.

Geopotential model

A geopotential model is a mathematical representation of Earth's gravitational potential, delivered as spherical harmonic coefficients from which geoid undulations, gravity anomalies, and satellite orbits are computed.

George Everest

Sir George Everest (1790–1866) was a Welsh military engineer who completed the Great Trigonometrical Survey of India; Mount Everest was named in his honor in 1865.

Geostatistical inversion

Geostatistical inversion is a stochastic seismic inversion method, introduced in 1993, that generates multiple equiprobable property models conditioned to seismic data and well logs, quantifying uncertainty.

Geothermal gradient

The geothermal gradient, also called the geothermic gradient, is the rate at which temperature rises with depth in the Earth, averaging about 25–30 °C per kilometer near the surface.

Gleysols

Gleysols are a Reference Soil Group (GL) of the World Reference Base for soils affected by shallow groundwater, with wetness limiting them largely to swamp vegetation and grazing.

Global Seismographic Network

The Global Seismographic Network (GSN) is a global network of about 150 seismic stations, established in 1988, that records earthquakes and provides its data free in real time.

Gravimetry

Gravimetry is the measurement of gravitational acceleration and its variations in space and time, using instruments on the ground, ships, aircraft, or satellites to infer Earth's mass distribution.

Gravity anomaly

A gravity anomaly is the difference between measured gravity, reduced to sea level, and the theoretical gravity of a uniform ellipsoidal Earth, revealing rock density variations.

Gravity gradiometer

A gravity gradiometer is an instrument that measures the rate at which gravity changes from one point to a nearby point, used in resource exploration and passive navigation.

Gravity gradiometry

Gravity gradiometry is a geophysical surveying method that measures how gravitational acceleration changes from point to point, revealing subsurface density structure from aircraft or satellites.

Gravity inversion

Gravity inversion is the geophysical process of estimating a subsurface density model from measured gravity anomalies. It is non-unique and unstable, so regularization and constraints are needed.

Gutenberg discontinuity

The Gutenberg discontinuity is the seismic boundary about 2,900 km down where Earth's solid mantle meets its liquid iron outer core, named for Beno Gutenberg.

Histosols

Histosols, commonly called bogs, moors, peats, or mucks, are soils formed in thick deposits of decaying plant material with no permafrost, one of the twelve USDA soil orders.

Hollow Earth

The Hollow Earth theory, or Hohlweltlehre, holds that Earth is hollow with habitable inner worlds; proposed by Edmond Halley, it was refuted by geophysics and survives in fiction.

Horizontal-to-vertical spectral ratio

The horizontal-to-vertical spectral ratio (HVSR) is a single-station seismological method that estimates a site's fundamental resonance frequency from ambient vibration or earthquake recordings using one three-component seismometer.

Hypocenter

A hypocenter, also called the focus, is the point within the Earth where an earthquake rupture starts; the epicenter lies directly above, and focal depth strongly affects shaking intensity.

Imaging condition

An imaging condition is the mathematical rule combining forward-propagated source and backward-propagated receiver wavefields into a subsurface reflectivity image, the final step of seismic migration.

Impact of the COVID-19 pandemic on the environment

The COVID-19 pandemic caused a 6.4% drop in global carbon dioxide emissions in 2020, alongside air quality changes, rising plastic waste, and mostly negative effects on conservation.

Inceptisols

Inceptisols are a major order of USDA Soil Taxonomy: weakly to moderately weathered soils covering about 17 percent of global ice-free land, more than any other soil order.

Induced polarization

Induced polarization is a geophysical method measuring the delayed voltage decay of subsurface materials after an injected current is cut off, reporting chargeability and phase, used mainly to explore for disseminated metallic sulfides.