Editing Geophysics

{{short description|Physics of the Earth and its vicinity}}
{{Use dmy dates|date=April 2014}}
{{Geophysics}}

'''Geophysics''' ({{IPAc-en|ˌ|dʒ|iː|oʊ|ˈ|f|ɪ|z|ɪ|k|s}}) is a subject of [[natural science]] concerned with the physical processes and [[Physical property|properties]] of [[Earth]] and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct investigations across a wide range of scientific disciplines. The term ''geophysics'' classically refers to [[solid earth]] applications: Earth's [[figure of the Earth|shape]]; its [[gravitational]], [[Earth's magnetic field|magnetic fields]], and [[electromagnetic field]]s; its [[structure of the Earth|internal structure]] and [[Earth#Chemical composition|composition]]; its [[geodynamics|dynamics]] and their surface expression in [[plate tectonics]], the generation of [[magma]]s, [[volcanism]] and rock formation.<ref name=Sheriff1991>{{harvnb|Sheriff|1991}}</ref> However, modern geophysics organizations and pure scientists use a broader definition that includes the [[water cycle]] including snow and ice; [[geophysical fluid dynamics|fluid dynamics]] of the oceans and the [[atmosphere]]; [[atmospheric electricity|electricity]] and [[magnetism]] in the [[ionosphere]] and [[magnetosphere]] and [[solar-terrestrial physics]]; and analogous problems associated with the [[Moon]] and other planets.<ref name=Sheriff1991/><ref name=IUGG>{{harvnb|IUGG|2011}}</ref><ref name=AGUscience>{{harvnb|AGU|2011}}</ref><ref name="Gutenberg (1929)">Gutenberg, B., 1929, Lehrbuch der Geophysik. Leipzig. Berlin (Gebruder Borntraeger).</ref><ref name="Runcorn">Runcorn, S.K, (editor-in-chief), 1967, International dictionary of geophysics:. Pergamon, Oxford, 2 volumes, 1,728 pp., 730 fig</ref><ref name="Geophysics">Geophysics, 1970, Encyclopaedia Britannica, Vol.10, p. 202-202</ref>

Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times. The first magnetic compasses were made from [[lodestone]]s, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. [[Isaac Newton]] applied his theory of mechanics to the tides and the [[precession of the equinox]]; and instruments were developed to measure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, and geophysics played an essential role in the development of the theory of plate tectonics.

Geophysics is pursued for fundamental understanding of the Earth its space environment. Geophysics often addresses societal needs, such as [[mineral resources]], assessment and [[Mitigation#Disaster_mitigation|mitigation]] of [[natural hazards]] and [[environmental impact assessment]].<ref name=IUGG/>  In [[exploration geophysics]], [[geophysical survey]] data are used to analyze potential petroleum reservoirs and mineral deposits, locate groundwater, find archaeological relics, determine the thickness of glaciers and soils, and assess sites for [[environmental remediation]].
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== Physical phenomena ==
Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of the [[Earth sciences]], while some geophysicists conduct research in the [[planetary sciences]]. To provide a more clear idea on what constitutes geophysics, this section describes phenomena that are studied in [[physics]] and how they relate to the Earth and its surroundings. Geophysicists also investigate the physical processes and properties of the Earth, its fluid layers, and magnetic field along with the near-Earth environment in the [[Solar System]], which includes other planetary bodies.

=== Gravity ===
[[Image:Earth gravity.png|thumb|A map of deviations in gravity from a perfectly smooth, idealized Earth|alt=Image of globe combining color with topography.]]
{{Main|Gravity of Earth}}
{{further|Physical geodesy|Gravimetry}}
The gravitational pull of the Moon and Sun gives rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide.<ref>{{harvnb|Ross|1995|pp=236–242}}</ref>

Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.<ref name=Poirier>{{harvnb|Poirier|2000}}</ref> Measurements of [[gravitational acceleration]] and [[gravitational potential]] at the Earth's surface and above it can be used to look for mineral deposits (see [[gravity anomaly]] and [[gravimetry]]).<ref name=Telford/> The surface gravitational field provides information on the dynamics of [[tectonic plates]]. The [[geopotential]] surface called the [[geoid]] is one definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).<ref name=Lowrie/>

=== Vibrations ===
{{Main|Seismology}}
[[Image:Pswaves.jpg|thumb|upright=1.3|Illustration of the deformations of a block by body waves and surface waves (see [[seismic wave]]) |alt=Deformed blocks with grids on surface.]]
[[Seismic wave]]s are vibrations that travel through the Earth's interior or along its surface.<ref>{{Cite web |date=2024-01-12 |title=Seismic wave {{!}} Earth's Interior Structure & Movement {{!}} Britannica |url=https://www.britannica.com/science/seismic-wave |access-date=2024-02-18 |website=www.britannica.com |language=en}}</ref> The entire Earth can also oscillate in forms that are called [[normal modes]] or [[seismic wave#Normal modes|free oscillations of the Earth]]. Ground motions from waves or normal modes are measured using [[seismograph]]s. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.<ref name=Shearer>{{cite book|last=Shearer|first=Peter M.|title=Introduction to seismology|year=2009|publisher=Cambridge University Press|location=Cambridge|isbn=9780521708425|edition=2nd}}</ref><ref name=Stein>{{harvnb|Stein|Wysession|2003}}</ref>

Recording of [[seismic wave]]s from controlled sources provides information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using [[Reflection seismology|Reflection Seismology]] can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas.<ref name=Telford>{{harvnb|Telford|Geldart|Sheriff|1990}}</ref> Changes in the travel direction, called [[Seismic refraction|refraction]], can be used to infer the [[Earth's interior|deep structure of the Earth]].<ref name=Stein/>

Earthquakes pose a [[Earthquakes#Effects/impacts of earthquakes|risk to humans]]. Understanding their mechanisms, which depend on the type of earthquake (e.g., [[Intraplate earthquake|intraplate]] or [[Deep focus earthquake|deep focus]]), can lead to better estimates of earthquake risk and improvements in [[earthquake engineering]].<ref name=Bozorgnia2004>{{harvnb|Bozorgnia|Bertero|2004}}</ref>

=== Electricity ===
Although we mainly notice electricity during [[thunderstorms]], there is always a downward electric field near the surface that averages 120 [[volt]]s per meter.<ref name=Harrison>{{harvnb|Harrison|Carslaw|2003}}</ref> Relative to the solid Earth, the ionization of the planet's atmosphere is a result of the galactic [[cosmic rays]] penetrating it, which leaves it with a net positive charge.<ref>{{Cite web |last=Nicoll |first=Keri |date=April 2016 |title=Earth's electric atmosphere |url=https://www.metlink.org/wp-content/uploads/2020/11/PhysRev25_4_Nicoll.pdf |access-date=February 18, 2024 |website=metlink.org}}</ref> A current of about 1800 [[ampere]]s flows in the global circuit.<ref name=Harrison/> It flows downward from the [[ionosphere]] over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds and [[sprite (lightning)|sprite]]s above.

A variety of electric methods are used in geophysical survey. Some measure [[spontaneous potential]], a potential that arises in the ground because of human-made or natural disturbances. [[Telluric current]]s flow in Earth and the oceans. They have two causes: [[electromagnetic induction]] by the time-varying, external-origin [[geomagnetic field]] and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field.<ref>{{harvnb|Lanzerotti|Gregori|1986}}</ref> The distribution of telluric current density can be used to detect variations in [[electrical resistivity]] of underground structures. Geophysicists can also provide the electric current themselves (see [[induced polarization]] and [[electrical resistivity tomography]]).

=== Electromagnetic waves ===
[[Electromagnetic waves]] occur in the ionosphere and magnetosphere as well as in [[Earth's outer core]]. [[Dawn chorus (electromagnetic)|Dawn chorus]] is believed to be caused by high-energy electrons that get caught in the [[Van Allen radiation belt]]. [[Whistler (radio)|Whistlers]] are produced by [[lightning]] strikes. [[Hiss (electromagnetic)|Hiss]] may be generated by both. [[Electromagnetic radiation|Electromagnetic]] waves may also be generated by earthquakes (see [[seismo-electromagnetics]]).

In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. [[Alfvén wave]]s are [[magnetohydrodynamic]] waves in the [[magnetosphere]] or the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic [[Rossby wave]]s may be one source of [[geomagnetic secular variation]].<ref name="Merrill">{{harvnb|Merrill|McElhinny|McFadden|1998}}</ref>

Electromagnetic methods that are used for geophysical survey include [[transient electromagnetics]], [[magnetotellurics]], [[surface nuclear magnetic resonance]] and electromagnetic seabed logging.<ref>{{cite book |last1=Stéphane |first1=Sainson |title=Electromagnetic seabed logging : a new tool for geoscientists |date=2017 |publisher=Springer |isbn=978-3-319-45355-2}}</ref>

=== Magnetism ===
{{further|Earth's magnetic field|Aeromagnetic survey|Paleomagnetism}}
The Earth's magnetic field protects the Earth from the deadly [[solar wind]] and has long been used for navigation. It originates in the fluid motions of the outer core.<ref name=Merrill/> The magnetic field in the upper atmosphere gives rise to the [[Aurora (astronomy)|auroras]].<ref name=Kivelson>{{harvnb|Kivelson|Russell|1995}}</ref>

[[Image:Geomagnetisme.svg|thumb|right|upright=1.1|Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line). |alt=Diagram with field lines, axes and magnet lines.]][[File:Geodynamo Between Reversals.gif|thumb|left|Computer simulation of the [[Earth's magnetic field]] in a period of normal polarity between [[Geomagnetic reversal|reversals]]<ref>{{cite news |title=Earth's Inconstant Magnetic Field |url=https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield |access-date=13 November 2018 |work=science@nasa |publisher=National Aeronautics and Space Administration |date=29 December 2003 |language=en}}</ref>]]
The Earth's field is roughly like a tilted [[dipole]], but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the [[geomagnetic pole]] stays near the [[geographic pole]], but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These [[geomagnetic reversals]], analyzed within a [[Geomagnetic reversal#Geomagnetic polarity time scale|Geomagnetic Polarity Time Scale]], contain 184 polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the [[Laschamp event]] occurring 41,000 years ago during the [[Glacial period#Last glacial period|last glacial period]]. Geologists observed [[Geomagnetic reversal#History|geomagnetic reversal recorded]] in volcanic rocks, through [[Magnetostratigraphy#Correlation and ages|magnetostratigraphy correlation]] (see [[natural remanent magnetization]]) and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on [[seafloor spreading]], a part of plate tectonics. They are the basis of [[magnetostratigraphy]], which correlates magnetic reversals with other [[Stratigraphic section|stratigraphies]] to construct geologic time scales.<ref name=Opdyke>{{harvnb|Opdyke|Channell|1996}}</ref> In addition, the [[paleomagnetism|magnetization in rocks]] can be used to measure the motion of continents.<ref name=Merrill/>

=== Radioactivity ===
{{further|Radiometric dating}}
[[Image:Thorium decay chain from lead-212 to lead-208.svg|thumb|Example of a radioactive decay chain (see [[Radiometric dating]]) |alt=Diagram with compound balls representing nuclei and arrows.]]
[[Radioactive decay]] accounts for about 80% of the Earth's [[internal heat]], powering the geodynamo and plate tectonics.<ref name=Turcotte>{{harvnb|Turcotte|Schubert|2002}}</ref> The main heat-producing [[isotopes]] are [[Potassium|potassium-40]], [[Uranium|uranium-238]], uranium-235, and [[Thorium|thorium-232]].<ref>{{harvnb|Sanders|2003}}</ref>
Radioactive elements are used for [[radiometric dating]], the primary method for establishing an absolute time scale in [[geochronology]].

Unstable isotopes decay at predictable rates, and the decay rates of different [[isotope]]s cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past [[Era (geology)|geologic eras]].<ref name=Renne>{{harvnb|Renne|Ludwig|Karner|2000}}</ref> Radiometric mapping using ground and airborne [[gamma spectrometry]] can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.<ref>{{cite web|title=Radiometrics|url=http://www.ga.gov.au/scientific-topics/disciplines/geophysics/radiometrics|website=Geoscience Australia|date=15 May 2014|publisher=Commonwealth of Australia|access-date=23 June 2014}}</ref><ref>{{cite web|title=Interpreting radiometrics|url=http://spatial.agric.wa.gov.au/geophysics/radiometrics.asp|archive-url=https://web.archive.org/web/20120321102348/http://spatial.agric.wa.gov.au/geophysics/radiometrics.asp |archive-date=21 March 2012|website=Natural Resource Management|publisher=Department of Agriculture and Food, Government of Western Australia|access-date=23 June 2014}}</ref>

=== Fluid dynamics ===
{{Main|Geophysical fluid dynamics}}
[[Fluid dynamics|Fluid motions]] occur in the magnetosphere, [[Earth's atmosphere|atmosphere]], ocean, mantle and core. Even the mantle, though it has an enormous [[viscosity]], flows like a fluid over long time intervals. This flow is reflected in phenomena such as [[isostasy]], [[post-glacial rebound]] and [[mantle plume]]s. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.<ref name=Merrill/>

Geophysical fluid dynamics is a primary tool in [[physical oceanography]] and [[meteorology]]. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the [[Coriolis effect]]. In the atmosphere, it gives rise to large-scale patterns like [[Rossby waves]] and determines the basic circulation patterns of storms. In the ocean, they drive large-scale circulation patterns as well as [[Kelvin waves]] and [[Ekman spirals]] at the ocean surface.<ref name=Pedlosky>{{harvnb|Pedlosky|1987}}</ref> In the Earth's core, the circulation of the molten iron is structured by [[Taylor columns]].<ref name=Merrill/>

Waves and other phenomena in the magnetosphere can be modeled using [[magnetohydrodynamics]].

=== Heat flow ===
{{Main|Geothermal gradient}}
[[File:Convection-snapshot.png|thumb|upright=1.4|A model of [[thermal convection]] in the [[Earth's mantle]]. The thin red columns are [[mantle plumes]]. |alt=Pseudocolor image in vertical profile.]]
The Earth is cooling, and the resulting [[heat flow]] generates the Earth's magnetic field through the [[geodynamo]] and plate tectonics through [[mantle convection]].<ref name=Davies>{{harvnb|Davies|2001}}</ref> The main sources of heat are: primordial heat due to Earth's cooling and [[radioactivity]] in the planets upper crust.<ref>{{Cite web |title=What is "Heat Flow"? |url=https://www.smu.edu/dedman/academics/departments/Earth-Sciences/Research/GeothermalLab/DataMaps/GeothermalMapofNorthAmerica/What-is-Heat-Flow |access-date=2024-02-18 |website=www.smu.edu |language=en}}</ref> There is also some contributions from [[phase transitions]]. Heat is mostly carried to the surface by [[thermal convection]], although there are two thermal boundary layers – the [[core–mantle boundary]] and the [[lithosphere]] – in which heat is transported by [[Conduction (heat)|conduction]].<ref name=Fowler>{{harvnb|Fowler|2005}}</ref> Some heat is carried up from the bottom of the [[Mantle (geology)|mantle]] by [[mantle plumes]]. The heat flow at the Earth's surface is about {{math| 4.2 × 10<sup>13</sup> W}}, and it is a potential source of [[Geothermal energy|geothermal]] energy.<ref name=Pollack>{{harvnb|Pollack|Hurter|Johnson|1993}}</ref>

=== Mineral physics ===
{{Main|Mineral physics}}
The physical properties of minerals must be understood to infer the composition of the Earth's interior from [[seismology]], the [[geothermal gradient]] and other sources of information. Mineral physicists study the [[elasticity (physics)|elastic]] properties of minerals; their high-pressure [[phase diagrams]], melting points and [[equations of state]] at high pressure; and the [[Rheology|rheological properties]] of rocks, or their ability to flow. Deformation of rocks by [[creep (deformation)|creep]] make flow possible, although over short times the rocks are brittle. The [[viscosity]] of rocks is affected by temperature and pressure, and in turn, determines the rates at which tectonic plates move.<ref name=Poirier/>

Water is a very complex substance and its unique properties are essential for life.<ref name=Sadava>{{harvnb|Sadava|Heller|Hillis|Berenbaum|2009}}</ref> Its physical properties shape the [[hydrosphere]] and are an essential part of the [[water cycle]] and [[climate]]. Its thermodynamic properties determine [[evaporation]] and the thermal gradient in the [[atmosphere]]. The many types of [[precipitation (meteorology)|precipitation]] involve a complex mixture of processes such as [[coalescence (physics)|coalescence]], [[supercooling]] and [[supersaturation]].<ref>{{harvnb|Sirvatka|2003}}</ref> Some precipitated water becomes [[groundwater]], and groundwater flow includes phenomena such as [[percolation]], while the [[conductivity (electrolytic)|conductivity]] of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as [[salinity]] have a large effect on its motion in the oceans.<ref name=Pedlosky/>

The many phases of ice form the [[cryosphere]] and come in forms like [[ice sheet]]s, [[glacier]]s, [[sea ice]], freshwater ice, snow, and frozen ground (or [[permafrost]]).<ref>{{harvnb|CFG|2011}}</ref>

== Regions of the Earth ==

=== Size and form of the Earth ===
Contrary to popular belief, the earth is not entirely spherical but instead generally exhibits an [[ellipsoid]] shape- which is a result of the centrifugal forces the planet generates due to its constant motion.<ref name=":0">{{Cite web |title=Is the Earth round? |url=https://oceanservice.noaa.gov/facts/earth-round.html#:~:text=While%20the%20Earth%20appears%20to,unique%20and%20ever-changing%20shape. |access-date=2024-02-18 |website=oceanservice.noaa.gov |language=en}}</ref> These forces cause the planets diameter to bulge towards the [[Equator]] and results in the [[Earth ellipsoid|ellipsoid shape]].<ref name=":0" /> Earth's shape is constantly changing, and different factors including [[Post-glacial rebound|glacial isostatic rebound]] (large ice sheets melting causing the Earth's crust to the rebound due to the release of the pressure<ref>{{Cite web |last=US Department of Commerce |first=National Oceanic and Atmospheric Administration |title=What is glacial isostatic adjustment? |url=https://oceanservice.noaa.gov/facts/glacial-adjustment.html |access-date=2024-02-18 |website=oceanservice.noaa.gov |language=EN-US}}</ref>), geological features such as [[mountain]]s or [[Oceanic trench|ocean trenches]], [[Plate tectonics|tectonic plate]] dynamics, and [[natural disaster]]s can further distort the planet's shape.<ref name=":0" />

=== Structure of the interior ===
{{Main|Structure of Earth}}
[[Image:Earthquake wave paths.svg|thumb|upright=1.3|Seismic velocities and boundaries in the interior of the [[Earth]] sampled by seismic waves |alt=Diagram with concentric shells and curved paths.]]
Evidence from [[seismology]], heat flow at the surface, and [[mineral physics]] is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior – its composition, density, temperature, pressure. For example, the Earth's mean [[specific gravity]] ({{math|5.515}}) is far higher than the typical specific gravity of rocks at the surface ({{math|2.7–3.3}}), implying that the deeper material is denser. This is also implied by its low [[moment of inertia]] ({{math| 0.33 <var>M R</var><sup>2</sup>}}, compared to {{math| 0.4 <var>M R</var><sup>2</sup>}} for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the [[Adams–Williamson equation]]. The conclusion is that pressure alone cannot account for the increase in density. Instead, we know that the Earth's core is composed of an alloy of iron and other minerals.<ref name=Poirier/>

Reconstructions of seismic waves in the deep interior of the Earth show that there are no [[S-waves]] in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field. [[Earth's inner core]], however, is solid because of the enormous pressure.<ref name=Lowrie>{{harvnb|Lowrie|2004}}</ref>

Reconstruction of seismic reflections in the deep interior indicates some major discontinuities in seismic velocities that demarcate the major zones of the Earth: [[Earth's inner core|inner core]], [[Earth's outer core|outer core]], mantle, [[lithosphere]] and [[crust (geology)|crust]]. The mantle itself is divided into the [[upper mantle (Earth)|upper mantle]], transition zone, lower mantle and ''D′′'' layer. Between the crust and the mantle is the [[Mohorovičić discontinuity]].<ref name=Lowrie/>

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the [[geotherm]] must also be determined. This requires physical theory for [[thermal conduction]] and [[convection]] and the heat contribution of [[radionuclides|radioactive elements]]. The main model for the radial structure of the interior of the Earth is the [[preliminary reference Earth model]] (PREM). Some parts of this model have been updated by recent findings in mineral physics (see [[post-perovskite]]) and supplemented by [[seismic tomography]]. The mantle is mainly composed of [[silicates]], and the boundaries between layers of the mantle are consistent with phase transitions.<ref name=Poirier/>

The mantle acts as a solid for seismic waves, but under high pressures and temperatures, it deforms so that over millions of years it acts like a liquid. This makes [[plate tectonics]] possible.

=== Magnetosphere ===
{{Main|Magnetosphere}}
[[Image:Structure of the magnetosphere mod.svg|upright=1.3|thumb|Schematic of Earth's magnetosphere. The [[solar wind]] flows from left to right. |alt=Diagram with colored surfaces and lines.]]
If a planet's [[magnetic field]] is strong enough, its interaction with the solar wind forms a magnetosphere. Early [[space probe]]s mapped out the gross dimensions of the Earth's magnetic field, which extends about 10 [[Earth radii]] towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the [[Magnetotail|magnetic tail]], hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles called the Van Allen radiation belts.<ref name=Kivelson/>

== Methods ==

===Geodesy===
{{Main|Geodesy}}

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province of [[geodesy]]. While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as the [[American Geophysical Union]], the [[Canadian Geophysical Union]] and the [[International Union of Geodesy and Geophysics]] encompass both.<ref name=NRC>{{harvnb|National Research Council (U.S.). Committee on Geodesy|1985}}</ref>

Absolute positions are most frequently determined using the [[global positioning system]] (GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the [[GRS 80|1980 Geodetic Reference System]]. An alternative, [[astro-geodetic|optical astronomy]], combines astronomical coordinates and the local gravity vector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficult to use than GPS. However, it is useful for measuring motions of the Earth such as [[nutation]] and [[Chandler wobble]]. Relative positions of two or more points can be determined using [[very-long-baseline interferometry]].<ref name=NRC/><ref>{{harvnb|Defense Mapping Agency|1984}}</ref><ref name=Torge>{{harvnb|Torge|2001}}</ref>

Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using [[gravimeters]] deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using [[Radar altimeter|radar altimetry]], contributing to a more accurate [[geoid]].<ref name=NRC/> In 2002, [[NASA]] launched the [[Gravity Recovery and Climate Experiment]] (GRACE), wherein two twin [[satellite]]s map variations in Earth's gravity field by making measurements of the distance between the two satellites using GPS and a microwave ranging system.  Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.<ref>{{harvnb|CSR|2011}}</ref>

===Satellites and space probes===
Satellites in space have made it possible to collect data from not only the visible light region, but in other areas of the [[electromagnetic spectrum]]. The planets can be characterized by their force fields: gravity and their [[magnetic field]]s, which are studied through geophysics and space physics.

Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the [[gravity field]]s of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above [[lunar maria]] were measured through [[Lunar Orbiter program|lunar orbiters]], which led to the discovery of concentrations of mass, [[mass concentration (astronomy)|mascons]], beneath the [[Mare Imbrium|Imbrium]], [[Mare Serenitatis|Serenitatis]], [[Mare Crisium|Crisium]], [[Mare Nectaris|Nectaris]] and [[Mare Humorum|Humorum]] basins.<ref>{{harvnb|Muller|Sjogren|1968}}</ref>

=== Global positioning systems (GPS) and geographical information systems (GIS) ===
{{further|GIS}}
Since geophysics is concerned with the shape of the Earth, and by extension the mapping of features around and in the planet, geophysical measurements include high accuracy GPS measurements. These measurements are processed to increase their accuracy through [[differential GPS]] processing. Once the geophysical measurements have been processed and inverted, the interpreted results are plotted using GIS. Programs such as [[ArcGIS]] and [[Geosoft]] were built to meet these needs and include many geophysical functions that are built-in, such as [[upward continuation]], and the calculation of the measurement [[derivative]] such as the first-vertical derivative.<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref> Many geophysics companies have designed in-house geophysics programs that pre-date ArcGIS and GeoSoft in order to meet the visualization requirements of a geophysical dataset.

=== Remote sensing ===
{{Main|Remote sensing}}
[[Exploration geophysics]] is a branch of applied geophysics that involves the development and utilization of different seismic or electromagnetic methods which the aim of investigating different energy, mineral and water resources.<ref>{{Cite web |title=Energy Geosciences |url=https://www.jsg.utexas.edu/research/themes/energy-geosciences/ |access-date=2024-02-18 |website=Jackson School of Geosciences |language=en}}</ref> This is done through the uses of various [[remote sensing]] platforms such as; [[satellite]]s, [[aircraft]], [[boat]]s, [[Unmanned aerial vehicle|drones]], [[borehole]] sensing equipment and [[Seismic source|seismic receivers]]. These equipment are often used in conjunction with different geophysical methods such as [[Magnetism|magnetic]], [[gravimetry]], [[Electromagnetism|electromagnetic]], [[Radiometry|radiometric]], [[Barometer|barometry]] methods in order to gather the data. The remote sensing platforms used in exploration geophysics are not perfect and need adjustments done on them in order to accurately account for the effects that the platform itself may have on the collected data. For example, when gathering [[Aeromagnetic survey|aeromagnetic]] data (aircraft gathered magnetic data) using a conventional fixed-wing aircraft- the platform has to be adjusted to account for the electromagnetic currents that it may generate as it passes through [[Earth's magnetic field]].<ref name="Telford" /> There are also corrections related to changes in measured potential field intensity as the Earth rotates, as the Earth orbits the Sun, and as the moon orbits the Earth.<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref>

=== Signal processing ===
{{Main|Signal processing}}
Geophysical measurements are often recorded as [[time-series]] with [[Satellite Navigation|GPS]] location. Signal processing involves the correction of time-series data for unwanted noise or errors introduced by the measurement platform, such as aircraft vibrations in gravity data. It also involves the reduction of sources of noise, such as diurnal corrections in magnetic data.<ref name="Telford"/><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref> In seismic data, electromagnetic data, and gravity data, processing continues after error corrections to include [[computational geophysics]] which result in the final interpretation of the geophysical data into a geological interpretation of the geophysical measurements<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref>

== History ==
{{Main|History of geophysics}}
Geophysics emerged as a separate discipline only in the 19th century, from the intersection of [[physical geography]], [[geology]], [[astronomy]], meteorology, and physics.<ref name=history_resources>{{harvnb|Hardy|Goodman|2005}}</ref><ref>{{cite journal|last=Schröder|first=W.|title=History of geophysics|journal=Acta Geodaetica et Geophysica Hungarica|year=2010|volume=45|issue=2|pages=253–261|doi=10.1556/AGeod.45.2010.2.9|bibcode=2010AGGH...45..253S |s2cid=122239663}}</ref> The first known use of the word ''geophysics'' was in German ("Geophysik") by [[Julius Fröbel]] in 1834.<ref>{{Cite journal |last=Varga |first=P. |date=2009 |title=Common roots of modern seismology and of earth tide research. A historical overview |url=https://linkinghub.elsevier.com/retrieve/pii/S0264370709000994 |journal=Journal of Geodynamics |language=en |volume=48 |issue=3–5 |pages=241–246 |doi=10.1016/j.jog.2009.09.032|bibcode=2009JGeo...48..241V |s2cid=129513373 }}</ref> However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the [[Ancient history|ancient era]].

=== Ancient and classical eras ===
[[Image:EastHanSeismograph.JPG|thumbnail|upright|Replica of [[Zhang Heng]]'s seismoscope, possibly the first contribution to [[seismology]] |alt=Picture of ornate urn-like device with spouts in the shape of dragons]]
The magnetic compass existed in China back as far as the fourth century BC. It was used as much for [[feng shui]] as for navigation on land. It was not until good steel needles could be forged that compasses were used for navigation at sea; before that, they could not retain their magnetism long enough to be useful. The first mention of a compass in Europe was in 1190 AD.<ref>{{harvnb|Temple|2006|pp=162–166}}</ref>

In circa 240 BC, [[Eratosthenes]] of Cyrene deduced that the Earth was round and measured the [[circumference of Earth]] with great precision.<ref name="russo273277">{{cite book |last=Russo |first=Lucio |author-link=Lucio Russo |date=2004 |title=The Forgotten Revolution |url=https://archive.org/details/forgottenrevolut00russ_217|url-access=limited |location=Berlin |publisher=Springer |page=[https://archive.org/details/forgottenrevolut00russ_217/page/n277 273]–277}}</ref> He developed a system of [[latitude]] and [[longitude]].<ref name=Eratosthenes>{{harvnb|Eratosthenes|2010}}</ref>

Perhaps the earliest contribution to seismology was the invention of a [[seismoscope]] by the prolific inventor [[Zhang Heng]] in 132 AD.<ref>{{harvnb|Temple|2006|pp=177–181}}</ref> This instrument was designed to drop a bronze ball from the mouth of a dragon into the mouth of a toad. By looking at which of eight toads had the ball, one could determine the direction of the earthquake. It was 1571 years before the first design for a seismoscope was published in Europe, by [[Jean de Hautefeuille|Jean de la Hautefeuille]]. It was never built.<ref name=Dewey>{{harvnb|Dewey|Byerly|1969}}</ref>

=== Beginnings of modern science ===
The 17th century had major milestones that marked the beginning of modern science. In 1600, [[William Gilbert (physicist)|William Gilbert]] release a publication titled ''[[De Magnete]]'' (1600) where he conducted  series of experiments on both natural magnets  (called [[Lodestone|'loadstones]]') and artificially magnetized iron.<ref name=":12">{{Cite web |title=Review of "De Magnete" |url=https://pwg.gsfc.nasa.gov/earthmag/DMGRev2.htm |access-date=2024-02-18 |website=pwg.gsfc.nasa.gov}}</ref> His experiments lead to observations involving a small compass needle ([[versorium]]) which replicated magnetic behaviours when subjected to a spherical magnet, along with it experiencing '[[magnetic dip]]s' when it was pivoted on a horizontal axis.<ref name=":12" /> HIs findings led to the deduction that compasses point north due to the Earth itself being a giant magnet.<ref name=":12" />

In 1687 [[Isaac Newton]] published his work titled ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'' which was pivotal in the development of modern scientific fields such as [[astronomy]] and [[physics]].<ref name=":22">{{Citation |last=Smith |first=George |title=Newton's Philosophiae Naturalis Principia Mathematica |date=2008 |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |url=https://plato.stanford.edu/archives/win2008/entries/newton-principia/ |access-date=2024-02-18 |edition=Winter 2008 |publisher=Metaphysics Research Lab, Stanford University}}</ref> In it, Newton both laid the foundations for [[classical mechanics]] and [[gravitation]], as well as explained different geophysical phenomena such as the [[Axial precession|precession of the equinox]] (the orbit of whole star patterns along an [[Ecliptical pole|ecliptic axis]].<ref>{{Cite web |last=Institute of Physics |date=February 18, 2024 |title=Precession of the equinoxes |url=https://spark.iop.org/precession-equinoxes#:~:text=The%20precession%20of%20the%20equinoxes%20is%20a%20slow%20rotation%20of,BC%20to%20~120%20BC). |access-date=February 18, 2024}}</ref> [[Newton's law of universal gravitation|Newton's theory of gravity]] had gained so much success, that it resulted in changing the main objective of physics in that era to unravel natures fundamental forces, and their characterizations in laws.<ref name=":22" />

The first [[seismometer]], an instrument capable of keeping a continuous record of seismic activity, was built by [[James David Forbes|James Forbes]] in 1844.<ref name=Dewey/>

== See also ==
{{Portal|Earth sciences|Geophysics|Physics}}

*[[International Union of Geodesy and Geophysics]] (IUGG)
*[[Sociedade Brasileira de Geofísica]]
*{{Annotated link|Earth system science}}
*{{Annotated link|List of geophysicists}}
*{{Annotated link|Outline of geophysics}}
*{{Annotated link|Geodynamics}}
*{{Annotated link|Planetary science}}
* [[Geological Engineering]]
* [[Physics]]
* [[Space physics]]
* [[Geosciences]]
* [[Geodesy]]

== Notes ==
{{Reflist}}

== References ==
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{{Refend}}

==External links==
{{Commons category}}
*[http://www.environmental-geophysics.co.uk/documents/ref_manual/TechRef.pdf A reference manual for near-surface geophysics techniques and applications] {{Webarchive|url=https://web.archive.org/web/20210218212208/http://www.environmental-geophysics.co.uk/documents/ref_manual/TechRef.pdf |date=18 February 2021 }}
*[https://web.archive.org/web/20190210154628/https://www.iugg-georisk.org/ Commission on Geophysical Risk and Sustainability (GeoRisk), International Union of Geodesy and Geophysics (IUGG)]
*[http://www.sedigroup.org/ Study of the Earth's Deep Interior, a Committee of IUGG]
*[http://www.iugg.org/about/commissions/ Union Commissions (IUGG)]
*[http://geomag.usgs.gov/ USGS Geomagnetism Program]
*[https://web.archive.org/web/20120121022344/http://careercrate.com/video/266/Seismic-processor Career crate: Seismic processor]
*[http://www.seg.org/ Society of Exploration Geophysicists]

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