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Geophysics

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(Redirected fromGeophysical)
For the journal, seeGeophysics (journal).
Not to be confused withPhysical geography.

false color image
Age of the sea floor. Much of the dating information comes from magnetic anomalies.[1]
Computer simulation of theEarth's magnetic fieldin a period of normal polarity betweenreversals[2]

Geophysics(/ˌdʒiːoʊˈfɪzɪks/) is a subject ofnatural scienceconcerned with the physical processes andphysical propertiesof theEarthand its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists, who usually study geophysics,physics,or one of theEarth sciencesat the graduate level, complete investigations across a wide range of scientific disciplines. The termgeophysicsclassically refers to solid earth applications: Earth'sshape;itsgravitational,magnetic fields,andelectromagnetic fields;itsinternal structureandcomposition;itsdynamicsand their surface expression inplate tectonics,the generation ofmagmas,volcanismand rock formation.[3]However, modern geophysics organizations and pure scientists use a broader definition that includes thewater cycleincluding snow and ice;fluid dynamicsof the oceans and theatmosphere;electricityandmagnetismin theionosphereandmagnetosphereandsolar-terrestrial physics;and analogous problems associated with theMoonand other planets.[3][4][5][6][7][8]

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 fromlodestones,while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD.Isaac Newtonapplied his theory of mechanics to the tides and theprecession 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 applied to societal needs, such asmineral resources,mitigation ofnatural hazardsandenvironmental protection.[4]Inexploration geophysics,geophysical surveydata 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 forenvironmental remediation.

Physical phenomena

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Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of theEarth sciences,while some geophysicists conduct research in theplanetary sciences.To provide a more clear idea on what constitutes geophysics, this section describes phenomena that are studied inphysicsand 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 theSolar System,which includes other planetary bodies.

Gravity

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Image of globe combining color with topography.
A map of deviations in gravity from a perfectly smooth, idealized Earth
Main article:Gravity of Earth
Further information:Physical geodesyandGravimetry

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.[9]

Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.[10]Measurements ofgravitational accelerationandgravitational potentialat the Earth's surface and above it can be used to look for mineral deposits (seegravity anomalyandgravimetry).[11]The surface gravitational field provides information on the dynamics oftectonic plates.Thegeopotentialsurface called thegeoidis 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).[12]

Heat flow

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Main article:Geothermal gradient
Pseudocolor image in vertical profile.
A model ofthermal convectionin theEarth's mantle.The thin red columns aremantle plumes.

The Earth is cooling, and the resultingheat flowgenerates the Earth's magnetic field through thegeodynamoand plate tectonics throughmantle convection.[13]The main sources of heat are: primordial heat due to Earth's cooling andradioactivityin the planets upper crust.[14]There is also some contributions fromphase transitions.Heat is mostly carried to the surface bythermal convection,although there are two thermal boundary layers – thecore–mantle boundaryand thelithosphere– in which heat is transported byconduction.[15]Some heat is carried up from the bottom of themantlebymantle plumes.The heat flow at the Earth's surface is about4.2 × 1013W,and it is a potential source ofgeothermalenergy.[16]

Vibrations

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Main article:Seismology
Deformed blocks with grids on surface.
Illustration of the deformations of a block by body waves and surface waves (seeseismic wave)

Seismic wavesare vibrations that travel through the Earth's interior or along its surface.[17]The entire Earth can also oscillate in forms that are callednormal modesorfree oscillations of the Earth.Ground motions from waves or normal modes are measured usingseismographs.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.[18][19]

Recording ofseismic wavesfrom 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 usingReflection Seismologycan 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.[11]Changes in the travel direction, calledrefraction,can be used to infer thedeep structure of the Earth.[19]

Earthquakes pose arisk to humans.Understanding their mechanisms, which depend on the type of earthquake (e.g.,intraplateordeep focus), can lead to better estimates of earthquake risk and improvements inearthquake engineering.[20]

Electricity

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Although we mainly notice electricity duringthunderstorms,there is always a downward electric field near the surface that averages 120voltsper meter.[21]Relative to the solid Earth, the ionization of the planet's atmosphere is a result of the galacticcosmic rayspenetrating it, which leaves it with a net positive charge.[22]A current of about 1800amperesflows in the global circuit.[21]It flows downward from theionosphereover most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds andspritesabove.

A variety of electric methods are used in geophysical survey. Some measurespontaneous potential,a potential that arises in the ground because of human-made or natural disturbances.Telluric currentsflow in Earth and the oceans. They have two causes:electromagnetic inductionby the time-varying, external-origingeomagnetic fieldand motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field.[23]The distribution of telluric current density can be used to detect variations inelectrical resistivityof underground structures. Geophysicists can also provide the electric current themselves (seeinduced polarizationandelectrical resistivity tomography).

Electromagnetic waves

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Electromagnetic wavesoccur in the ionosphere and magnetosphere as well as inEarth's outer core.Dawn chorusis believed to be caused by high-energy electrons that get caught in theVan Allen radiation belt.Whistlersare produced bylightningstrikes.Hissmay be generated by both.Electromagneticwaves may also be generated by earthquakes (seeseismo-electromagnetics).

In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction.Alfvén wavesaremagnetohydrodynamicwaves in themagnetosphereor the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magneticRossby wavesmay be one source ofgeomagnetic secular variation.[24]

Electromagnetic methods that are used for geophysical survey includetransient electromagnetics,magnetotellurics,surface nuclear magnetic resonanceand electromagnetic seabed logging.[25]

Magnetism

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Further information:Earth's magnetic field,Aeromagnetic survey,andPaleomagnetism

The Earth's magnetic field protects the Earth from the deadlysolar windand has long been used for navigation. It originates in the fluid motions of the outer core.[24]The magnetic field in the upper atmosphere gives rise to theauroras.[26]

Diagram with field lines, axes and magnet lines.
Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line).

The Earth's field is roughly like a tilteddipole,but it changes over time (a phenomenon called geomagnetic secular variation). Mostly thegeomagnetic polestays near thegeographic pole,but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. Thesegeomagnetic reversals,analyzed within aGeomagnetic 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 theLaschamp eventoccurring 41,000 years ago during thelast glacial period.Geologists observedgeomagnetic reversal recordedin volcanic rocks, throughmagnetostratigraphy correlation(seenatural remanent magnetization) and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information onseafloor spreading,a part of plate tectonics. They are the basis ofmagnetostratigraphy,which correlates magnetic reversals with otherstratigraphiesto construct geologic time scales.[27]In addition, themagnetization in rockscan be used to measure the motion of continents.[24]

Radioactivity

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Further information:Radiometric dating
Diagram with compound balls representing nuclei and arrows.
Example of a radioactive decay chain (seeRadiometric dating)

Radioactive decayaccounts for about 80% of the Earth'sinternal heat,powering the geodynamo and plate tectonics.[28]The main heat-producingisotopesarepotassium-40,uranium-238,uranium-235, andthorium-232.[29] Radioactive elements are used forradiometric dating,the primary method for establishing an absolute time scale ingeochronology.

Unstable isotopes decay at predictable rates, and the decay rates of differentisotopescover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in pastgeologic eras.[30]Radiometric mapping using ground and airbornegamma spectrometrycan be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.[31][32]

Fluid dynamics

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Main article:Geophysical fluid dynamics

Fluid motionsoccur in the magnetosphere,atmosphere,ocean, mantle and core. Even the mantle, though it has an enormousviscosity,flows like a fluid over long time intervals. This flow is reflected in phenomena such asisostasy,post-glacial reboundandmantle plumes.The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.[24]

Geophysical fluid dynamics is a primary tool inphysical oceanographyandmeteorology.The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to theCoriolis effect.In the atmosphere, it gives rise to large-scale patterns likeRossby wavesand determines the basic circulation patterns of storms. In the ocean, they drive large-scale circulation patterns as well asKelvin wavesandEkman spiralsat the ocean surface.[33]In the Earth's core, the circulation of the molten iron is structured byTaylor columns.[24]

Waves and other phenomena in the magnetosphere can be modeled usingmagnetohydrodynamics.

Mineral physics

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Main article:Mineral physics

The physical properties of minerals must be understood to infer the composition of the Earth's interior fromseismology,thegeothermal gradientand other sources of information. Mineral physicists study theelasticproperties of minerals; their high-pressurephase diagrams,melting points andequations of stateat high pressure; and therheological propertiesof rocks, or their ability to flow. Deformation of rocks bycreepmake flow possible, although over short times the rocks are brittle. Theviscosityof rocks is affected by temperature and pressure, and in turn, determines the rates at which tectonic plates move.[10]

Water is a very complex substance and its unique properties are essential for life.[34]Its physical properties shape thehydrosphereand are an essential part of thewater cycleandclimate.Its thermodynamic properties determineevaporationand the thermal gradient in theatmosphere.The many types ofprecipitationinvolve a complex mixture of processes such ascoalescence,supercoolingandsupersaturation.[35]Some precipitated water becomesgroundwater,and groundwater flow includes phenomena such aspercolation,while theconductivityof water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such assalinityhave a large effect on its motion in the oceans.[33]

The many phases of ice form thecryosphereand come in forms likeice sheets,glaciers,sea ice,freshwater ice, snow, and frozen ground (orpermafrost).[36]

Regions of the Earth

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Size and form of the Earth

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Contrary to popular belief, the earth is not entirely spherical but instead generally exhibits anellipsoidshape- which is a result of the centrifugal forces the planet generates due to its constant motion.[37]These forces cause the planets diameter to bulge towards theEquatorand results in theellipsoid shape.[37]Earth's shape is constantly changing, and different factors includingglacial isostatic rebound(large ice sheets melting causing the Earth's crust to the rebound due to the release of the pressure[38]), geological features such asmountainsorocean trenches,tectonic platedynamics, andnatural disasterscan further distort the planet's shape.[37]

Structure of the interior

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Main article:Structure of Earth
Diagram with concentric shells and curved paths.
Seismic velocities and boundaries in the interior of theEarthsampled by seismic waves

Evidence fromseismology,heat flow at the surface, andmineral physicsis 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 meanspecific gravity(5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its lowmoment of inertia(0.33M R2,compared to0.4M R2for 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 theAdams–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.[10]

Reconstructions of seismic waves in the deep interior of the Earth show that there are noS-wavesin 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.[12]

Reconstruction of seismic reflections in the deep interior indicates some major discontinuities in seismic velocities that demarcate the major zones of the Earth:inner core,outer core,mantle,lithosphereandcrust.The mantle itself is divided into theupper mantle,transition zone, lower mantle andD′′layer. Between the crust and the mantle is theMohorovičić discontinuity.[12]

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 thegeothermmust also be determined. This requires physical theory forthermal conductionandconvectionand the heat contribution ofradioactive elements.The main model for the radial structure of the interior of the Earth is thepreliminary reference Earth model(PREM). Some parts of this model have been updated by recent findings in mineral physics (seepost-perovskite) and supplemented byseismic tomography.The mantle is mainly composed ofsilicates,and the boundaries between layers of the mantle are consistent with phase transitions.[10]

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 makesplate tectonicspossible.

Magnetosphere

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Main article:Magnetosphere
Diagram with colored surfaces and lines.
Schematic of Earth's magnetosphere. Thesolar windflows from left to right.

If a planet'smagnetic fieldis strong enough, its interaction with the solar wind forms a magnetosphere. Earlyspace probesmapped out the gross dimensions of the Earth's magnetic field, which extends about 10Earth radiitowards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind themagnetic tail,hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles called the Van Allen radiation belts.[26]

Methods

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Geodesy

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Main article:Geodesy

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province ofgeodesy.While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as theAmerican Geophysical Union,theCanadian Geophysical Unionand theInternational Union of Geodesy and Geophysicsencompass both.[39]

Absolute positions are most frequently determined using theglobal positioning system(GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the1980 Geodetic Reference System.An alternative,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 asnutationandChandler wobble.Relative positions of two or more points can be determined usingvery-long-baseline interferometry.[39][40][41]

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 usinggravimetersdeployed 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 usingradar altimetry,contributing to a more accurategeoid.[39]In 2002,NASAlaunched theGravity Recovery and Climate Experiment(GRACE), wherein two twinsatellitesmap 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.[42]

Satellites and space probes

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

Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of thegravity fieldsof the planets to be mapped. For example, in the 1970s, the gravity field disturbances abovelunar mariawere measured throughlunar orbiters,which led to the discovery of concentrations of mass,mascons,beneath theImbrium,Serenitatis,Crisium,NectarisandHumorumbasins.[43]

Global positioning systems (GPS) and geographical information systems (GIS)

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Further information: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 throughdifferential GPSprocessing. Once the geophysical measurements have been processed and inverted, the interpreted results are plotted using GIS. Programs such asArcGISandGeosoftwere built to meet these needs and include many geophysical functions that are built-in, such asupward continuation,and the calculation of the measurementderivativesuch as the first-vertical derivative.[11][44]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

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Main article:Remote sensing

Exploration geophysicsis 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.[45]This is done through the uses of variousremote sensingplatforms such as;satellites,aircraft,boats,drones,boreholesensing equipment andseismic receivers.These equipment are often used in conjunction with different geophysical methods such asmagnetic,gravimetry,electromagnetic,radiometric,barometrymethods 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 gatheringaeromagneticdata (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 throughEarth's magnetic field.[11]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.[11][44]

Signal processing

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Main article:Signal processing

Geophysical measurements are often recorded astime-serieswithGPSlocation. 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.[11][44]In seismic data, electromagnetic data, and gravity data, processing continues after error corrections to includecomputational geophysicswhich result in the final interpretation of the geophysical data into a geological interpretation of the geophysical measurements[11][44]

History

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Main article:History of geophysics

Geophysics emerged as a separate discipline only in the 19th century, from the intersection ofphysical geography,geology,astronomy,meteorology, and physics.[46][47]The first known use of the wordgeophysicswas in German ( "Geophysik" ) byJulius Fröbelin 1834.[48]However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since theancient era.

Ancient and classical eras

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Picture of ornate urn-like device with spouts in the shape of dragons
Replica ofZhang Heng's seismoscope, possibly the first contribution toseismology

The magnetic compass existed in China back as far as the fourth century BC. It was used as much forfeng shuias 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.[49]

In circa 240 BC,Eratosthenesof Cyrene deduced that the Earth was round and measured thecircumference of Earthwith great precision.[50]He developed a system oflatitudeandlongitude.[51]

Perhaps the earliest contribution to seismology was the invention of aseismoscopeby the prolific inventorZhang Hengin 132 AD.[52]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, byJean de la Hautefeuille.It was never built.[53]

Beginnings of modern science

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The 17th century had major milestones that marked the beginning of modern science. In 1600,William Gilbertrelease a publication titledDe Magnete(1600) where he conducted series of experiments on both natural magnets (called'loadstones') and artificially magnetized iron.[54]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 dips' when it was pivoted on a horizontal axis.[54]HIs findings led to the deduction that compasses point north due to the Earth itself being a giant magnet.[54]

In 1687Isaac Newtonpublished his work titledPrincipiawhich was pivotal in the development of modern scientific fields such asastronomyandphysics.[55]In it, Newton both laid the foundations forclassical mechanicsandgravitation,as well as explained different geophysical phenomena such as theprecession of the equinox(the orbit of whole star patterns along anecliptic axis.[56]Newton's theory of gravityhad 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.[55]

The firstseismometer,an instrument capable of keeping a continuous record of seismic activity, was built byJames Forbesin 1844.[53]

See also

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Notes

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  1. ^Müller, R. Dietmar; Sdrolias, Maria; Gaina, Carmen; Roest, Walter R. (April 2008)."Age, spreading rates, and spreading asymmetry of the world's ocean crust".Geochemistry, Geophysics, Geosystems.9(4): Q04006.Bibcode:2008GGG.....9.4006M.doi:10.1029/2007GC001743.S2CID15960331.
  2. ^"Earth's Inconstant Magnetic Field".science@nasa.National Aeronautics and Space Administration. 29 December 2003.Retrieved13 November2018.
  3. ^abSheriff 1991
  4. ^abIUGG 2011
  5. ^AGU 2011
  6. ^Gutenberg, B., 1929, Lehrbuch der Geophysik. Leipzig. Berlin (Gebruder Borntraeger).
  7. ^Runcorn, S.K, (editor-in-chief), 1967, International dictionary of geophysics:. Pergamon, Oxford, 2 volumes, 1,728 pp., 730 fig
  8. ^Geophysics, 1970, Encyclopaedia Britannica, Vol.10, p. 202-202
  9. ^Ross 1995,pp. 236–242
  10. ^abcdPoirier 2000
  11. ^abcdefgTelford, Geldart & Sheriff 1990
  12. ^abcLowrie 2004
  13. ^Davies 2001
  14. ^"What is" Heat Flow "?".smu.edu.Retrieved18 February2024.
  15. ^Fowler 2005
  16. ^Pollack, Hurter & Johnson 1993
  17. ^"Seismic wave | Earth's Interior Structure & Movement | Britannica".britannica.12 January 2024.Retrieved18 February2024.
  18. ^Shearer, Peter M. (2009).Introduction to seismology(2nd ed.). Cambridge: Cambridge University Press.ISBN9780521708425.
  19. ^abStein & Wysession 2003
  20. ^Bozorgnia & Bertero 2004
  21. ^abHarrison & Carslaw 2003
  22. ^Nicoll, Keri (April 2016)."Earth's electric atmosphere"(PDF).metlink.org.Retrieved18 February2024.
  23. ^Lanzerotti & Gregori 1986
  24. ^abcdeMerrill, McElhinny & McFadden 1998
  25. ^Stéphane, Sainson (2017).Electromagnetic seabed logging: a new tool for geoscientists.Springer.ISBN978-3-319-45355-2.
  26. ^abKivelson & Russell 1995
  27. ^Opdyke & Channell 1996
  28. ^Turcotte & Schubert 2002
  29. ^Sanders 2003
  30. ^Renne, Ludwig & Karner 2000
  31. ^"Radiometrics".Geoscience Australia.Commonwealth of Australia. 15 May 2014.Retrieved23 June2014.
  32. ^"Interpreting radiometrics".Natural Resource Management.Department of Agriculture and Food, Government of Western Australia. Archived fromthe originalon 21 March 2012.Retrieved23 June2014.
  33. ^abPedlosky 1987
  34. ^Sadava et al. 2009
  35. ^Sirvatka 2003
  36. ^CFG 2011
  37. ^abc"Is the Earth round?".oceanservice.noaa.gov.Retrieved18 February2024.
  38. ^US Department of Commerce, National Oceanic and Atmospheric Administration."What is glacial isostatic adjustment?".oceanservice.noaa.gov.Retrieved18 February2024.
  39. ^abcNational Research Council (U.S.). Committee on Geodesy 1985
  40. ^Defense Mapping Agency 1984
  41. ^Torge 2001
  42. ^CSR 2011
  43. ^Muller & Sjogren 1968
  44. ^abcdReynolds 2011
  45. ^"Energy Geosciences".Jackson School of Geosciences.Retrieved18 February2024.
  46. ^Hardy & Goodman 2005
  47. ^Schröder, W. (2010). "History of geophysics".Acta Geodaetica et Geophysica Hungarica.45(2): 253–261.doi:10.1556/AGeod.45.2010.2.9.S2CID122239663.
  48. ^Varga, P. (2009)."Common roots of modern seismology and of earth tide research. A historical overview".Journal of Geodynamics.48(3–5): 241–246.Bibcode:2009JGeo...48..241V.doi:10.1016/j.jog.2009.09.032.S2CID129513373.
  49. ^Temple 2006,pp. 162–166
  50. ^Russo, Lucio(2004).The Forgotten Revolution.Berlin: Springer. p.273–277.
  51. ^Eratosthenes 2010
  52. ^Temple 2006,pp. 177–181
  53. ^abDewey & Byerly 1969
  54. ^abc"Review of" De Magnete "".pwg.gsfc.nasa.gov.Retrieved18 February2024.
  55. ^abSmith, George (2008),"Newton's Philosophiae Naturalis Principia Mathematica",in Zalta, Edward N. (ed.),The Stanford Encyclopedia of Philosophy(Winter 2008 ed.), Metaphysics Research Lab, Stanford University,retrieved18 February2024
  56. ^Institute of Physics (18 February 2024)."Precession of the equinoxes".Retrieved18 February2024.

References

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