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 Newton applied 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 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 inphysics 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 theSolar System, which includes other planetary bodies.
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.[7]
Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.[8] Measurements ofgravitational acceleration andgravitational potential at the Earth's surface and above it can be used to look for mineral deposits (seegravity anomaly andgravimetry).[9] The surface gravitational field provides information on the dynamics oftectonic plates. Thegeopotential surface called thegeoid 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).[10]
Illustration of the deformations of a block by body waves and surface waves (seeseismic wave)
Seismic waves are vibrations that travel through the Earth's interior or along its surface.[11] The entire Earth can also oscillate in forms that are callednormal modes orfree 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.[12][13]
Recording ofseismic waves 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 usingReflection 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.[9] Changes in the travel direction, calledrefraction, can be used to infer thedeep structure of the Earth.[13]
Although we mainly notice electricity duringthunderstorms, there is always a downward electric field near the surface that averages 120volts per meter.[15] Relative to the solid Earth, the ionization of the planet's atmosphere is a result of the galacticcosmic rays penetrating it, which leaves it with a net positive charge.[16] A current of about 1800amperes flows in the global circuit.[15] It flows downward from theionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds andsprites above.
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 currents flow in Earth and the oceans. They have two causes:electromagnetic induction by the time-varying, external-origingeomagnetic field and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field.[17] The distribution of telluric current density can be used to detect variations inelectrical resistivity of underground structures. Geophysicists can also provide the electric current themselves (seeinduced polarization andelectrical resistivity tomography).
In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction.Alfvén waves aremagnetohydrodynamic waves in themagnetosphere 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 magneticRossby waves may be one source ofgeomagnetic secular variation.[18]
The Earth's magnetic field protects the Earth from the deadlysolar wind and has long been used for navigation. It originates in the fluid motions of the outer core.[18] The magnetic field in the upper atmosphere gives rise to theauroras.[20]
Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line).Computer simulation of theEarth's magnetic field in a period of normal polarity betweenreversals[21]
Unstable isotopes decay at predictable rates, and the decay rates of differentisotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in pastgeologic eras.[25] Radiometric mapping using ground and airbornegamma 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.[26][27]
Fluid motions occur 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 rebound andmantle plumes. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.[18]
Geophysical fluid dynamics is a primary tool inphysical oceanography andmeteorology. 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 waves and determines the basic circulation patterns of storms. In the ocean, they drive large-scale circulation patterns as well asKelvin waves andEkman spirals at the ocean surface.[28] In the Earth's core, the circulation of the molten iron is structured byTaylor columns.[18]
Waves and other phenomena in the magnetosphere can be modeled usingmagnetohydrodynamics.
The Earth is cooling, and the resultingheat flow generates the Earth's magnetic field through thegeodynamo and plate tectonics throughmantle convection.[29] The main sources of heat are: primordial heat due to Earth's cooling andradioactivity in the planets upper crust.[30] 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 boundary and thelithosphere – in which heat is transported byconduction.[31] Some heat is carried up from the bottom of themantle bymantle plumes. The heat flow at the Earth's surface is about 4.2 × 1013 W, and it is a potential source ofgeothermal energy.[32]
The physical properties of minerals must be understood to infer the composition of the Earth's interior fromseismology, thegeothermal gradient and other sources of information. Mineral physicists study theelastic properties of minerals; their high-pressurephase diagrams, melting points andequations of state at high pressure; and therheological properties of rocks, or their ability to flow. Deformation of rocks bycreep make flow possible, although over short times the rocks are brittle. Theviscosity of rocks is affected by temperature and pressure, and in turn, determines the rates at which tectonic plates move.[8]
Water is a very complex substance and its unique properties are essential for life.[33] Its physical properties shape thehydrosphere and are an essential part of thewater cycle andclimate. Its thermodynamic properties determineevaporation and the thermal gradient in theatmosphere. The many types ofprecipitation involve a complex mixture of processes such ascoalescence,supercooling andsupersaturation.[34] Some precipitated water becomesgroundwater, and groundwater flow includes phenomena such aspercolation, while theconductivity of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such assalinity have a large effect on its motion in the oceans.[28]
Contrary to popular belief, the earth is not entirely spherical but instead generally exhibits anellipsoid shape- which is a result of the centrifugal forces the planet generates due to its constant motion.[36] These forces cause the planets diameter to bulge towards theEquator and results in theellipsoid shape.[36] 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[37]), geological features such asmountains orocean trenches,tectonic plate dynamics, andnatural disasters can further distort the planet's shape.[36]
Seismic velocities and boundaries in the interior of theEarth sampled by seismic waves
Evidence fromseismology, heat flow at the surface, andmineral 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 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 to 0.4M R2 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 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.[8]
Reconstructions of seismic waves in the deep interior of the Earth show that there are noS-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.[10]
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,lithosphere andcrust. The mantle itself is divided into theupper mantle, transition zone, lower mantle andD′′ layer. Between the crust and the mantle is theMohorovičić discontinuity.[10]
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 thegeotherm must also be determined. This requires physical theory forthermal conduction andconvection and 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.[8]
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 tectonics possible.
Schematic of Earth's magnetosphere. Thesolar wind flows from left to right.
If a planet'smagnetic field is strong enough, its interaction with the solar wind forms a magnetosphere. Earlyspace probes mapped out the gross dimensions of the Earth's magnetic field, which extends about 10Earth radii towards 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.[20]
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 Union and theInternational Union of Geodesy and Geophysics encompass both.[38]
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 asnutation andChandler wobble. Relative positions of two or more points can be determined usingvery-long-baseline interferometry.[38][39][40]
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 usinggravimeters 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 usingradar altimetry, contributing to a more accurategeoid.[38] In 2002,NASA launched theGravity Recovery and Climate Experiment (GRACE), wherein two twinsatellites 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.[41]
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 fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances abovelunar maria were measured throughlunar orbiters, which led to the discovery of concentrations of mass,mascons, beneath theImbrium,Serenitatis,Crisium,Nectaris andHumorum basins.[42]
Global positioning systems (GPS) and geographical information systems (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 GPS processing. Once the geophysical measurements have been processed and inverted, the interpreted results are plotted using GIS. Programs such asArcGIS andGeosoft were built to meet these needs and include many geophysical functions that are built-in, such asupward continuation, and the calculation of the measurementderivative such as the first-vertical derivative.[9][43] 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.
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.[44] This is done through the uses of variousremote sensing platforms such as;satellites,aircraft,boats,drones,borehole sensing equipment andseismic receivers. These equipment are often used in conjunction with different geophysical methods such asmagnetic,gravimetry,electromagnetic,radiometric,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 gatheringaeromagnetic 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 throughEarth's magnetic field.[9] 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.[9][43]
Geophysical measurements are often recorded astime-series withGPS 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.[9][43] In seismic data, electromagnetic data, and gravity data, processing continues after error corrections to includecomputational geophysics which result in the final interpretation of the geophysical data into a geological interpretation of the geophysical measurements[9][43]
Geophysics emerged as a separate discipline only in the 19th century, from the intersection ofphysical geography,geology,astronomy, meteorology, and physics.[45][46] The first known use of the wordgeophysics was in German ("Geophysik") byJulius Fröbel in 1834.[47] However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since theancient era.
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 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.[48]
Perhaps the earliest contribution to seismology was the invention of aseismoscope by the prolific inventorZhang Heng in 132 AD.[51] 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.[52]
The 17th century had major milestones that marked the beginning of modern science. In 1600,William Gilbert release a publication titledDe Magnete (1600) where he conducted series of experiments on both natural magnets (called'loadstones') and artificially magnetized iron.[53] 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.[53] HIs findings led to the deduction that compasses point north due to the Earth itself being a giant magnet.[53]
In 1687Isaac Newton published his work titledPrincipia which was pivotal in the development of modern scientific fields such asastronomy andphysics.[54] In it, Newton both laid the foundations forclassical mechanics andgravitation, as well as explained different geophysical phenomena such as theprecession of the equinox (the orbit of whole star patterns along anecliptic axis.[55]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.[54]
The firstseismometer, an instrument capable of keeping a continuous record of seismic activity, was built byJames Forbes in 1844.[52]
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