Introduction of Geophysics

10/5/2014 Geophysics - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Geophysics 1/13

Age of the sea floor. Much of the dating information

comes from magnetic anomalies.

GeophysicsFrom Wikipedia, the free encyclopedia

Geophysics /di ofzks/ is the physics of the Earthand its environment in space; also the study of theEarth using quantitative physical methods. The termgeophysics sometimes refers only to the geologicalapplications: Earth's shape; its gravitational andmagnetic fields; its internal structure and composition;its dynamics and their surface expression in platetectonics, the generation of magmas, volcanism and

rock formation.[1] However, modern geophysicsorganizations use a broader definition that includes thehydrological cycle including snow and ice; fluiddynamics of the oceans and the atmosphere; electricityand magnetism in the ionosphere and magnetosphereand solar-terrestrial relations; and analogous problems

associated with the Moon and other planets.[1][2][3]

Although geophysics was only recognized as a separate discipline in the 19th century, its origins go back to ancienthistory. The first magnetic compasses were lodestones, appearing in written records, found in early survivingdescriptions from China, India and Greece, with a modern magnetic compass dating back to the fourth century BCand the first seismoscope was built in 132 BC. Geophysical methods were developed for navigation; Isaac Newtonapplied his theory of mechanics to the tides and the precession of the equinox; and instruments were developed tomeasure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20thcentury, geophysical methods were developed for remote exploration of the solid Earth and the ocean, andgeophysics played an essential role in the development of the theory of plate tectonics.

Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental

protection.[2] 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 forenvironmental remediation.

Contents

1 Physical phenomena

1.1 Gravity

1.2 Heat flow

1.3 Vibrations

1.4 Electricity

1.5 Electromagnetic waves

1.6 Magnetism



1.7 Radioactivity

1.8 Fluid dynamics

1.9 Mineral physics

2 Regions of the Earth

2.1 Size and form of the Earth

2.2 Structure of the Earth

2.3 Magnetosphere

3 Methods

3.1 Geodesy

3.2 Space probes

4 History

4.1 Ancient and classical eras

4.2 Beginnings of modern science

5 See also

6 Notes

7 References

8 External links

Physical phenomena

Geophysics is a highly interdisciplinary subject and geophysicists contribute to every area of the Earth sciences. Toprovide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physicsand how they relate to the Earth and its surroundings.

Gravity

Main article: Gravity of Earth

Further information: Physical geodesy, Gravimetry

The gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and

between every low tide.[4]

Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.[5]

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).[6] The surface gravitational field providesinformation on the dynamics of tectonic plates. The geopotential surface called the geoid is one definition of theshape 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).[7]



A map of deviations in gravity from a

perfectly smooth, idealized Earth.

A model of thermal convection in the Earth's

mantle. The thin red columns are mantle

plumes.

Heat flow

Main article: Geothermal gradient

The Earth is cooling, and the resulting heat flow generates theEarth's magnetic field through the geodynamo and plate tectonics

through mantle convection.[8] The main sources of heat are theprimordial heat and radioactivity, although there are alsocontributions from phase transitions. Heat is mostly carried to thesurface by thermal convection, although there are two thermalboundary layers the core-mantle boundary and the lithosphere

in which heat is transported by conduction.[9] Some heat is carriedup from the bottom of the mantle by mantle plumes. The heat flow

at the Earth's surface is about 4.2 1013 W, and it is a

potential source of geothermal energy.[10]

Vibrations

Main article: Seismology

Seismic waves are vibrations that travel through the Earth's interioror along its surface. The entire Earth can also oscillate in forms thatare called normal modes or free oscillations of the Earth. Groundmotions from waves or normal modes are measured usingseismographs. If the waves come from a localized source such asan 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.[11][12]

Measurements of seismic waves are a source of information on the region that the waves travel through. If thedensity or composition of the rock changes suddenly, some of the waves are reflected. Reflections can provide

information on near-surface structure.[6] Changes in the travel direction, called refraction, can be used to infer the

deep structure of the Earth.[12]

Earthquakes pose a risk to humans. Understanding their mechanisms, which depend on the type of earthquake(e.g., intraplate or deep focus), can lead to better estimates of earthquake risk and improvements in earthquake

engineering.[13]

Electricity

Although we mainly notice electricity during thunderstorms, there is always a downward electric field near the

surface that averages 120 V m1.[14] Relative to the solid Earth, the atmosphere has a net positive charge due to

bombardment by cosmic rays. A current of about 1800 A flows in the global circuit.[14] It flows downward fromthe ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightningbelow the clouds and sprites above.



Illustration of the deformations of a block by

body waves and surface waves (see seismic

wave).

A variety of electric methods are used in geophysical survey.Some measure spontaneous potential, a potential that arises in theground because of man-made or natural disturbances. Telluriccurrents 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.[15] Thedistribution of telluric current density can be used to detectvariations in electrical resistivity of underground structures.Geophysicists can also provide the electric current themselves (seeinduced polarization and electrical resistivity tomography).

Electromagnetic waves

Electromagnetic waves occur in the ionosphere andmagnetosphere as well as the Earth's outer core. Dawn chorus isbelieved to be caused by high-energy electrons that get caught inthe Van Allen radiation belt. Whistlers are produced by lightningstrikes. Hiss may be generated by both. Electromagnetic wavesmay also be generated by earthquakes (see seismo-electromagnetics).

In the Earth's outer core, electric currents in the highly conductiveliquid iron create magnetic fields by electromagnetic induction (seegeodynamo). Alfvn waves are magnetohydrodynamic waves inthe magnetosphere or the Earth's core. In the core, they probably have little observable effect on the geomagnetic

field, but slower waves such as magnetic Rossby waves may be one source of geomagnetic secular variation.[16]

Electromagnetic methods that are used for geophysical survey include transient electromagnetics andmagnetotellurics.

Magnetism

Further information: Earth's magnetic field and 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 Earth's outer core (see geodynamo).[16] The magnetic field in the upper

atmosphere gives rise to the auroras.[17]

The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called geomagnetic secularvariation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging 440,000to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals, analyzed within aGeomagnetic Polarity Time Scale, contain 184 polarity intervals in the last 83 million years, with change infrequency over time, with the most recent brief complete reversal of the Laschamp event occurring 41,000 yearsago during the last glacial period. Geologists observed geomagnetic reversal recorded in volcanic rocks, throughmagnetostratigraphy correlation (see natural remanent magnetization) and their signature can be seen as parallellinear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor



Earth's dipole axis (pink line) is

tilted away from the rotational axis

(blue line).

Example of a radioactive decay chain

(see Radiometric dating).

spreading, a part of plate tectonics. They are the basis of magnetostratigraphy, which correlates magnetic reversals

with other stratigraphies to construct geologic time scales.[18] In addition, the magnetization in rocks can be used to

measure the motion of continents.[16]

Radioactivity

Further information: Radiometric dating and geotherm

Radioactive decay accountsfor about 80% of the Earth'sinternal heat, powering thegeodynamo and plate

tectonics.[19] The main heat-producing isotopes arepotassium-40, uranium-238,uranium-235, and thorium-

232.[20] Radioactive elementsare used for radiometric dating,the primary method forestablishing an absolute timescale in geochronology.Unstable isotopes decay atpredictable rates, and the

decay rates of different isotopes cover several orders of magnitude,so radioactive decay can be used to accurately date both recent

events and events in past geologic eras.[21]

Fluid dynamics

Main article: Geophysical fluid dynamics

Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has anenormous viscosity, flows like a fluid over long time intervals (see geodynamics). This flow is reflected inphenomena such as isostasy, post-glacial rebound and mantle plumes. The mantle flow drives plate tectonics and

the flow in the Earth's core drives the geodynamo.[16]

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earthhas profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise tolarge-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.[22] In the

Earth's core, the circulation of the molten iron is structured by Taylor columns.[16]

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

Mineral physics



Further information: Mineral physics

The physical properties of minerals must be understood to infer the composition of the Earth's interior fromseismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic propertiesof minerals; their high-pressure phase diagrams, melting points and equations of state at high pressure; and therheological properties of rocks, or their ability to flow. Deformation of rocks by creep make flow possible, althoughover 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 (see geodynamics).[5]

Water is a very complex substance and its unique properties are essential for life.[23] Its physical properties shapethe hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determineevaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture

of processes such as coalescence, supercooling and supersaturation.[24] Some of the precipitated water becomesgroundwater, and groundwater flow includes phenomena such as percolation, while the conductivity of watermakes 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.[22]

The many phases of ice form the cryosphere and come in forms like ice sheets, glaciers, sea ice, freshwater ice,

snow, and frozen ground (or permafrost).[25]

Regions of the Earth

Size and form of the Earth

Main article: Figure of the Earth

The Earth is roughly spherical, but it bulges towards the Equator, so it is roughly in the shape of an ellipsoid (seeEarth ellipsoid). This bulge is due to its rotation and is nearly consistent with an Earth in hydrostatic equilibrium. Thedetailed shape of the Earth, however, is also affected by the distribution of continents and ocean basins, and to

some extent by the dynamics of the plates.[7]

Structure of the Earth

Main article: Structure of the Earth

Evidence from seismology, heat flow at the surface, and mineral physics is combined with the Earth's mass andmoment of inertia to infer models of the Earth's interior its composition, density, temperature, pressure. Forexample, the Earth's mean specific gravity (5.515) is far higher than the typical specific gravity of rocks at thesurface (2.73.3), implying that the deeper material is denser. This is also implied by its low moment of inertia (

0.33 M R2, compared to 0.4 M R2 for a sphere of constant density). However, some of the density increase iscompression under the enormous pressures inside the Earth. The effect of pressure can be calculated using theAdamsWilliamson 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.[5]



Seismic velocities and boundaries in the

interior of the Earth sampled by seismic

waves.

Schematic of Earth's magnetosphere. The solar

wind flows from left to right.

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 themotion of this highly conductive fluid generates the Earth's field (see geodynamo). The inner core, however, is solid

because of the enormous pressure.[7]

Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities thatdemarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself isdivided into the upper mantle, transition zone, lower mantle and D layer. Between the crust and the mantle is the

Mohorovii discontinuity.[7]

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model ofthe Earth, mineral physics is needed to interpret seismic velocitiesin terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. Thisrequires physical theory for thermal conduction and convectionand the heat contribution of radioactive elements. The main modelfor the radial structure of the interior of the Earth is the preliminaryreference Earth model (PREM). Some parts of this model havebeen updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography. The mantleis mainly composed of silicates, and the boundaries between layers

of the mantle are consistent with phase transitions.[5]

The mantle acts as a solid for seismic waves, but under highpressures and temperatures it deforms so that over millions ofyears it acts like a liquid. This makes plate tectonics possible.Geodynamics is the study of the fluid flow in the mantle and core.

Magnetosphere

Main article: Magnetosphere

If a planet's magnetic field is strong enough, its interactionwith the solar wind forms a magnetosphere. Early spaceprobes mapped out the gross dimensions of the Earth'smagnetic field, which extends about 10 Earth radii towardsthe Sun. The solar wind, a stream of charged particles,streams out and around the terrestrial magnetic field, andcontinues behind the magnetic tail, hundreds of Earth radiidownstream. Inside the magnetosphere, there are relativelydense regions of solar wind particles called the Van Allen

radiation belts.[17]

Methods

Geodesy



Main article: Geodesy

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, alongwith 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.[26]

Absolute positions are most frequently determined using the global positioning system (GPS). A three-dimensionalposition is calculated using messages from four or more visible satellites and referred to the 1980 GeodeticReference System. An alternative, optical astronomy, combines astronomical coordinates and the local gravityvector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficultto 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.[26][27][28]

Gravity measurements became part of geodesy because they were needed to related measurements at the surfaceof the Earth to the reference coordinate system. Gravity measurements on land can be made using gravimetersdeployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has beenmeasured by analyzing the motion of satellites. Sea level can also be measured by satellites using radar altimetry,

contributing to a more accurate geoid.[26] In 2002, NASA launched the Gravity Recovery and Climate Experiment(GRACE), wherein two twin satellites map variations in Earth's gravity field by making measurements of thedistance between the two satellites using GPS and a microwave ranging system. Gravity variations detected byGRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets

and glaciers.[29]

Space probes

Space probes 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 their magnetic 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 the gravityfields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria weremeasured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the

Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.[30]

History

Main article: 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.[31][32] However, many geophysical phenomena such as theEarth's magnetic field and earthquakes have been investigated since the ancient era.

Ancient and classical eras



Replica of Zhang Heng's

seismoscope, possibly the

first contribution to

seismology.

The magnetic compass existed in China back as far as the fourth century BC. It was used as much for feng shui asfor navigation on land. It was not until good steel needles could be forged thatcompasses were used for navigation at sea; before that, they could not retain theirmagnetism long enough to be useful. The first mention of a compass in Europe

was in 1190 AD.[33]

In circa 240 BC, Eratosthenes of Cyrene deduced that the Earth was round andmeasured the circumference of the Earth, using trigonometry and the angle of theSun at more than one latitude in Egypt. He developed a system of latitude and

longitude.[34]

Perhaps the earliest contribution to seismology was the invention of a

seismoscope by the prolific inventor Zhang Heng in 132 AD.[35] This instrumentwas designed to drop a bronze ball from the mouth of a dragon into the mouth ofa toad. By looking at which of eight toads had the ball, one could determine thedirection of the earthquake. It was 1571 years before the first design for aseismoscope was published in Europe, by Jean de la Hautefeuille. It was never

built.[36]

Beginnings of modern science

One of the publications that marked the beginning of modern science was William Gilbert's De Magnete (1600), areport of a series of meticulous experiments in magnetism. Gilbert deduced that compasses point north because the

Earth itself is magnetic.[16]

In 1687 Isaac Newton published his Principia, which not only laid the foundations for classical mechanics andgravitation but also explained a variety of geophysical phenomena such as the tides and the precession of the

equinox.[37]

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James

Forbes in 1844.[36]

See also

List of geophysicists

Outline of geophysics

Notes

1. ^a b Sheriff 1991

2. ^a b IUGG 2011

3. ^ AGU 2011

4. ^ Ross 1995, pp. 236242



5. â b c d Poirier 2000

6. â b Telford, Geldart & Sheriff 1990

7. â b c d Lowrie 2004

8. ^ Davies 2001

9. ^ Fowler 2005

10. ^ Pollack, Hurter & Johnson 1993

11. ^ Shearer, Peter M. (2009). Introduction to seismology (2nd ed.). Cambridge: Cambridge University Press.

ISBN 9780521708425.

12. â b Stein & Wysession 2003

13. ^ Bozorgnia & Bertero 2004

14. â b Harrison & Carslaw 2003

15. ^ Lanzerotti & Gregori 1986

16. â b c d e f Merrill, McElhinny & McFadden 1996

17. â b Kivelson & Russell 1995

18. ^ Opdyke & Channell 1996

19. ^ Turcotte & Schubert 2002

20. ^ Sanders 2003

21. ^ Renne, Ludwig & Karner 2000

22. â b Pedlosky 1987

23. ^ Sadava et al. 2009

24. ^ Sirvatka 2003

25. ^ CFG 2011

26. â b c National Research Council (U.S.). Committee on Geodesy 1985

27. ^ Defense Mapping Agency 1984

28. ^ Torge 2001

29. ^ CSR 2011

30. ^ Muller & Sjogren 1968

31. ^ Hardy & Goodman 2005

32. ^ Schrder, W. (2010). "History of geophysics". Acta Geodaetica et Geophysica Hungarica 45 (2): 253261.

doi:10.1556/AGeod.45.2010.2.9 (http://dx.doi.org/10.1556%2FAGeod.45.2010.2.9).

33. ^ Temple 2006, pp. 162166

34. ^ Eratosthenes 2010

35. ^ Temple 2006, pp. 177181

36. â b Dewey & Byerly 1969

37. ^ Newton 1999 Section 3



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External links

A reference manual for near-surface geophysics techniques and applications (http://www.environmental-

geophysics.co.uk/documents/ref_manual/TechRef.pdf)

Commission on Geophysical Risk and Sustainability (GeoRisk), International Union of Geodesy and

Geophysics (IUGG) (http://www.iugg-georisk.org/)

Study of the Earth's Deep Interior, a Committee of IUGG (http://www.sedigroup.org/)

Union Commissions (IUGG) (http://www.iugg.org/about/commissions/)

USGS Geomagnetism Program (http://geomag.usgs.gov/)

Career crate: Seismic processor (http://careercrate.com/video/266/Seismic-processor)

Society of Exploration Geophysicists (http://www.seg.org/)

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Stein, Seth; Wysession, Michael (2003). An introduction to seismology, earthquakes, and earth structure. Wiley-

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Telford, William Murray; Geldart, L. P.; Sheriff, Robert E. (1990). Applied geophysics. Cambridge University

Press. ISBN 978-0-521-33938-4.

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Turcotte, Donald Lawson; Schubert, Gerald (2002). Geodynamics (http://books.google.com/books?id=-

nCHlVuJ4FoC&pg=PA286) (2nd ed.). Cambridge University Press. ISBN 0-521-66624-4.

Verhoogen, John (1980). Energetics of the Earth (http://books.google.com/?id=yTsrAAAAYAAJ). National

Academy Press. ISBN 978-0-309-03076-2.

Introduction of Geophysics

Documents

geophysicsfrom wikipedia

geophysical methods

free encyclopediahttp

quantitative physical

modern magnetic compass

physical geodesy

mineral deposits

mineral resources