Geophysics

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(Redirected from
Geophysical
)
For the journal, see Geophysics (journal).
Not to be confused with Physical geography.

false color image
Age of the sea floor. Much of the dating information comes from magnetic anomalies.[1]
Computer simulation of the Earth's magnetic field in a period of normal polarity between reversals[2]

Geophysics (

solar-terrestrial physics; and analogous problems associated with the Moon and 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 from

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 applied to societal needs, such as

natural hazards and environmental protection.[4] 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
.

Physical phenomena

Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of 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 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 geodesy and 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.[9]

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

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).[12]

Heat flow

Main article: Geothermal gradient
mantle plumes
.

The Earth is cooling, and the resulting

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.[16]

Vibrations

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

seismographs. 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 of

deep structure of the Earth.[19]

Earthquakes pose a

deep focus), can lead to better estimates of earthquake risk and improvements in earthquake engineering.[20]

Electricity

Although we mainly notice electricity during

cosmic rays penetrating it, which leaves it with a net positive charge.[22] A current of about 1800 amperes flows in the global circuit.[21] 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 sprites
above.

A variety of electric methods are used in geophysical survey. Some measure

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 is believed to be caused by high-energy electrons that get caught in the Van Allen radiation belt. Whistlers are produced by lightning strikes. Hiss may be generated by both. 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.

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 waves may be one source of geomagnetic secular variation.[24]

Electromagnetic methods that are used for geophysical survey include transient electromagnetics, magnetotellurics, surface nuclear magnetic resonance and electromagnetic seabed logging.[25]

Magnetism

Further information: Earth's magnetic field, Aeromagnetic survey, and Paleomagnetism

The Earth's magnetic field protects the Earth from the deadly

auroras.[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 tilted

geomagnetic reversals, analyzed within a 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 last glacial period. Geologists observed geomagnetic reversal recorded in volcanic rocks, through 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 stratigraphies to construct geologic time scales.[27] In addition, the magnetization in rocks can be used to measure the motion of continents.[24]

Radioactivity

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

isotopes are potassium-40, uranium-238, uranium-235, and thorium-232.[29]
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

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.[31][32]

Fluid dynamics

Main article: Geophysical fluid dynamics

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 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 in

Taylor columns.[24]

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

Mineral physics

Main article: Mineral physics

The physical properties of minerals must be understood to infer the composition of the Earth's interior from

equations of state at high pressure; and the rheological properties of rocks, or their ability to flow. Deformation of rocks by 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.[10]

Water is a very complex substance and its unique properties are essential for life.

precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation.[35] Some precipitated water becomes groundwater, and groundwater flow includes phenomena such as percolation, while the 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.[33]

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).[36]

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.[37] These forces cause the planets diameter to bulge towards the Equator and results in the ellipsoid shape.[37] Earth's shape is constantly changing, and different factors including glacial 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 as mountains or ocean trenches, tectonic plate dynamics, and natural disasters can further distort the planet's shape.[37]

Structure of the interior

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

Evidence from

specific 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 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 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.[10]

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.[12]

Reconstruction of seismic reflections in the deep interior indicates some major discontinuities in seismic velocities that demarcate the major zones of the Earth:

upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić 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 the

silicates, 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 makes plate tectonics possible.

Magnetosphere

Main article: Magnetosphere
Diagram with colored surfaces and lines.
Schematic of Earth's magnetosphere. The solar wind flows from left to right.

If a planet's

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.[26]

Methods

Geodesy

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 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.[39]

Absolute positions are most frequently determined using the

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.[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 using

Gravity Recovery and Climate Experiment (GRACE), wherein two twin satellites 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.[42]

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 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

lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.[43]

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

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 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.[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

Main article: 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.[45] This is done through the uses of various remote sensing platforms such as; satellites, aircraft, boats, drones, borehole sensing equipment and seismic receivers. These equipment are often used in conjunction with different geophysical methods such as magnetic, 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 gathering 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.[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

Main article: Signal processing

Geophysical measurements are often recorded as

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.[11][44] 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[11][44]

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.[46][47] The first known use of the word geophysics was in German ("Geophysik") by Julius Fröbel in 1834.[48] However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the ancient era.

Ancient and classical eras

Picture of ornate urn-like device with spouts in the shape of dragons
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 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.[49]

In circa 240 BC,

circumference of Earth with great precision.[50] He developed a system of latitude and longitude.[51]

Perhaps the earliest contribution to seismology was the invention of a

seismoscope by the prolific inventor Zhang Heng in 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, by Jean de la Hautefeuille. It was never built.[53]

Beginnings of modern science

The 17th century had major milestones that marked the beginning of modern science. In 1600, William Gilbert release a publication titled De 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 1687

ecliptic axis[56]). 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.[55]

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes in 1844.[53]

See also

Notes

  1. S2CID 15960331
    .
  2. ^ "Earth's Inconstant Magnetic Field". science@nasa. National Aeronautics and Space Administration. 29 December 2003. Retrieved 13 November 2018.
  3. ^ a b Sheriff 1991
  4. ^ a b IUGG 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. ^ a b c d Poirier 2000
  11. ^ a b c d e f g Telford, Geldart & Sheriff 1990
  12. ^ a b c Lowrie 2004
  13. ^ Davies 2001
  14. ^ "What is "Heat Flow"?". www.smu.edu. Retrieved 18 February 2024.
  15. ^ Fowler 2005
  16. ^ Pollack, Hurter & Johnson 1993
  17. ^ "Seismic wave | Earth's Interior Structure & Movement | Britannica". www.britannica.com. 12 January 2024. Retrieved 18 February 2024.
  18. ISBN 9780521708425
    .
  19. ^ a b Stein & Wysession 2003
  20. ^ Bozorgnia & Bertero 2004
  21. ^ a b Harrison & Carslaw 2003
  22. ^ Nicoll, Keri (April 2016). "Earth's electric atmosphere" (PDF). metlink.org. Retrieved 18 February 2024.
  23. ^ Lanzerotti & Gregori 1986
  24. ^ a b c d e Merrill, McElhinny & McFadden 1998
  25. ISBN 978-3-319-45355-2
    .
  26. ^ a b Kivelson & 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. Retrieved 23 June 2014.
  32. ^ "Interpreting radiometrics". Natural Resource Management. Department of Agriculture and Food, Government of Western Australia. Archived from the original on 21 March 2012. Retrieved 23 June 2014.
  33. ^ a b Pedlosky 1987
  34. ^ Sadava et al. 2009
  35. ^ Sirvatka 2003
  36. ^ CFG 2011
  37. ^ a b c "Is the Earth round?". oceanservice.noaa.gov. Retrieved 18 February 2024.
  38. ^ US Department of Commerce, National Oceanic and Atmospheric Administration. "What is glacial isostatic adjustment?". oceanservice.noaa.gov. Retrieved 18 February 2024.
  39. ^ a b c National Research Council (U.S.). Committee on Geodesy 1985
  40. ^ Defense Mapping Agency 1984
  41. ^ Torge 2001
  42. ^ CSR 2011
  43. ^ Muller & Sjogren 1968
  44. ^ a b c d Reynolds 2011
  45. ^ "Energy Geosciences". Jackson School of Geosciences. Retrieved 18 February 2024.
  46. ^ Hardy & Goodman 2005
  47. S2CID 122239663
    .
  48. S2CID 129513373
    .
  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. ^ a b Dewey & Byerly 1969
  54. ^ a b c "Review of "De Magnete"". pwg.gsfc.nasa.gov. Retrieved 18 February 2024.
  55. ^ a b Smith, 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, retrieved 18 February 2024
  56. ^ Institute of Physics (18 February 2024). "Precession of the equinoxes". Retrieved 18 February 2024.

References

External links

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