사용자:Geosapiens1369/번역장1

위키백과, 우리 모두의 백과사전.

지구물리학(영어: geophysics)은 물리적인 방법을 이용해 지구를 연구하는 학문 분야이다. 지구물리학은 대기 물리학, 기상학, 해양학, 지진학, 화산학 , 지구 전자기학, 지구 중력학 등으로 구분할 수 있다. 지질학과 중복되는 부분이 많기 때문에 엄밀하게 구별하기는 어렵다. 넓은 의미에서는 우주과학도 포함되지만, 우주 일반이나 태양계 외의 천체에 관한 학문은 천체물리학의 범주로 분류하는 경우가 많다.

보통 지질학과(또는 지구과학과)의 커리큘럼 중 하나의 과목으로 편성되어 있으나, 미국에는 독립된 학부로서 지구물리학부(Department of Geophysics)가 있는 대학도 있다.

물리적인 현상들[편집]

지구물리학은 여러 학문 분야가 접하는 학문이며 지구물리학자들은 지구과학의 모든 분야에 기여한다. 지구과학과 물리학이 서로 어떻게 기여하는지 더 명확한 아이디어를 얻기 위해 물리학에서 연구되는 지구과학적 현상을 알아보자.

중력[편집]

Image of globe combining color with topography.
지오이드의 중력과 측정중력치의 편차를 나타낸 중력이상도.

달과 태양의 중력의 영향으로 매 태음일 마다 각각 두 번의 썰물과 밀물이 발생한다.[1]

  • 중력에 의한 암석의 깊이 효과

지구의 중력은 지구 내부 암반에 힘을 가하여 깊이에 따라 암석의 밀도를 증감시킨다. 이상적으로 암반이 연속적이며 균질하고 정역학적 봉압인 환경하에 있다고 상정했을 때 지면으로 부터의 깊이 h에 있는 암석에 연직 방향으로 가해지는 압력을 정수압방정식과 같은 p = ρgh로 표현할 수 있다.(단, 중력가속도g는 h의 함수이다.)[2]

  • 중력탐사

지표면 혹은 대기중에서의 중력가속도와 중력퍼텐셜의 측정은 광상탐사(mineral exploration)에 도움을 줄 수 있다(중력 이상, gravimetry참고).[3] 또한 지표면 중력장에 대한 정보는 판 구조론의 동역학적 기작에 관한 단서를 제공한다. 한편 중력탐사를 위해 지구의 중력장에 대한 표준 모델이 요구된다. 지오이드에서의 등지오퍼텐셜(geopotential)면은 측지학 지구모형의 정의가 된다. 지오이드는 바다에서는 평균 해수면으로 정의하고, 육지에서는 바다에서 시작하여 가상의 수로(canal)를 팠을 때, 수로의 수면으로 정의한다.[4]

열[편집]

Pseudocolor image in vertical profile.
맨틀 열대류 모델. 오랜지색 기둥은 맨틀플룸을 나타낸다.

지구 내부에는 지구 생성초기부터 간직되어 온 열과 방사성 동위원소의 붕괴와 압력 변화에 의한 열역학 퍼텐셜 차이로 인해 형성된 열이 존재한다. 이 열이 지표면으로 전달되는 과정에서 맨틀과 외핵은 열대류가, 지각에서는 열전도가 발생한다. 부가적으로 외핵의 열대류로 인해 지구의 자기장이 형성되며 맨틀 대류에 의해 판구조운동이 야기된다.[5]

진동[편집]

지구에서 발생하는 대표적인 진동은 지진파와 자유 진동이다. 지진파는 지구의 내부나 지표면을 통해 전달되는 진동이다. 또한 지구내부나 표면에서 대규모 지진에 의해 큰 충격이 발생한 후에는 지구 전체가 조화 진동자의 한 형태로 자유 진동(Free oscillations of the Earth)을 하기도 한다.

지진파와 자유 진동으로 인한 지표의 움직임은 지진계를 사용해 측정된다. 지진이나 폭발과 같이 진원이 국지적일 경우 다수의 지진계를 통해 진원의 위치나 파열된 단층의 형태, 발생한 에너지 등을 파악할 수 있다. 이러한 정보는 판 구조론과 맨틀 대류 연구에 중요한 단서를 제공한다.[6][7]

지진파의 측정을 통해 지진파가 통과해 온 지층의 정보를 파악할 수 있다. 암석의 밀도나 구성이 갑자기 변화한다면 지진파가 일부분 반사되는데 이러한 지진파 반사(Reflection seismology)를 이용해 천부 지각의 구조에 대한 정보를 얻을 수 있다.[3] 또한 지진파가 지구 내부 물질의 밀도와 구성 변화에 따라 굴절되는 지진파 굴절(Seismic refraction)을 이용해 지구의 구조를 추론할 수 있다.[7]

판 경계 혹은 심발에서 발생하는 지진의 메커니즘을 이해하는 것은 지진 예보 기술의 달발과 지진공학 기술의 진보를 야기할 것이다.[8]

전기[편집]

지구의 자연적인 전기적 현상이라고 하면 흔히 번개를 떠올린다. 하지만 우리는 이미 자연적으로 형성되는 전기장속에서 살아가고 있는데 그 정체는 다음과 같다. 지구의 대기는 우주선(宇宙線, cosmic rays)의 충격에 의해 전체적으로 양전하를 띄고있다. 지표면은 대기에 비해 상대적으로 음전하를 띄고있기 때문에 평균 약 120 V m−1의 연직방향으로의 전기장이 형성된다.[9] 따라서 대기와 지표를 하나의 거대한 회로로 볼 수 있으며 이 회로에는 평균 1800 A의 전류가 흐른다.[9] 그러한 대기의 전리층에서 지표면으로의 전하의 이동은 지구 전체에서 이루어지고 있으며 뇌우를 통해 다시 전리층으로 이동하게 된다. 뇌우를 통한 전류의 흐름은 구름 위아래의 번개의 형태로 나타난다.

다양한 전기적 현상들이 지구물리탐사에 이용된다. 대표적으로 다음과 같다.

  • 자연전위(spontaneous potential) : 인공적 혹은 자연적인 교란에 의해 지면에 형성되는 전위
  • 지전류(地電流, telluric current) : 지구의 영구적인 자기장에 더불어 외부 기원의 지자기장(geomagnetic field) 혹은 바닷물과 같은 전도체의 움직임에 의한 전자기유도에 따른 자기장의 효과로 발생하는 전류.[10] 지전류 밀도의 분포는 지하 구조의 전기저항의 차이를 감지하는데 이용될 수 있다. 또한 지구물리학자들은 인공적으로 자연전위를 발생시켜 연구나 탐사에 이용할 수 있다.(유도분극, 전기 고유 저항 단층 찰영(electrical resistivity tomography) 참고)


전자기장[편집]

Electromagnetic waves occur in the ionosphere and magnetosphere as well as the 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 Earth's outer core, electric currents in the highly conductive liquid iron create magnetic fields by electromagnetic induction (see geodynamo). Alfvén waves are magnetohydrodynamic waves in the 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.[11]

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

자성[편집]

<nowiki /> 이 부분의 본문은 Earth's magnetic field, Aeromagnetic survey 및 Paleomagnetism입니다.

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).[11] The magnetic field in the upper atmosphere gives rise to the auroras.[12]

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 dipole, but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals, analyzed within a Geomagnetic 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.[13] In addition, the magnetization in rocks can be used to measure the motion of continents.[11]

방사능[편집]

<nowiki /> 이 부분의 본문은 Radiometric dating 및 geotherm입니다.
Diagram with compound balls representing nuclei and arrows.
Example of a radioactive decay chain (see Radiometric dating).

Radioactive decay accounts for about 80% of the Earth's internal heat, powering the geodynamo and plate tectonics.[14] The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232.[15] 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 isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras.[16] Radiometric mapping using ground and airborne gamma spectrometry can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.[17][18]

유체역학[편집]

<nowiki /> 이 부분의 본문은 Geophysical fluid dynamics입니다.

Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has an enormous viscosity, flows like a fluid over long time intervals (see geodynamics). 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.[11]

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface.[19] In the Earth's core, the circulation of the molten iron is structured by Taylor columns.[11]

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

광물물리학[편집]

<nowiki /> 이 부분의 본문은 Mineral physics입니다.

The physical properties of minerals must be understood to infer the composition of the Earth's interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals; their high-pressure phase diagrams, melting points and 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 (see geodynamics).[2]

Water is a very complex substance and its unique properties are essential for life.[20] Its physical properties shape the hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation.[21] 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.[19]

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).[22]

지구의 물리적 특성[편집]

지구의 크기와 형태[편집]

<nowiki /> 이 부분의 본문은 Figure of the Earth입니다.

The Earth is roughly spherical, but it bulges towards the Equator, so it is roughly in the shape of an ellipsoid (see Earth ellipsoid). This bulge is due to its rotation and is nearly consistent with an Earth in hydrostatic equilibrium. The detailed 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.[4]

내부 구조[편집]

<nowiki /> 이 부분의 본문은 Structure of the Earth입니다.
Diagram with concentric shells and curved paths.
Seismic velocities and boundaries in the interior of the Earth sampled by seismic waves.

Evidence from seismology, heat flow at the surface, and mineral physics is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior – its composition, density, temperature, pressure. For example, the Earth's mean specific gravity (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.[2]

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 (see geodynamo). The inner core, however, is solid because of the enormous pressure.[4]

Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić discontinuity.[4]

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. This requires physical theory for thermal conduction and convection and the heat contribution of radioactive elements. The main model for the radial structure of the interior of the Earth is the preliminary reference Earth model (PREM). Some parts of this model have been updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography. The mantle is mainly composed of silicates, and the boundaries between layers of the mantle are consistent with phase transitions.[2]

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. Geodynamics is the study of the fluid flow in the mantle and core.

자기권[편집]

<nowiki /> 이 부분의 본문은 Magnetosphere입니다.
Diagram with colored surfaces and lines.
Schematic of Earth's magnetosphere. The solar wind flows from left to right.

If a planet's magnetic field is strong enough, its interaction with the solar wind forms a magnetosphere. Early space probes mapped out the gross dimensions of the Earth's magnetic field, which extends about 10 Earth radii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the 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.[12]

연구 방법[편집]

측지학[편집]

<nowiki /> 이 부분의 본문은 Geodesy입니다.

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.[23]

Absolute positions are most frequently determined using the global positioning system (GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the 1980 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 as nutation and Chandler wobble. Relative positions of two or more points can be determined using very-long-baseline interferometry.[23][24][25]

Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using gravimeters deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using radar altimetry, contributing to a more accurate geoid.[23] 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 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.[26]

인공 위성[편집]

Space probes 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 gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above 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.[27]

역사[편집]

<nowiki /> 이 부분의 본문은 History of geophysics입니다.

Geophysics emerged as a separate discipline only in the 19th century, from the intersection of physical geography, geology, astronomy, meteorology, and physics.[28][29] However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the ancient era.

고대 ＆ 중세[편집]

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.[30]

In circa 240 BC, Eratosthenes of Cyrene deduced that the Earth was round and measured the circumference of the Earth, using trigonometry and the angle of the Sun at more than one latitude in Egypt. He developed a system of latitude and longitude.[31]

Perhaps the earliest contribution to seismology was the invention of a seismoscope by the prolific inventor Zhang Heng in 132 AD.[32] 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.[33]

현대 과학의 시작[편집]

One of the publications that marked the beginning of modern science was William Gilbert's De Magnete (1600), a report of a series of meticulous experiments in magnetism. Gilbert deduced that compasses point north because the Earth itself is magnetic.[11]

In 1687 Isaac Newton published his Principia, which not only laid the foundations for classical mechanics and gravitation but also explained a variety of geophysical phenomena such as the tides and the precession of the equinox.[34]

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes in 1844.[33]

See also[편집]

Portal icon지질학 포털
Portal icon지리학 포털
Portal icon물리학 포털

Notes[편집]

  1. ↑ Ross 1995, 236–242쪽
  2. ↑ 가 나 다 라 Poirier 2000
  3. ↑ 가 나 Telford, Geldart & Sheriff 1990
  4. ↑ 가 나 다 라 Lowrie 2004
  5. ↑ Davies 2001
  6. ↑ Shearer, Peter M. (2009). 《Introduction to seismology》 2판. Cambridge: Cambridge University Press. ISBN 9780521708425. 
  7. ↑ 가 나 Stein & Wysession 2003
  8. ↑ Bozorgnia & Bertero 2004
  9. ↑ 가 나 Harrison & Carslaw 2003
  10. ↑ Lanzerotti & Gregori 1986
  11. ↑ 가 나 다 라 마 바 Merrill, McElhinny & McFadden 1996
  12. ↑ 가 나 Kivelson & Russell 1995
  13. ↑ Opdyke & Channell 1996
  14. ↑ Turcotte & Schubert 2002
  15. ↑ Sanders 2003
  16. ↑ Renne, Ludwig & Karner 2000
  17. ↑ “Radiometrics”. 《Geoscience Australia》. Commonwealth of Australia. 2014년 6월 23일에 확인함. 
  18. ↑ “Interpreting radiometrics”. 《Natural Resource Management》. Department of Agriculture and Food, Government of Western Australia. 2012년 3월 21일에 원본 문서에서 보존된 문서. 2014년 6월 23일에 확인함. 
  19. ↑ 가 나 Pedlosky 1987
  20. ↑ Sadava 등. 2009
  21. ↑ Sirvatka 2003
  22. ↑ CFG 2011
  23. ↑ 가 나 다 National Research Council (U.S.). Committee on Geodesy 1985
  24. ↑ Defense Mapping Agency 1984
  25. ↑ Torge 2001
  26. ↑ CSR 2011
  27. ↑ Muller & Sjogren 1968
  28. ↑ Hardy & Goodman 2005
  29. ↑ Schröder, W. (2010). “History of geophysics”. 《Acta Geodaetica et Geophysica Hungarica》 45 (2): 253–261. doi:10.1556/AGeod.45.2010.2.9. 
  30. ↑ Temple 2006, 162–166쪽
  31. ↑ Eratosthenes 2010
  32. ↑ Temple 2006, 177–181쪽
  33. ↑ 가 나 Dewey & Byerly 1969
  34. ↑ Newton 1999 Section 3

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