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Free-air Anomaly
In geophysics, the free-air gravity anomaly, often simply called the free-air anomaly, is the measured gravity anomaly after a free-air correction is applied to account for the elevation at which a measurement is made. It does so by adjusting these measurements of gravity to what would have been measured at a reference level, which is commonly taken as mean sea level or the geoid. Applications Studies of the subsurface structure and composition of the earth's crust and mantle employ surveys using gravimeters to measure the departure of observed gravity from a theoretical gravity value to identify anomalies due to geologic features below the measurement locations. The computation of anomalies from observed measurements involves the application of corrections that define the resulting anomaly. The free-air anomaly can be used to test for isostatic equilibrium over broad regions. Survey methods The free-air correction adjusts measurements of gravity to what would have been meas ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geophysics
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' sometimes refers only to solid earth applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets. Gutenberg, B., 1929, Lehrbuch der Geophysik. Leipzig. Berlin (Gebruder Borntraeger). Runcorn, S.K, (editor-in-chief), 1967, International ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bouguer Anomaly
In geodesy and geophysics, the Bouguer anomaly (named after Pierre Bouguer) is a gravity anomaly, corrected for the height at which it is measured and the attraction of terrain. The height correction alone gives a free-air gravity anomaly. Definition The Bouguer anomaly g_B defined as: g_B = g_ - \delta g_B + \delta g_T Here, * g_F is the free-air gravity anomaly. * \delta g_B is the ''Bouguer correction'' which allows for the gravitational attraction of rocks between the measurement point and sea level; * \delta g_T is a ''terrain correction'' which allows for deviations of the surface from an infinite horizontal plane The free-air anomaly g_F, in its turn, is related to the observed gravity g_ as follows: g_F = g_ - g_\lambda + \delta g_F where: * g_\lambda is the correction for latitude (because the Earth is not a perfect sphere; see normal gravity); * \delta g_F is the free-air correction. Reduction A Bouguer reduction is called ''simple'' (or ''incomplete'') if the t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reference Ellipsoid
An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations. It is a spheroid (an ellipsoid of revolution) whose minor axis (shorter diameter), which connects the geographical North Pole and South Pole, is approximately aligned with the Earth's axis of rotation. The ellipsoid is defined by the ''equatorial axis'' (''a'') and the ''polar axis'' (''b''); their radial difference is slightly more than 21 km, or 0.335% of ''a'' (which is not quite 6,400 km). Many methods exist for determination of the axes of an Earth ellipsoid, ranging from meridian arcs up to modern satellite geodesy or the analysis and interconnection of continental geodetic networks. Amongst the different set of data used in national surveys are several of special importance: the Bessel ellipsoid of 1841, the international H ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Earth's Gravity
The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector quantity, whose direction coincides with a plumb bob and strength or magnitude is given by the norm g=\, \mathit\, . In SI units this acceleration is expressed in metres per second squared (in symbols, m/ s2 or m·s−2) or equivalently in newtons per kilogram (N/kg or N·kg−1). Near Earth's surface, the gravity acceleration is approximately , which means that, ignoring the effects of air resistance, the speed of an object falling freely will increase by about per second every second. This quantity is sometimes referred to informally as ''little '' (in contrast, the gravitational constant is referred to as ''big ''). The precise strength of Earth's gravity varies depending on location. The nominal "average" value at Earth's surface, known ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Seafloor Spreading
Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "plowed" through the fixed and immovable seafloor. The idea that the seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S. Naval Electronics Laboratory in San Diego in the 1960s. The phenomenon is known today as plate tectonics. In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading. Significance Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mid-ocean Ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin. The production of new seafloor and oceanic lithosphere results from mantle upwelling in response to plate separation. The melt rises as magma at the linear weakness between the separating plates, and emerges as lava, creating new oceanic crust and lithosphere upon cooling. The first discovered mid-ocean ridge was the Mid-Atlantic Ridge, which is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are traditionally called mid-ocean rid ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Prentice Hall
Prentice Hall was an American major educational publisher owned by Savvas Learning Company. Prentice Hall publishes print and digital content for the 6–12 and higher-education market, and distributes its technical titles through the Safari Books Online e-reference service. History On October 13, 1913, law professor Charles Gerstenberg and his student Richard Ettinger founded Prentice Hall. Gerstenberg and Ettinger took their mothers' maiden names, Prentice and Hall, to name their new company. Prentice Hall became known as a publisher of trade books by authors such as Norman Vincent Peale; elementary, secondary, and college textbooks; loose-leaf information services; and professional books. Prentice Hall acquired the training provider Deltak in 1979. Prentice Hall was acquired by Gulf+Western in 1984, and became part of that company's publishing division Simon & Schuster. S&S sold several Prentice Hall subsidiaries: Deltak and Resource Systems were sold to National Education ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Inverse Square Law
In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space. Radar energy expands during both the signal transmission and the reflected return, so the inverse square for both paths means that the radar will receive energy according to the inverse fourth power of the range. To prevent dilution of energy while propagating a signal, certain methods can be used such as a waveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to one dimension in order to prevent loss of energy transfer to a bullet. Formula In mathematical notation the inverse square law can be expressed as an intensity (I) varying as a function of distance (d) from some centre. The intensity is pr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gravitational Acceleration
In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodies accelerate in vacuum at the same rate, regardless of the masses or compositions of the bodies; the measurement and analysis of these rates is known as gravimetry. At a fixed point on the surface, the magnitude of Earth's gravity results from combined effect of gravitation and the centrifugal force from Earth's rotation. At different points on Earth's surface, the free fall acceleration ranges from , depending on altitude, latitude, and longitude. A conventional standard value is defined exactly as . Locations of significant variation from this value are known as gravity anomalies. This does not take into account other effects, such as buoyancy or drag. Relation to the Universal Law Newton's law of universal gravitation states that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theoretical Gravity
In geodesy and geophysics, theoretical gravity or normal gravity is an approximation of the true gravity on Earth's surface by means of a mathematical model representing Earth. The most common model of a smoothed Earth is a rotating Earth ellipsoid of revolution (i.e., a spheroid). Principles The type of gravity model used for the Earth depends upon the degree of fidelity required for a given problem. For many problems such as aircraft simulation, it may be sufficient to consider gravity to be a constant, defined as: :g=g_= based upon data from ''World Geodetic System 1984'' (WGS-84), where g is understood to be pointing 'down' in the local frame of reference. If it is desirable to model an object's weight on Earth as a function of latitude, one could use the following: :g=g_ - \tfrac(g_-g_) \cos\left(2\, \varphi \cdot \frac\right) where * g_ = * g_ = * g_ = * \varphi = latitude, between −90° and +90° Neither of these accounts for changes in gravity with changes in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theoretical Gravity
In geodesy and geophysics, theoretical gravity or normal gravity is an approximation of the true gravity on Earth's surface by means of a mathematical model representing Earth. The most common model of a smoothed Earth is a rotating Earth ellipsoid of revolution (i.e., a spheroid). Principles The type of gravity model used for the Earth depends upon the degree of fidelity required for a given problem. For many problems such as aircraft simulation, it may be sufficient to consider gravity to be a constant, defined as: :g=g_= based upon data from ''World Geodetic System 1984'' (WGS-84), where g is understood to be pointing 'down' in the local frame of reference. If it is desirable to model an object's weight on Earth as a function of latitude, one could use the following: :g=g_ - \tfrac(g_-g_) \cos\left(2\, \varphi \cdot \frac\right) where * g_ = * g_ = * g_ = * \varphi = latitude, between −90° and +90° Neither of these accounts for changes in gravity with changes in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
