From Wikipedia, the free encyclopediaGeophysics is the physics of the Earth and its environment in space. Its subjects include the shape of the Earth, its gravitational and magnetic fields, the dynamics of the Earth as a whole and of its component parts, the Earth's internal structure, composition and tectonics, the generation of magmas, volcanism and rock formation, the hydrological cycle including snow and ice, all aspects of the oceans, the atmosphere, ionosphere, magnetosphere and solar-terrestrial relations, and analogous problems associated with the Moon and other planets.[1]
Geophysics is also applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection.[1] Geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, to locate groundwater, to locate archaeological finds, to find the thicknesses of glaciers and soils, and for environmental remediation.
GEOELECTRIC
The purpose of the geoelectrical survey was to determine the subsurface resistivity distribution by making measurements at ground level. From the measurements, resistivity is actually below the earth's surface can be estimated. Soil resistivity associated with the various geological parameters such as mineral and fluid content, porosity and degree of saturation of water in the rock. Electrical resistivity surveys have been used for decades in hidrogeological, mining, and the investigation geothecnical. Recently, it has been used for environmental surveys. (Dr. M. H. Loke, 1996-2004)
Broadly speaking geoelectric method is divided into two kinds, namely:
1. Geoelectric passive
Geoelectric where existing energy required in advance so that does not provide any injection / revenue stream first. Geoelectric this kind is called the Self Potential (SP).
SP measurements performed on a certain path in order to measure the potential difference between two different points as the V1 and V2. way of measurement by using two porouspot where prisoners are always arranged as small as possible. Errors in the measurement of SP usually occurs because of fluid flow under the surface that lead to leaps suddenly to the value of potential difference. Therefore this method is very good for geothermal exploration.
2. That are active geoelectrical
Geoelectric where there is the energy required for the injection currents into the earth first. Geoelectric this kind there are two methods, the method of resistivity (resistivity) and Induced Polarization (induced Polarization).
Which will be discussed further is the geoelectric that are active. The method described is known by the name of geoelectrical resistivity or called resistivity method (resistivity).
Each medium has different properties of electric current through it, this depends on the type prisoners. In this method, electrical current is injected into the earth through two current electrodes and potential difference that occurs is measured through two potential electrodes. From the results of current and potential difference measurements for each different electrode distances can then be lowered barriers to price variations of each type of layers beneath the surface of the earth, below the measuring point (Sounding Point).
This method is more effective when used for exploration of its relatively shallow. This method rarely provides information layer depth of more than 1000 or 1500 feet. Therefore this method is rarely used for hydrocarbon exploration, but more widely used for the field of engineering geology as determining the depth of base rock, reservoir water searches, geothermal exploration, and also for environmental geophysics.
So this resistivity method to learn about rock resistivity difference by determining the change of resistivity with depth. Each medium has essentially the electrical properties are influenced by the rock composer / mineral composition, the homogeneity of rock, mineral content, water content, permeability, texture, temperature, and geologic age. Some electrical properties of these are electric potential and electrical resistivity.
Geoelectrical resistivity utilizing conductivity properties to detect the state of rock below the surface. The nature of the resistivity of the rock itself there are 3 kinds, namely:
1. Conductive medium
Easy medium that delivers an electric current. Big resistivitasnya is 10-8 ohm m to 1 ohm m.
2. Medium semikonduktif
Medium is quite easy to conduct electrical current. Big resistivitasnya is 1 ohm to 107 ohm m m.
3. Resistive medium
Medium is difficult to conduct electrical current. Large resistivitasnya is greater 107 ohm m.
GRAVITY
The mechanism of Newton's law of universal gravitation.
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 a day.[6] Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.[7] Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can be used to look for mineral deposits (see also gravity anomaly and gravimetry). They also reflect the dynamics of 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).
HEAT FLOW
Main article: Geothermal gradient
A model of thermal convection in the Earth's mantle.
The Earth is cooling, and the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are the primordial heat and radioactivity, although there are also contributions from phase transitions. Heat is mostly carried to the surface by thermal convection, although there are two thermal boundary layers - the core-mantle boundary and the lithosphere - in which heat is transported by conduction. Some heat is carried up 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.
VIBRATION
Body waves and surface waves (see seismic wave).
Seismic waves are vibrations that travel through the Earth's interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes. One such mode is the "breathing mode", a uniform expansion and contraction of the Earth.
Ground motions from waves or normal modes are measured using 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.
Seismic waves can also provide information on the region that the waves travel through. If the density or composition of the rock changes suddenly, some of the waves are reflected. Reflections can provide information on near-surface structure. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth.
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.
RADIOACTIVITY
Further information: Radiometric dating and geotherm
Example of a radioactive decay chain (see Radiometric dating).
Radioactive decay, in addition to being the main source of heat in the Earth (see geotherm), is an invaluable tool for 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.
[edit]Electricity
Further information: Natural electric field of the Earth
Although we mainly notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 V m-1.[8] 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.[8] 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 spontaneous potential, a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in Earth and the Oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field.[9] The distribution of telluric current density can be used to detect variations in electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves (see induced polarization and electrical resistivity tomography).
ELECTROMAGNETIC
Electromagnetic waves occur in the ionosphere and magnetosphere as well as the Earth's outer core. dawn chorus is 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 magnetic 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 secular variation.[10]
Electromagnetic methods that are used for geophysical survey include transient electromagnetics and magnetotellurics.
MAGNETISM
Further information: Geomagnetism and Paleomagnetism
The variation between magnetic north and "true" north (see Earth's magnetic field).
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 core (see geodynamo). The magnetic field in the upper atmosphere gives rise to the auroras.[5]
The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called secular variation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals are recorded in rocks (see natural remanent magnetization) and their signature can be seen in striped magnetic anomalies on the seafloor. These stripes provide quantitative information on seafloor spreading, a part of plate tectonics. In addition, the magnetization in rocks can be used to measure the motion of continents (see paleomagnetism).[5]
FLUIDS DYNAMIC
Main article: Geophysical fluid dynamics
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 and post-glacial rebound. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.
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. In the Earth's core, the circulation of the molten iron is structured by Taylor columns.
Waves and other phenomena in the magnetosphere can be modeled using magnetohydrodynamics.
CONDENSED MATTER PHYSICS
Further information: Mineral physics
The physical properties of minerals must be understood to infer the composition of the Earths' interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals as well as their high-pressure phase diagrams, melting points and equations of state at high pressure. Studies of creep determine how rocks that are brittle at the surface can flow deep down. These properties determine the rheology that determines the geodynamics.[7]
Further information: Hydrology and Physical Oceanography
Water is a very complex substance and its unique properties are essential for life. 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. Some of the 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.
Further information: Cryosphere
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).[11]
STRUCTURE OF THE EARTH
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. The Earth's mass is M = 5.975 × 1024 kg and its mean radius is R = 6371 km , so its mean specific gravity is < ρ > = 5.515. This is substantially higher than the typical specific gravity (2.7–3.3) of rocks at the surface. Its moment of inertia is 0.33 M R2, whereas it would be 0.4 M R2 if the earth was a sphere of constant density. Both lines of evidence point to a concentration of mass near the center. However, the density of the rock will increase with depth because of the increasing pressure. To determine how large this effect is, the Adams–Williamson equation is used to determine how density increases with pressure. 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.[7]
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.[12]
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.[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 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 probably due to phase transitions.[7]
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.
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