№ 1.7. Geophysics

 Geophysics is a branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth; such phenomena include the earth’s magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity. The scope of geophysics also broadly includes outer-space phenomena that influence the earth; the effects of the sun on the earth’s magnetic field; and manifestations of cosmic radiation and the solar wind.

This discipline embraces all fields devoted to researching the earth’s interior, atmosphere, hydrosphere (waters), and ionosphere (ionized upper atmosphere). Related fields are included in the following descriptions.

Solid Earth Physics.  Embracing all fields devoted to the earth’s interior, solid earth physics involves studying the behavior of earth materials from the crust down to the core, particularly as they relate to the earth’s size and shape, gravity, magnetism, and seismicity. The specialized field of geodesy is concerned with determining the earth’s size and shape and locating precise points on its surface.

Terrestrial Magnetism.  Geomagnetism refers to the study of magnetic phenomena exhibited by the earth and its atmosphere. Generation of the magnetic field seems to be related to the motion of fluid, electrically conducting material within the earth, so that the planet acts as a self-exciting dynamo. The conducting material and the geomagnetic field may mutually control each other. Study of this problem is known as magnetohydrodynamics or hydromagnetics. The study of how the magnetic field has changed throughout the earth’s history, called paleomagnetism, provided the first strong evidence for the theory of plate tectonics.

Gravity and Tides.  Gravity is the attractive force exerted by the mass of the earth. The gradient of the gravitational potential - that is, the force of gravity—is perpendicular to the surface of the earth, which means that the force acts in the vertical direction. Gravimeters are highly sensitive balances used to make relative gravity measurements.

The rotation of the earth in the gravity fields of the moon and sun imposes periodicities in the gravitational potential at any point on the earth’s surface. Tides are the most obvious effect; in addition to marine tides, solid earth tides occur as slight crustal deformations.

Seismology.  Understanding of global seismic activity became possible with the recognition that major earthquakes are triggered by movement of the earth’s tectonic plates. In addition, much of what we are able to surmise about the earth’s mantle and core has been gained by studying the passage of earthquake waves through the center of the earth.

Hydrology.  This is the principal science dealing with continental water on and under the earth’s surface and in the atmosphere.

Volcanology. Volcanologic studies are concerned with the surface eruption of gas-charged magmas from within the earth and with the structures, deposits, and landforms associated with such activity.

Terrestrial Electricity.  Static or alternating electric currents that flow through the ground are induced by natural or artificial electric or magnetic fields. Geophysicists have determined from effects of induced currents or geomagnetic variations that, in general, conductivity increases with depth in the mantle.

Atmospheric Phenomena.  Physics of the lower atmosphere, where air is dense enough to be subject to the laws of fluid, is the province of meteorology. Phenomena of the upper atmosphere are the subject of aeronomy and magnetospheric physics.

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NOTES:

• core – керн, колонка породы, ядро;

• alternating electric current - (эл.) переменный ток;

• conductivity - (эл.) проводимость.