Geophysics

I

INTRODUCTION

Geophysics, 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, even in subtle ways; the effects of the sun on the earth’s magnetic field; and manifestations of cosmic radiation and the solar wind.

II

AREAS OF STUDY

Subdivision of the wide-ranging subject matter of geophysics into various branches involves categorizing specific endeavors. Strictly speaking, however, the 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.

A

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 (see Earth), 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. Involved in this study are the determination of the earth’s gravitational field and observation of variations in the earth’s rotation, the location of the poles, and tides. Two new techniques for making geodetic measurements, very long baseline interferometry (VLBI) and satellite laser ranging (SLR), have been used to determine, within a fraction of a centimeter, the rates at which the continents are moving toward or away from each other. See Plate Tectonics.

B

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. See Earth: The Core and Earth's Magnetism.

C

Gravity and Tides

Gravity (see Gravitation) 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. Differences in relative gravity due to variations in the earth’s density below the measurement site are referred to as Bouguer anomalies.

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 (see Tide).

D

Seismology

Comprehensive 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. In this decade, geophysicists have also made great strides in understanding the structure of the crust and upper mantle, a zone known as the lithosphere (see Earth: Plate Tectonics). Major accomplishments in lithospheric research have been made possible through the use of an echo-sounding technique originally developed for finding oil and gas: seismic reflection profiling. See Seismology.

E

Hydrology

This is the principal science dealing with continental water on and under the earth’s surface and in the atmosphere. The constant circulation of water from land and sea through the biosphere and atmosphere by evaporation, evapotranspiration (loss of water from the soil by evaporation and transpiration by plants), and precipitation and runoff constitutes the so-called hydrologic cycle. See Water.

F

Volcanology

Volcanologic studies are concerned with the surface eruption of gas-charged magmas (molten rock materials) from within the earth and with the structures, deposits, and landforms associated with such activity (see Volcano).

Although no single set of volcanic activities reliably indicates future volcanic events, certain processes provide geophysicists with clues to possible forthcoming eruptions. Such phenomena include changes in the strength and orientation of the earth’s magnetic field; swarms of microearthquakes; increased heat flow in the earth, sometimes detected by thermocouples (see Thermoelectricity) or from infrared aerial photos; variations in local electrical currents within the earth; increased exhalations of gases from fumaroles and vents; and the tumescence (bulging upward) of magma domes.

G

Terrestrial Electricity

Static or alternating electric currents that flow through the ground are induced by natural or artificial electric or magnetic fields. Electrical resistivity deep within the earth is explored by so-called magnetotelluric probing. Geophysicists have determined from effects of induced currents or geomagnetic variations that, in general, conductivity increases with depth in the mantle.

H

Atmospheric Phenomena

Physics of the lower atmosphere, where air is dense enough to be subject to the laws of fluid dynamics (see Fluid Mechanics), is the province of meteorology. In recent years, the techniques of remote sensing have begun to play a major role in monitoring storms and other transient atmospheric phenomena, such as lightning.

Phenomena of the upper atmosphere are the subject of aeronomy and magnetospheric physics (see Ionosphere). The earth’s magnetic field reacts with the solar wind (see Sun) to form a sort of sheath, called the magnetosphere, that acts as a gigantic natural dynamo, more than 100,000 km (more than 60,000 mi) across. When high-energy particles streaming from the sun penetrate this sheath and enter the Van Allen belts, the phenomena known as aurora are created (see Radiation Belts).

III

GEOPHYSICAL SURVEYS

Geophysical exploration, commonly called applied geophysics or geophysical prospecting, is conducted to locate economically significant accumulations of oil, natural gas, and other minerals, including groundwater. Geophysical investigations are also employed with engineering objectives in mind, such as predicting the behavior of earth materials in relation to foundations for roads, railways, buildings, tunnels, and nuclear power plants. Surveys are generally identified by the property being measured—namely, electrical, gravity, magnetic, seismic, thermal, or radioactive properties.

Used primarily in the search for oil, gas, and base metals, electrical and electromagnetic surveys map variations in the conductivity or capacitance of rocks (see Electricity). Measured by special tools lowered into holes drilled for oil and gas, conductivity variations provide geophysicists with clues from which they can judge the hydrocarbon-bearing potential of rock strata. Direct and alternating electrical currents are measured in ground surveys, but the lower radio frequencies are used both in ground and in airborne electromagnetic surveys.

Gravity surveys measure density variations in local rock masses. Used mainly in petroleum exploration, these surveys are based on use of a device called a gravimeter. Gravity surveys are made on land, at sea, and down boreholes.

In ground magnetic surveys, variations in the earth’s magnetic field are measured at stations placed closely together; aeromagnetic surveys may also be conducted, especially in petroleum exploration. Devices called magnetometers, towed by aircraft or behind a seismic research ship, help to detect magnetic anomalies or to distinguish geologic features that might appear similar from seismic data alone.

Measurement of seismic-wave travel time is one of the most common geophysical methods used in surveys. Seismic exploration is divided into refraction and reflection surveys, depending on whether the predominant portion of the seismic waves’ travel is horizontal or vertical. Refraction seismic surveys are used in engineering geophysics and petroleum exploration, and to locate groundwater or buried stream channels containing placer mineral deposits. Seismic reflection surveys, on the other hand, detect boundaries between different kinds of rocks; this detection assists in the mapping of geologic structures. Seismic energy is detected on land by using devices called geophones, which react to on-site ground motions; and in water by using piezometric devices, which measure hydrostatic pressure changes.

Geothermal surveys concentrate on temperature variations and the generation, conduction, and loss of heat within the earth. Geothermometry is also important to volcanologic studies as well as to locating geothermal energy resources. See Geothermics.

Radioactivity surveys, conducted on the ground and from the air, measure natural radiation from the earth. Geiger and scintillation counters (see Particle Detectors) are used in searching for ores of uranium as well as in searching for rare earth metals, potash deposits, and other radioactive materials.

IV

ORGANIZATIONS AND PROGRAMS

Major national and international geophysical bodies that publish mainly in English include the American Geophysical Union (AGU), American Meteorological Society (AMS), International Union of Geodesy and Geophysics (IUGG), International Council of Scientific Unions (ICSU), Royal Astronomical Society, Seismological Society of America, Society of Exploration Geophysicists, and World Meteorological Organization (WMO). Such organizations take part in extensive research and exploration efforts, as well as gathering and publishing the results of research.

Programs of geophysics are designed to collect, exchange, analyze, and synthesize data from many sites over extended periods of time. The International Geophysical Year (IGY, 1957-58), for example, was an international program that concentrated on exploration of the solar and terrestrial atmospheres. It was followed in 1964-65 by the International Years of the Quiet Sun (IQSY) to compare times of maximum and minimum solar activity and their effects on terrestrial phenomena. Solid earth geophysical programs of the same decade included the World Magnetic Survey Board of IUGG and the Upper Mantle Program coordinated by ICSU. The International Hydrological Decade (IHD, 1965-74) was launched by the United Nations Educational, Scientific and Cultural Organization (UNESCO) to deal with a variety of water-related topics of practical significance to humanity. The International Indian Ocean Expedition (IIOE, 1961-66) was one of several oceanographic geophysical programs. Geological programs for drilling far into the earth’s crust, such as the Deep Sea Drilling Program completed in 1983 and the succeeding Ocean Drilling Program, are also of importance to geophysicists (see Ocean and Oceanography); the world’s deepest well—already more than 12,000 m (40,000 ft) deep—is being drilled in the Kola Peninsula, in northern Russia. Geodetic data provided by satellites of the U.S. Navstar Global Positioning System are helping to measure seismic and plate-tectonic movements. The World Weather Watch (WWW), an ongoing atmospheric science program managed by ICSU and WMO, is a global data collecting, processing, and dissemination system serving all nations. The Global Atmospheric Research Program (GARP) is a research endeavor geared to quantitative weather prediction.