history of geophysics

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history of geophysics Geophysics employs the principles and methods of physics in the study of the character and history of the Earth. Its modern offshoot, geophysical prospecting or exploration geophysics, applies these principles and methods to the search for natural resources to obtain information about subsurface structure for engineering purposes. Archaeological information, especially the location of buried remains, is also obtainable by these means.

As early as ad 132 Chang Heng set up a seismic instrument in China to indicate not only when an earthquake occurred but also the direction of the first shock waves. During the Renaissance, Leonardo da Vinci (1452–1519) acquired a considerable knowledge of geology, the Earth's gravity field, and wave propagation and reflection. It was William Gilbert (1540–1603) who founded the sciences of magnetism and electricity and noted that the magnet dipped at an angle dependent on latitude. Slightly later, in Italy, Galileo Galilei (1564–1642) developed the correct formula for the motion of a pendulum, so important in the study of gravity. A Dutchman, Christian Huygens (1629–95), was able to explain the behaviour of waves impinging upon an interface, and also diffraction. Sir Isaac Newton (1642–1727) produced the concept of calculus, the basis laws of motion, and explained the true cause of ocean tides.

Experiments and instruments were developed in the eighteenth century and enabled Pierre Bouguer (1698–1758) to compare the regional mass of the Earth with the local mass of mountains by pendulum experiments. The Bouguer correction is used today for gravity measurements at an elevation above a datum. In 1798 an early Woodwardian Professor of Geology at Cambridge, John Michell, described a method for measuring a gravity field by a ‘torsion balance’, and in the same year Charles Coulomb built a similar device to study magnetic and electrical attraction. During the nineteenth century much progress resulted from the invention of the Eötvös torsion balance and the setting up of global observational networks. Magnetic stations were established in many places throughout the world and gravity was measured in a wide variety of topographical settings.

Seismic observations on a network basis had to await the invention of accurate seismometers. The first such instruments were made in 1880 and seismographs were installed at the University of California in 1887. A co-ordinated network of 27 seismic stations was soon operating on a global basis and in 1903 the International Seismological Association was formed. Then began a period of intense seismological study during which discontinuities deep within the Earth were discovered by A. Mohorovicić (the Moho discontinuity) and Beno Gutenberg (the Guten̆berg discontinuity).

During the two world wars location by acoustic and seismic means achieved some success on land and at sea. Between the wars the search for natural resources became intense, using gravity, seismic, electric, and magnetic methods. The petroleum industry in particular invested heavily in these advances, being largely concerned with seismic surveying. After the Second World War progress continued with the aid of microelectronics and computers. Airborne and marine patrols during the ‘cold war’ period were equipped with advanced means of locating nuclear explosions, and remote sensing was developed. The rise of plate tectonic concepts after the Second World War was largely a consequence of the exploration of the deep oceans (again, defence needs were paramount), using increasingly sophisticated instruments.

D. L. Dineley

Bibliography

Bates, C. C.,, Gaskell, T. F.,, and and Rice, R. B. (1982) Geophysics in the affairs of man. Pergamon Press, Oxford.

geophysics, history of see history of geophysics