3rd GeoPhysical Examples

3rd GeoPhysical Examples - ~,,,I " Fig. 4.17....

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Unformatted text preview: ~,,,I " Fig. 4.17. SeIsmIc reflection shooting offshore as carried out In the Gulf of Mexico. (Socony Mobil Oil Co., Inc.1 ~ r , ~-- ,> . INSTRUMENTS FOR MEASU'RING (;RAVity 201 two decades. In addition, pendulum apparatus was operated inten- sively by one company for a period of several years. Although both instruments are now obsolete, data obtained by them may still be reviewed long after the original surveys were made. It is therefore desirable that we consider the principles of the torsion balance and pendulum briefly in this chapter. For more comprehensive and:quan- titative treatments of these instruments, the reader is referred to Nettle- ton's, Jakosky's, or Heiland's texts or to the first five references given at the end of this chapter. To maintain proper historical sequence, the two obsolete instruments will be discussed before we take up the more modern types. 10-1. The Eotvos Torsion Balance (2) (I) In Chap. 8, the concepts of gradient and curvature of the gravitational potential were introduced, and it was shown that these quantities, when Fig. 10-1. Schematicdiagrams for four types of torsion balance. (1) Cavendish balance. (2)Standard EOtvostorsion balance. (3) Variation of standard type. (4) TIlt-beam balance. measured on the earth's surface, exhibit characteristic patterns in the presence of subsurface density anomalies such as those associated with s/11tdomes and other geologic features. The Eotvos torsion balance is used to measure gradients and curvatures. The precision workmanship involved in its design and construction has been exceeded in few if any field instruments. Principles of Design. Basically the Eotvos balance consists of two equal weights at different heights connected by a rigid frame, the system being suspended by a torsion wire in such a way that it is free to rotate in a horizontal plane about the wire. Usually the frame is a lighrhori- zontal beam, with one of the weights attached directly to one end and the other weight suspended from the opposite end. In principle, the instrument is similar to the Cavendish balance, first used to establish the value of the gravitational constant 1 ', as described in Chap. 8; how- ever, in the Eotv6s balance both weights are not in the same horizontal plane as is the case with the Cavendish instrument. Figure 10-1 shows r----..r----....
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3rd GeoPhysical Examples - ~,,,I " Fig. 4.17....

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