610 



SEISMIC METHODS 



[Chap. 9 



The Imperial Geophysical Experimental Survey used an inductive seis- 

 mograph in which the mass consisted of an electromagnet suspended from 

 a diaphragm. The pickup coil was stationary between the poles of this 

 electromagnet (see Fig. 259 of Edge and Laby) and was connected to a 

 Cambridge string galvanometer. Illustrations and diagrams of a multiple- 

 string galvanometer used for refraction shooting by the I.E.G.S. are found 

 in Figs. 153, 154, 156, and 253 of Edge and Laby's book. 



Fig. 9-1136 illustrates an inductive seismometer with single coil moving 

 in the field of a permanent (pot-type) magnet. Fig. 9-114 represents a 

 portable four-channel refraction seismic apparatus for shallow depth prob- 

 lems in which inductive detectors 



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to emfifffier 



Fig. 9-1136. Inductive seismometer. 



with electromagnetic damping are 

 employed. 



Reluctance electromagnetic seis- 

 mographs have been developed 

 most extensively for reflection 

 shooting. Two balanced-arma- 

 ture types are shown in Fig. 9-115. 

 They are very efficient, well 

 adapted to intermediate and high 

 natural frequencies, and may be 

 used for both refraction and reflec- 

 tion work. 



(6) Capacitive seismographs. The 

 condenser (or radio) seismograph 

 has been applied in various countries for refraction work, and it is used in 

 this country for reflection work by one or two companies. Various "radio" 

 seismographs have been described by Haeno*^ and H^.** A condenser 

 seismograph, suitable for application in reflection work, is shown diagram- 

 matically in Fig. 9-116. The oscillatory circuit and tube are closed in the 

 detector box, with the seismometer mass acting as the movable plate of a 

 variable condenser. A quadruple cable supplying A and B voltage goes 

 from the detector to a three-stage-resistance coupled amplifier, which is 

 coupled through a" transformer to an oscillograph or string galvanometer. 

 A detailed discussion of the condenser type detectors has been given by 

 Irland. Various mechanical seismometers of the horizontal and vertical 

 type were equipped with a capacitive transducer changing the phase of 

 two coupled oscillators. Simultaneous shaking table and instrument 



«" Japan. J. Astron. and Geophys., 8(2), 39-50 (1931). 

 «2 Union Geophys. Trav. Sci. A (9), 1933. 



** G. A. Irland, A Study of Some Seismometers, U. S. Bur, Mines Tech. Paper No. 

 556 (1934). 



« U. S. Bur. Mines Tech. Paper No. 518 (1932). 



