580 



SEISMIC METHODS 



[Chap. 9 



9 



iwjm 



Fig. 9-94. Horizontal seismographs 

 (schematic): (1) Ewing, (2) Rebeur- 

 Paschwitz, (3) Zoellner, (4) Milne, (5) 

 Wiechert-astatic, (6) Wood-Anderson, 

 (7) Galitzin, (8) Schweydar. 



According to application, seismo- 

 meters may be divided into (1) 

 station seismographs, (2) vibro- 

 graphs, and (3) prospecting seis- 

 mographs. These differ in respect 

 to mass, magnification, natural 

 frequency, and method of record- 

 ing, as shown in Table 59. 



Of the vertical seismographs (Fig. 

 9-95) the simplest consists of a mass 

 suspended from a coil spring (1). 

 The three seismometers in 2, 3, and 

 4 use levers with coil springs; the 

 pallograph (4) is employed for the 

 measurement of ship vibrations. 

 The designs in 2, 5, 6, 7, and 8 

 are used in seismic prospecting de- 

 tectors. Universal seismographs 

 for the recording of all ground com- 

 ponents have been made by com- 

 bining the coil type (Fig. 9-95, 1) 

 with the Ewing pendulum (Fig. 

 9-94, 1). This is known as the De 

 Quervain universal seismograph. 

 They have not found application as 

 prospecting detectors. 



B. Elementary Theory 







1. Geometric and physical char- 

 acteristics of seismometers. Assume 

 a simple pendulum seismograph, 

 as shown in Fig. 9-96, its mass 

 m being suspended at a distance 

 I' from the axis. Attached to it 

 is a magnifying lever whose equiv- 

 alent length is J — V. According 

 to the well-known theory of the 

 mathematical pendulum, 



Fig. 9-95. Vertical seismographs (sche- 

 matic): (1) coil spring, (2) Gray spring- 

 lever, .(3) Tanakadate, (4) pallograph, (5) 

 dual spring, (6) geophone, (7) combina- 

 tion of (1) and (5), (8) Vicentini. 



^o Vertical seismographs, particu- 

 larly the astatic types, are also em- 

 ployed as gravimeters (see pp. 125, 126, 

 132, and 134). 



