Chap. 9] 



SEISMIC METHODS 



607 



E. Review of Prospecting Seismographs 



Prospecting seismographs may be divided into mechanical and electrical 

 types. The latter, in turn, fall into three groups: electromagnetic, ca- 

 pacitive, and pressure detectors, as was discussed previously in connection 

 with the description of reflection instruments. A reflection detector may 

 be used for refraction work, but not vice versa. Almost all such detectors 

 record the vertical component of the ground motion. 



Fig. 9-111. Comparison of calculated and experimental response of detector, 

 amplifier, and galvanometer for given tuning factors and damping ratios. 



1. Mechanical seismographs. Much of the first refraction work was 

 done with the Mintrop mechanical seismograph^" (see Fig. 9-112). Pen- 

 dulum and recorder are separate as in most other mechanical seismographs. 

 The former is a vertical-component instrument, consisting of a spherical 

 mass attached to a leaf spring. The mass carries a long, cone-shaped 

 extension with a thin spring at its end which rubs against a spindle carrying 

 a mirror. The movement of the lever is damped electromagnetically. In 

 the recorder the unexposed paper is housed below and fed through a 



•« C. A. Heiland, Eng. and Min. J., 121(2), Fig. 18 (Jan. 9, 1926). 



