Chap. 9] 



SEISMIC METHODS 



497 



vanometers are extensively employed.^^ They are generally coupled to 

 the receiver by a suitable step-down transformer; the regular seismic 

 oscillographs (coil galvanometers or string galvanometers) may be used 

 when a separate indicator is undesirable. In that case the output trans- 

 former of the radio receiver is connected in series or in parallel to the 

 secondary of the output transformer of the seismic amplifier. A disad- 

 vantage of all oscillographs and mirror devices is their inertia, which re- 

 quires damping. This is avoided in the glow-tube oscillograph shown in 

 Fig. 7-25. The latter ignites with about 2 -10"* amperes for optimum 

 plate voltage. Its cathode is a hollow slotted cylinder whose end surrounds 



—aa*^ 



T»B*^l 



Harvey Radio Laboratories, Inc. 



Fig. 9-35. Radio transceiver. 



the anode. The current controls the length of the glow in the slot. This 

 glow is projected on the photographic paper. 



5. Review of seismic methods (fan shooting, refraction, reflection). All 

 seismic prospecting methods have in common the generation of an in- 

 stantaneous shock and the measurement of resultant surface vibrations at 

 one or more distant points. The phj^sical parameters which may be deter- 

 mined for any vibratory motion are (1) frequency; (2) intensity or ampli- 

 tude; (3) velocity and travel time. Practical experience has shown that 

 the first two parameters are too complex to be used for interpretations in 



*' The regular seismic oscillographs are discussed on pp. 552; 598-601. 



