802 



EXPLORATION GEOPHYSICS 



proportional to the acceleration of the ground rather than to the velocity, 

 as in the case of the electromagnetic detector. The chief disadvantages 

 of the piezoelectric detector are its low sensitivity and the variation of 

 sensitivity with temperature and with moisture. The last factor makes 

 it necessary to keep the inside of the case free of all moisture, and usually 

 some type of desiccator is employed. The low sensitivity requires an 

 additional stage of amplification which must be housed in the seismometer 

 case in order to raise the signal energy above the noise level of the cable, 

 which preferably is of the shielded type. Also, the fragility of the crystal 

 necessitates some form of clamping device which will protect the crystal 

 from mechanical damage during transportation. 



Figure 489 shows the exterior and interior views of one type of piezo- 

 electric seismometer. The mass acting on the crystal comprises the entire 

 interior of the seismometer, including transformer, pre-amplifying stage, 

 and chassis. A clamp screw is provided for locking the mechanism during 

 transport to prevent injury to the crystal. The crystal is of composite 

 construction having four separate crystals electrically connected in parallel, 

 with their "Z" axes vertical. The output of the crystal is impressed across 

 a high resistance in the grid circuit of the pre-amplifying tube. 



Capacity or Electrostatic Seismometer 



hniniiniii)ii)iiiiinnin)ii)fif/i 



Fig. 490. — Condenser type seismom- 

 eter. A, waterproof connector plug; B, 

 case; C and C', condenser plates; D, 

 diaphragm; E, bakelite support ring; 

 F, carrying handle. 



The coupling between the mechan- 

 ical and electrical systems of a capacity 

 seismometer (Figure 490) is accom- 

 plished by means of a condenser so de- 

 signed that the displacement of the earth's 

 surface during the arrival of seismic 

 waves produces a relative displacement 

 of the plates of the condenser.f The 

 varying capacity may be made to induce 

 electrostatically a varying electromotive 

 force in an auxiliary circuit.* 



In one type, the varying capacity causes 

 a current flow in a high potential tuned 

 circuit. In another type of capacity seis- 

 mometer, the change in capacity is made 

 to vary the frequency of an oscillating 

 circuit. This circuit is usually heterodyned 



t O. S. Petty, "Seismic Apparatus and Method," U. S. Patent 2,408,478, Oct. 1, 1946. 



* The relation between the time rate of change of electric charge on the condenser 

 and the relative displacement of the condenser plates may be obtained from the 

 Lagrangian equations in a manner similar to that outlined above for the reluctance 

 type seismometer. 



