916 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chaf. 12 



number of tubular openings on the circumference of the vibrator drums 

 into which cylindrical masses of various sizes can be fitted. A soil test 

 vibrator must be capable of producing vertical forces up to 40 to 50 cycles 

 (2400 to 3000 r.p.m.). The larger commercial vibrators weigh from a few 

 hundred to 1000 pounds (without ballast) and are equipped with shunt- 

 wound D.C. motors to permit accurate speed control. The smaller vibrat- 

 ors are driven by gasoline engines or motors supplied from storage bat- 

 teries; the more primitive models are merely brought up to speed manually 

 and are allowed to run down. 



Undoubtedly, the greatest flexibility is obtained by a D.C. motor sup- 

 plied from a gasoline-engine driven D.C. generator, in which case the 



Heiland Research Corp. 

 Fig. 12-17. Vibrator and recording truck. 



speed may be accurately controlled by varying the fields of both the 

 generator and the motor. The frequency is adjusted by a tachometer and 

 evaluated accurately by timing the vibration records. For the measure- 

 ment of the phase shift between force and displacement it is convenient 

 to provide an electrical impulse transmitter on one of the revolving drums. 

 This will record the vibrator phase, together with the oscillations picked 

 up by one or more vibration detectors. A vibrator with recording truck 

 in the background is shov/n in Fig. 12-17. For certain applications it may 

 be convenient to insert a wattmeter in the vibrator-motor circuit for the 

 measurement of power consumption. As shown below, the power taken 

 from the vibrator is at a maximum near the resonance point of the ground. 



