SEISMIC METHODS 805 



proper mechanical coupling to a suitable mass, an excellent seismometer 

 is obtained. For shallow work, the output is sufficient to operate the gal- 

 vanometer, without additional amplification. 



The amplification factor of the mechano-electronic transducer is greater 

 than that of the carbon-grain microphone type, and is also free of the high 

 noise level so characteristic of the carbon-grain microphone. 



EXHAUST TIP 



a 





^SS\\ 



Fig. 493. — Cross-sectional view of mechano-electric 

 transducer. (Courtesy of International Geophysics 

 Company.) 



A seismometer utilizing this transducer requires a local filament battery 

 of 6 volts and .15 amperes. The DC source for plate potential at 300 volts 

 and 5 milliamperes may be located at the instrument truck and transmitted 

 to the seismometer through a two-wire cable. By suitable transformer 

 coupling in series with each transducer, variations in the plate current will 

 actuate the galvanometer. 



Carrier Current Systems. — -Various schemes have been proposed to 

 utilize carrier frequency systems. f By having each seismometer operating 

 on a different carrier frequency, only a simple two-conductor cable would 

 be required between the instrument truck and the various seismometers. 

 At the instrument truck tuned circuits would be employed to separate each 

 of the carrier frequencies, and the amplified output of each circuit would 

 actuate its recording galvanometer. Systems of this type have not been 

 widely adopted, probably because the saving in cable is offset by the more 

 complicated carrier system. 



Mechanical Seismometers 



At present, the mechanical seismometers are seldom used in prospecting. 

 They are reviewed here chiefly because of their historical interest. 



Type Instruments 



The Schweydar instrument! consists of two units so designed that 

 one unit responds to the vertical component of the earth's motion and 



t E. E. Minor, "Vibration Measuring Apparatus," U. S. Patent 2,305,267, Dec. 15, 1942. 



t W. Schweydar and H. Reich, "Kiinstliche elastische Bodenwellen als Hilfsmittel geologischer 

 Forschung," Gerland's Beitrdge sur Geophysik, 1927, 17, 121. Broughton Edge and Laby, Geo- 

 physical Prospecting (Cambridge Univ. Press, 1931), pp. 214-216. 



