462 



EXPLORATION GEOPHYSICS 



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prises: (1) low frequency (25 cycle) gasoline-driven alternator, (2) or- 

 dinary iron electrodes for both the energizing and the measuring circuits, 

 and (3) one or two stages of audio-frequency amplification feeding a 

 vacuum tube voltmeter. 



This type of apparatus, while more complicated than the simple D.C. 

 galvanometer method, eliminates the errors and difficulties due to earth 

 currents and electrode-contact phenomena. For shallow investigations, the 

 frequency is low enough to avoid bothersome inductive effects and phase- 

 shift phenomena. In addition, no 

 electrical connection need be em- 

 ployed between the energizing cir- 

 cuit or power supply and the detect- 

 ing or measuring circuit. 



The complete apparatus is illus- 

 trated in Figure 276. The alternator 

 for supplying power may be of con- 

 ventional design, with an output of 

 about 1000 watts at 110 volts. The 

 output is controlled by the rheostat 

 in the excitor circuit. The gasoline 

 engine should preferably be of the 

 standard 2-cycle type which is made 

 of cast iron and has water-bath 

 cooling. The light - weight, high - 

 speed, air-cooled engines, frequently advocated because of their portability, 

 usually are not satisfactory for continuous, heavy duty work. An output 

 transformer must be employed for proper matching of the load impedance, 

 which varies over very wide limits, due to variations in stake resistance. 

 The vacuum tube voltmeter consists of one stage of 25-cycle amplifi- 

 cation and a rectifying tube or detector which functions as a vacuum tube 

 voltmeter. A grid-bias control is provided for proper initial adjustment 

 of the meter. Details of vacuum tube voltmeter design and theory are 

 given in many texts,t and excellent meters are available on the market. 



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 Fig. 276. — Low frequency voltmeter method. 



INTERPRETATION OF EQUIPOTENTIAL DATA 



Simple calculations suffice for converting the field readings into usable 

 data. Usually, the only corrections necessary are those for natural earth 

 potentials (when using the D.C. methods) and instrumental or meter 

 characteristics. The stations occupied during the survey are plotted ac- 

 curately on a map, preferably with alidade and plane table while the survey 

 is in progress. The equipotential lines are plotted later by drawing in the 

 best-fitting contours. 



t J. H. Morecroft, Principles of Radio Communication (John Wiley & Sons, 1927). 

 A. Hund, Phenomena in High-Frequency Systems (McGraw-Hill, 1936). 

 F. E. Terman, Measurements in Radio Engineering (McGraw-Hill, 1936). 



