Chap. 10] 



ELECTRICAL METHODS 



761 



and distortion network are in opposite arms of the bridge, the former syn- 

 chronizing the latter. An amphfier and oscilloscope are connected to the 

 detector arm. Two potentiometers in series are connected across the 

 input to the distortion network. The tapped portion of one potentiometer 

 carries a resistance and a capacitance in parallel, while the tapped portion 

 of the other potentiometer has a capacitance in series and a resistance in 

 parallel. Transients can be simulated with sufficient accuracy by the four 

 variable resistances in this network. 



A technique referred to under (4) above, that uses a controlled distor- 

 tion of the received impulse to produce a linear saw-tooth transient (Saw- 

 tran), has likewise been described 

 by Klipsch. In this case the 

 distortion network consists sim- 

 ply ,of a tapped resistance with 

 capacitance in series, both across 

 the input line. By adjusting the 

 sliding contact on the resistor 

 until the resultant wave consists 

 of a straight-line saw tooth, the 

 time constant (RC) of the tran- 

 sient can be determined. 



Opinions in the literature differ 

 concerning the superiority of the 

 transient over standard resis- 

 tivity methods. Some authors 

 claim that transient indications 

 cannot be duplicated by resis- 

 tivity measurements, that data 



on hitherto unobservable rock properties may be obtained, and that 

 depth penetration is much greater. Others contend that the depth 

 penetration is no greater than for other electrical methods under similar 

 field conditions and that the observed anomalies are essentially due to 

 shallow stratigraphic variations.^^ It is possible that the Eltran method 

 and related electrical methods designed to measure ground circuit react- 

 ance, will reveal formation properties of diagnostic value not obtainable 

 with the straight resistivity method. As was stated on page 639, the 

 capacitive reactance of the ground is probably related to its electrolytic 

 polarization properties. 



An attempt has been made to measure this polarization directly by a 

 so-called "electrochemical" method," likewise based on the observation 



1 mile 



Fig. 10-87. Eltran contours, Sandy Point 

 oil field, Brazoria County, Texas (after 

 Rosaire). 



"E. E. Rosaire, Geophysics, 111(2), 96-115 (March, 1938). 

 "M. Muller, Beitr. angew. Geophys., 4(3), 302-315 (1932). 



