ELECTRICAL METHODS 



553 



to build up a well exposed negative. If no extraneous potentials are 

 present, the reoccurring- wave pattern will produce a sharp, dense wave 

 image, while if undesirable potentials are present (caused by earth cur- 

 rents, stray or line leakage, induction, etc.), the picture of the wave form 

 will be a less distinct pattern, with the greatest exposure along the path 

 of the reoccurring wave form. Analysis is based upon the more dense 

 wave pattern thereby minimizing the effects of extraneous non-cyclic 

 potentials. 



A manual method of measuring the characteristics of the transient has 

 been developed wherein the distortion of the wave form produced by the 

 shallow surface layers is determined by employing an auxiliary circuit to 

 transform the incoming potential to an easily recognized form (straight 

 line) and measuring the additional distortion required, t 



Studies have been made by various investigators to determine the rela- 

 tive merits of the transient and the direct-current steady-state resistivity 

 measurements. The steady-state measurements usually give more reproduce- 

 able results, due to the absence of inductive and current redistribution 

 effects necessarily present with the use of transients. Figure 340 shows 



B 



D 



Fig. 340. — Comparison of a transient and a direct-current 

 resistivity survey in Louisiana. A, transient data; B, C, D, direct 

 current resistivity data for shallow, medium, and deep penetra- 

 tions, respectively. (Blondeau, Geophysics.) 



the comparison obtained by Blondeau X between a transient and a direct- 

 current resistivity survey in Louisiana. Curve A shows the transient 

 results, while curves B, C, and D show the direct-current resistivity results 

 at increasingly greater depths of penetration. The electrode configuration 

 for curve B gave a current penetration of approximately 200 feet, which 

 compares favorably with the results obtained by the transient method in 

 curve A. The electrode spacing employed for the Eltran work would 



t R. Saibara, S. Bilinsky and W. G. McLarry, "Exploration by Incremented Wave Distortion," 

 U. S. Patent 2,177,346, issued Oct. 24, 1939. 



t E. E. Blondeau, "Shallow Resistivity Survey at South Elton, Louisiana," Geophysics, Oct. 

 1939, Vol. IV, No. 4, pp. 271-278. 



