Chap. 10] 



ELECTRICAL METHODS 



759 



2^' 



the time gradient of the surge or by measuring the maximum height of 

 the transient and dividing it by the total charge of the current surge. 

 These transient voltage maxima, when contoured across a structure, give 

 a picture very similar to a resistivity contour map. 



The difficulties encountered in the evaluation of oscillographic records 

 lead to the adoption of neutralization and compensation methods to deter- 

 mine magnitude and shape 

 of the transients. Statham" f^^\_^ ^^^^ 



developed a method based it/ I ' ' ■» H'I'I' 



aAaa-1 — ^AAA 

 on a comparison oi tran- 

 sients in adjacent ground 

 intervals. In Fig. 10-86a, 

 I and III are the two pri- 

 mary circuits of opposing 

 polarity energized simulta- 

 neously through two screen- 

 grid thyratrons by closing 

 the switch 5 in their parallel 

 grid circuits. II is the 

 potential circuit connected 

 through a two-stage direct- 

 coupled amplifier to the 

 vertical plates of a cathode- 

 ray oscillograph on which 



a 4-inch deflection corresponds to a (ground) potential difference of 4 

 millivolts, while the horizontal plates are actuated hy a tuning-fork-con- 

 trolled hnear sweep circuit. With currents flowing in circuits I and III, 

 resistors 1 and 2 are first adjusted so that no steady potential occurs 

 in circuit II. If the switch >S is then closed and the transients due to 

 circuits I and III are equal, no transient will be recorded in circuit II. 

 When transients appear, they are compensated by moving the potential 

 electrodes. The direction and distance required for cancellation are in- 

 dicative of direction and rate of increase in effective conductivity. 

 Statham'has published a map showing the effect of a deep-seated Gulf 

 coast oil field on the conductivity vectors thus determined. 



Another group of Eltran methods involves a determination of the shape 

 of the transient by compensation Avith a simulating network. This net- 

 work may be synchronized by using a reference lead to the generator. 

 For large electrode separations, a local oscillator feeding the simulating 





zooo- 



-fOOO 



1000'- 



FiG. 10-S 



z. Circuit for comparison of electrical 

 transients (after Statham). 



^3 Louis Statham, Geophysics, 1(2), 271-277 (June, 1936), and U. S. Patent No. 

 2,113,749. 



