Chap. 10] 



ELECTRICAL METHODS 



757 



ratios greater than 1 are to the right, and those less than 1 to the left. 

 A peak to the right signifies a transition from a good to a poor conductor, 

 and a peak to the left a transition from a poor to a good conductor. The 



Feet 



jO 



50 



^m 



W 10 to to to to 



Fig. 10-85. Potential-drop-ratio curves indicating water level in glacial moraine and 

 bedrock, Newfoundland (after Lundberg and Zuschlag). 



electrical indications were interpreted as showing the effects of both water 

 and bedrock, the dry upper portion of the moraine being the poor, and its 

 water-bearing portion above bedrock being the good conductor. 



VII. ELECTRICAL TRANSIENT ("ELTRAN") METHODS 



Electrical methods previously discussed involve the measurement of 

 stationary potentials or potential differences. In electrical transient meth- 

 ods, on the other hand, their variation with time is observed. The recorded 

 time constants of the ground are primarily related to the resistance of the 

 ground circuit; it is likely that capacitive and electrolytic-polarization 

 effects introduce a reactive component. 



When a ground circuit is closed or opened, the equilibrium values of 

 voltage and current are not reached immediately. Because of the change 

 of the current with time, induction currents are generated. The greater 

 inductance of the lower paths causes the current to flow at first near the 

 surface (skin effect). The variation of the e.m.f. with time may be ex- 

 pressed by a relation of the form 



tp 



Et-Eo= {Em... - Eo)e ^^b^, 



(10-45a) 



where £^max. is the maximum initial potential difference between the ground 

 electrodes when the circuit is closed, Eo is the steady state value, t is time, 

 p is resistivity in ohm-cm, y is permeability, h is the distance between the 

 electrodes, and c is a constant*^ equal to 2.32- 10~^ With y = 1, the time 

 required for the difference E'max. — Eo to drop to 1/e of its value (time 

 constant T) is given by 



p 



T = 



(10-456) 



«8 T. M. Pearson, A.LM.E. Geophys. Pros., 34 (1934); C. and M. Schlumberger, 

 ibid., 139 (1932). 



