548 EXPLORATION GEOPHYSICS 



the grid of the control tube, and the correspondingly varying output across 

 a resistor in the plate circuit is connected to electrodes 4 and 5. 



Continuous Recording Apparatus. — When continuous recording 

 apparatus is used, the values of potential, current, and electrode move- 

 ment are recorded photographically on a moving film. Upon completion 

 of the run, the record is developed and the values of potential and current 

 are scaled from the record and substituted into appropriate resistivity 

 formulas for the particular electrode spacing employed. Graphs are then 

 made of the apparent resistivity values at various electrode separations, 

 ordinarily at regular intervals of about 50 feet, versus the traverse distance. 



A portion of a typical field record which shows instantaneous values 

 of potential E and corresponding values of the current / is given in 

 Figure 336. The vertical lines mark each ten-foot movement of the mobile 

 electrode. 



Fig. 336. — Typical record showing instantaneous values of potential 

 E and corresponding values of the current /. 



The current for energizing the ground is a uni-directional pulse having 

 a controlled time interval t to govern polarization. During the intervals 

 of time when the energizing current is off, the potential recorded is that 

 created by the natural ground currents. Variations in natural ground 

 potentials are shown by a curve which connects points obtained when 

 the energizing circuit is open. 



A view of the complete equipment for continuous profiling is shown 

 in Figure 337.$ Cabinet 1 contains controls for the recording camera, 

 the potential and current galvanometers, and a carrier-current relay- 



t J. J. Jakosky, "Method for Determining Underground Structure," U. S. Patent 2,015,401. 

 issued Scot 24 1935 



t J. J. Jakosky, "Continuous Electrical Profiling," Geophysics, March 1938, pp. 130-153. 



