ELECTRICAL METHODS 



525 



connected to a power take-off on the truck motor, a steel rod V/i inches 

 in diameter is forced into the ground to a depth of penetration which 

 depends on contact conditions and varies from one foot to three feet. 

 Upon completion of the measurement, the power is reversed and the rod 

 is withdrawn from the earth. The truck is then moved forward to the 

 next point of measurement and the procedure repeated. The reel which 

 contains the field wire is mounted upon the truck, and the wire is auto- 



FiG. 317. — Removing steel electrodes after completing electrical 

 measurements. A, hollow-stem hydraulic jack; B, electrode; C, 

 spare electrodes; D, plunger pump; E, electrode clamp. (Courtesy 

 of Union Oil Company of California.) 



matically played-out as the truck moves forward. Contact is maintained 

 by means of a suitable slip ring on the reel. The first measurement 

 is made at some predetermined electrode separation, and subsequent meas- 

 urements are made at uniform increments of distance. This type of 

 arrangement has proved to be somewhat faster than the stake method 

 described above. 



The points where contact is to be made are usually measured and 

 marked on the ground by means of stakes (pieces of lath 12 inches long) 

 on which are written the traverse distance. (Because the resistivity for- 

 mulas for calculating resistivity values include "distance," it is necessary 

 to measure the electrode separations with considerable accuracy.) 



