Subsurface Methods as Applied in Geophysics 



1113 



potential at points many miles apart are very similar in form. The pref- 

 erential direction of the currents is determined along with the ratio of 

 the intensity at the base station to the intensities at the mobile stations. 

 The magnitude and direction of telluric currents is governed partly by 

 variations in the electrical resistance of the earth's crust; they avoid 

 rocks of high resistivity and tend to flow over or around them, thus 



Figure 589. Self-potential anomaly at Malachite mine, Jefferson County, Colorado. 

 (After Heiland, Tripp, and Wantland. Reproduced permission Am. Inst. Min. 

 Met. Eng.) 



varying the surface-potential gradient. Amplitude ratios are known to be 

 high over uplifts and low over synclines and basins. 



An experimental survey was conducted over the Haynesville salt 

 dome in Texas. (See fig. 590.) Salt, a nonconductor of electricity, deflects 

 the current over and around the salt plug, causing a high current density 

 over the plug. The same effect would exist over basement uplifts. Ratios 

 of current density of as great as one to five may be noted on the Haynes- 

 ville map. Remarkable correspondence between the outline of the salt 

 plug and the contours of equal magnitude of the telluric field is shown. 



Controlled Currents 



In the several controlled-current methods the ground is energized 

 either by means of electrodes inserted into the ground (galvanic methods) , 

 or the current may flow in the ground by electromagnetic induction from 

 alternating currents flowing in lines, loops, or coils at or above the surface 

 of the ground (inductive methods). The power may be provided either 

 by batteries or by mobile generators. 



