ELECTRICAL METHODS 



S77 



definite boundaries as if deposited in stream channels, likewise suggests 

 such an origin. These features are significant in estimating yardage, as 

 the continuity of the rock laterally and in thickness cannot be safely 

 assumed except over short distances or between closely spaced control points. 



TOP HARD S5. 

 SILICEOUS 



SOFT S.S. 

 CLAY & ASH 



^NOTE: DEPTH & THICKNESS OF HIGH 

 RESISTIVITY SILICEOUS ROCK POUND BY 

 ANALYSIS OF RESISTIVITY MEASUREMENTS. 



400 500 600 



RESISTIVITY OHM-FEET 



Fig. 358. — Resistivity Depth Curve No. 1, near Drill Hole 1, Fremont Butte, Colorado. 

 (Courtesy of U. S. Department of the Interior, Bureau of Reclamation.) 



The applicability of the resistivity method to the problem is based on 

 the fact that the resistivity of the silicified sandstone is some 5 to 6 times 

 greater than that of the material surrounding it. 



The field program at Fremont Butte consisted of resistivity depth 

 measurements at the 8 core drill holes to secure control data for determin- 



SHA'-E^io 



40 



note: ABSENCE OF SILICEOUS ROCK 

 SHOWN BY LOW VALUES OF RESISTIVITY 



400 



100 200 300 



RESISTIVITY OHM -FEET 



Fig. 359. — Resistivity depth curve No. 2, near Drill Hole 2, 

 Fremont Butte, Colorado. (Courtesy of U. S. Department of the 

 Interior, Bureau of Reclamation.) 



