ELECTRICAL METHODS 



575 



deposit. The prospect contains an estimated 35,000 yards of sand and 

 gravel. 



The time required to map the entire project by the resistivity method 

 was less than that needed to put down one such hole as No. 2 to a depth 

 of about 18 feet. The observer and one helper made the geophysical survey 

 in two days in the field. After the drill hole control was established, the 

 resistivity depth measurements were used to determine the amount of 

 overburden stripping necessary and the thickness of the gravel. 



RESISTIVITY CONTOURS 



too 200 30 

 SCALE IN FEET 



= EQU I -RESISTIVITY CONTOURS OHM-FEET. 



• = TEST HOLE 

 ELECTRODE INTERVAL = 20 FEET 



LOGS OF TEST HOLES 

 7 6 5 4 3 2 1 



Fig. 356. — Resistivity contour map, Wilmar Gravel Project, Wilmar, Minn. 

 (After S. W. Wilcox.) 



The conventional method of mapping gravel deposits for engineering 

 projects is to auger or test pit the likely locations. This technique yields 

 many negative results, especially in the glaciated areas of the United States, 

 where the geologic criteria for the occurrence of gravel are often misleading. 

 In such areas, gravel and sand are usually costly and critical items in high- 

 way building. Under these conditions the resistivity method provides a 

 very economical and rapid means of prospecting for gravel. 



Resistivity Surveys of Riprap Deposits near Fremont Butte, Colo- 

 rado.^ — Resistivity investigations were made during April, 1948, at the 

 Fremont Butte "quartzite" deposit in northeastern Colorado. The rock in 

 this locality is petrographically a silicified sandstone, although not a true 

 quartzite. It was sufficiently hard to be considered as potential riprap for 

 two proposed dams by the U. S. Bureau of Reclamation in the Missouri 

 River Basin. 



The objectives of the geophysical work were as follows: (1) to check 

 the resistivity method against existing drill holes, for determining the 

 depth and thickness of the silicified rock and locating its boundaries; (2) 

 to define the extent of the rock in a part of the deposit where no drilling 

 had been done; and (3) to determine the speed and cost of resistivity 

 work as compared with drilling for exploring such riprap sources. 



t Dart Wantland, personal communication. 



