ELECTRICAL METHODS 579 



drilling. If it can be assumed that two core holes can be drilled per day 

 to a depth of 50 feet (such silicified rock is often hard drilling), resistivity 

 depth measurements are twice as fast, since four 50-foot-spacing depth 

 curves were run per day. 



For the location of the covered boundaries of the rock layer the 

 resistivity procedure has a considerable advantage. It can be assumed that 

 it requires 3 drill holes as a minimum to locate the boundary of the rock. 

 At the rate of 2 holes per day, 11/2 days work would be necessary for 

 one boundary determination. With the resistivity traverse this could be 

 done in one-fourth of a day, or 6 times as rapidly. 



Supplemental geophysical investigations and additional drilling were 

 done at Fremont Butte during July and August, 1948, to secure data for 

 an accurate estimate of the yardage of rock in the entire deposit. In 9 days, 

 21 resistivity depth curves were measured and approximately 1100 feet of 

 constant-depth traverse were run. In the supplemental program, resistivity 

 depth measurements were first made at selected sites to determine the 

 approximate thickness of the rock zone, and, as before, constant-depth 

 traverses were run back from the outcrop to establish the width of the 

 deposit. Core drill locations were then selected on the basis of the geo- 

 physical findings at those places where it would be most advantageous to 

 know the exact thickness of the rock and its quality, and which at the 

 same time would give the best control for further resistivity tests. Useless 

 drilling in the barren areas was minimized by use of the resistivity work. 



The procedure of supplementing and guiding the selection of drill 

 locations secured the information required for estimating the yardage in 

 the shortest possible time and with the minimum number of drill holes. 

 For example, the first drilling at the Fremont Butte area (December, 1947) 

 consisted of 9 core holes. The test resistivity survey was carried on in 

 April, 1948, as described above. These two programs explored about 2,000 

 feet of outcrop in the southern end of the deposit. The supplemental 

 program required only 15 drill holes to investigate over 4,000 feet of 

 outcrop or about twice as much as was covered in the original work. 



As a result of the investigations it was estimated that approximately 

 147,000 cubic yards of silicified sandstone (in place) were available at the 

 Fremont Butte deposit. The figure was considered accurate within 10 per 

 cent, indicating quantities of from 130,000 to 160,000 yards. 



During the 1948 field season the resistivity method was used in con- 

 nection with drilling to map 5 other potential riprap deposits. Including 

 the Fremont Butte area, it is estimated that a total of over 800,000 cubic 

 yards of silicified rock was mapped. 



