918 



EXPLORATION GEOPHYSICS 



a geophysicist, an observer, a surveyor and one or two helpers. Computa- 

 tion of records is usually done in the evening. The speed of the work varies 

 with terrain and other working conditions, but generally averages 4 to 10 

 stations or 2 to 4 spreads per working day. The cost of a crew is from 

 $2,500 to $4,000 for a 20-working-day month (1949 base price). A geolo- 

 gist or mining engineer familiar with the area usually can render valuable 

 aid in planning the work and in the interpretation of the geophysical data 

 in terms of the economic geology. 



Del Norte Dam Site, Colorado 



A seismic refraction survey of the Del Norte Dam Site was made 

 under the direction of E. L. Current, of the United States Bureau of 

 Reclamation, in October, 1943. f 



The details of this survey illustrate some of the numerous factors 

 which must be considered by the geologist and the geophysicist in the 

 exploration of dam sites, and they are presented for that reason. 



The site is located at the town of Del Norte on the western edge of 

 the San Luis Valley. The axis of the proposed dam cuts across the edge 



CONTOUR INTeRVAL 100 FT. 



Fig. 570. — Map of Del Norte Dam Site, Rio Grande County, Colorado. 

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



of the town itself. Details of the location are shown in Figure 570. An 

 earth dam with a crest length of 4,400 feet was proposed, to impound one 

 million acre-feet of water. Two dike sections approximately 1,000 and 

 2,000 feet long respectively were involved, in addition to the main dam, 

 which would require a total of some 10.6 million yards for the entire project. 



Geology, — The dam site and the reservoir area cover rocks of 

 Miocene age, represented by the Sheep Mountain and Conejos formations. 



t E. L. Current, "Report on Geophysical Survey of the Del Norte Dam Site, San Luis Valley 

 Project, Upper Rio Grande Basin," Oct., 1943. (Files of Geology Section, U. S. Bureau of Recla- 

 mation, Denver, Colorado.) 



