560 



EXPLORATION GEOPHYSICS 



trace of the fault was covered by alluvium over practically the whole 

 property. After discovery of the fault by geophysical methods, its trace 

 was found in a gulch at one point where erosion had slightly exposed it. 

 The knowledge of this escarpment materially affected plans for hydrau- 

 lic mining. Before the geophysical survey was made, the mining company 

 had perfected plans for beginning hydraulic operations in the northeast 

 part of the area, because surface topography indicated that the bedrock 

 most likely sloped toward this gulch. When the results of the geophysical 

 work were known, the operations at that point were abandoned, and plans 

 were made for work in the southern part of the area. 



W^ Grovels 

 Fig. 344. 



Feet scale Feet, Horaony Scale 



Cross sections of placer deposit along traverse lines. {Engineering 

 and Mining Journal, Feb. 1934.) 



The complete bedrock contours predicted on the basis of the data 

 obtained in the geophysical investigations are shown in Figure 345. 



Water Supply Problems, t — As regards the application of geo- 

 physical work to water supply, the geological considerations are somewhat 

 similar to those involved in placer investigations. In general, the problems 

 may be divided into two classes : ( 1 ) the determination of the existence 

 of a permanent subsurface water table either in fill material (gravels, 

 sands, etc.) or more consolidated rock; (2) the determination of sub- 

 surface structure that would afifect ground water accumulations. In water 

 supply investigations, as in other problems, the geophysical work is 

 facilitated by direct correlation of the geophysical data with well logs. 



t J. J. Jakosky and C. H. Wilson, "Geophysical Studies in Placer and Water Supply Problems," 

 A.I.M.E. Geophysical Prospecting, Tech. Pub. 515, February, 1934. 



