562 



EXPLORATION GEOPHYSICS 



canyon. The two areas have somewhat similar surface geologic and 

 topographic features. The object of the geophysical work in the Florida 

 canyon area was a determination of the depth and contour of the bedrock 

 across the canyon and the electrical characteristics of the overlying fill 

 material in order to establish the presence, or absence, of a water supply 

 comparable to the known supply in Saw Mill canyon. 



Resistivity measurements in both areas produced results of the type 

 shown in Figure 346. The curves showed inflections corresponding to 



o go 40 so oo too izo I40 ifo leo 



DEPTH ~ FEET 



Fig. 346. — Curve showing effects of water-level and bedrock on resistivity. 

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



(1) change from dry fill and stream wash to wet fill and top of water 

 horizon and (2) change from fill material to the underlying bedrock. At 

 this station the geophysical work placed the depth to the water level at 

 20 feet and the depth to bedrock at 29 feet. Direct measurement in an 

 adjacent drill hole gave the water level as 20 feet; the depth to bedrock, 

 26 feet. 



A combination of structural work and direct location of water table 

 is illustrated by a survey conducted near Yuma, Arizona, for a mining 

 company. The presence of a permanent water table in this area was 

 known. The problem was to locate the nearest point to the mine at which 

 a well would intersect the permanent water table in porous alluvium above 

 the impervious bedrock which sloped downward from the foothills. Figure 

 347 summarizes the general results secured by the electrical survey. 

 A well drilled near station 10 encountered the water table as predicted 

 by curve correlation. According to the mine management, the well is 

 capable of producing in excess of 500 gallons per minute. 



In 1931 a municipality in Southern California was threatened with 

 litigation by a group of land owners near the mouth of a river on which 



