ELECTRICAL METHODS 



563 



upstream development by the city was removing considerable water from 

 the surface and subsurface stream flow. 



It was the contention of the land owners that removal of the water 

 from the subsurface channel would allow infiltration of sea water from 

 the ocean which would be detrimental to their crops. The city claimed 

 that a natural subsurface rock barrier existed across the narrow portion 

 of the river valley and that this would effectively halt any damaging 

 influx of sea water. 



Fig. 347. — Location of water table 'and development of water supply, Yuma, Arizona. 



It was necessary to obtain data on the depth to bedrock and char- 

 acter of the subsurface contour of an old stream channel. The most 

 obvious method was to secure data allowing the plotting of a subsurface 

 contour map. These data could be obtained by a drilling program, or by 

 employment of a geophysical survey over the area. The latter was chosen, 

 and at 50 selected stations covering the area, geoelectrical data were 

 obtained showing depth to bedrock, and depth to the boundary of coarse 

 gravel, and finer silt and clay. These "electrical logs" were obtained at 

 a small fraction of the cost that would have been required for ordinary 

 drilling. In addition, magnetic work over the area as a part of the survey 

 disclosed evidence on the character of the probable subsurface conditions 

 underlying the stream valley. 



This survey also illustrates the use of electrical methods in determining 

 distribution of pervious and impervious materials. Figure 348 shows a 

 typical profile of the subsurface conditions as disclosed by the geophysical 

 work. The upper soil and clays are relatively impervious, while the lower 

 gravels are very pervious and allow flow of a subterranean stream during 

 the dryer portion of the year. 



