ELECTRICAL METHODS 559 



first determined by alidade and plane table and were then plotted in detail, 

 as shown in Figure 343. 



Magnetic measurements of the vertical intensity were made at 242 

 stations over the area, at intervals from 300 to 400 feet. Magnetic equip- 

 ment for this work was the conventional Schmidt-type field balance. A 

 study of the geological and magnetic maps did not allow a unique inter- 

 pretation either as regards major concentration zones of the "black sands" 

 or of bedrock conditions. Hence, the magnetic and geological work was 

 supplemented with electrical studies. 



Electrical resistivity measurements were made at Z7 stations distributed 

 at regular intervals. Proper spacing of stations is a matter of experience 

 and is governed by the local geologic and topographic conditions. In 

 this instance, most of the stations were located at 600-foot intervals along 

 traverses having a northeast-southwest direction. In the southwestern 

 portion, however, the stations were placed at 800-foot intervals, because 

 location of the gravel contact was known on three sides from the surface 

 evidence, and closer spacing was unnecessary. Resistivity measurements 

 were made with the constant potential apparatus and the five-electrode 

 configuration previously described. Three curves were plotted for each 

 station. All electrical curves showed the variation in apparent resistivity 

 from the surface to an indicated depth of 600 feet. The first or main curve 

 showed the variation in apparent resistivity along the traverse line. The 

 two auxiliary resistivity curves showed the variations in two directions 

 from the station and were used for evaluating topographic and near- 

 surface effects. 



Because of the difference in resistivity between the gravels and the 

 more compact formations, such as the bedrock, the boundary or contact 

 between the two materials at depth is indicated on the curves by a change 

 in trend. The depth to bedrock at each station was determined by Tagg's 

 method (p. 491) and correlation of curves (p. 509) using known geo- 

 logical conditions encountered in exploratory openings and projection of 

 outcrops as control. By correlating the computed depths with the surface 

 trace of the gravel-bedrock contact and with the surface elevation shown 

 by the topographic survey, it was possible to plot subsurface bedrock 

 contours.* The maximum thickness of gravels on this property was found 

 to be about 300 feet. 



The results of the electrical survey are illustrated in Figure 344 which 

 shows cross sections of the deposit along the traverse lines. From these 

 sections, the approximate total yardage of gravels was calculated. A strik- 

 ing feature of these sections is the evidence of faulting in the bedrock. 

 In particular, sections D-D, F-F, and E-E indicate an escarpment in the 

 bedrock of more than 100 feet average vertical displacement. Prior to 

 the geophysical work, this faulting had not been suspected, because the 



* The depth to bedrock given by electrical work is an average or effective depth 

 over an area having dimensions comparable to the depth of penetration. 



