Chap. 10] 



ELECTRICAL METHODS 



741 



both resistivity mapping and sounding. Fig. 10-73 shows the application 

 of resistivity mapping in a section consisting of glacial drift at the surface 

 and limestone with ore bodies below. The limestone is underlain by con- 

 glomerates. The three curves represent: (solid curve) actual subdrift 

 topography; (dotted curve) *apparent conductivity obtained with 40-foot 

 electrode separation; (dashed curve) apparent conductivity with 120-foot 

 electrode separation. The conductivity curve obtained with the 40-foot 

 separation does not give a true picture of the subdrift topography because 

 the average thickness of the glacial drift is greater than this separation, 

 and the results also reflect irregularities in the composition of the drift. 

 The closest relation between subdrift topography and apparent conductiv- 

 ity was obtained with 120-foot electrode separation, which is the greatest 

 thickness of the drift encountered in the area investigated. 



„-mw,__--f Surface of rock 



-"-Trr^T-- CondudMty at 120 Ft interrd 



» n 4(f H II 



)ft. 



Horizontal Scale 



_i ' Vertical Scale 



—I ' Condueh'yittf Scale 





^T^^^ 



I^SWS 



yn i I >l>)l llllinii 



Trrrrrrrr 



^Ore 



-^Lime 



• ^ 



~Conglom. 



Fig. 10-73. Apparent resistivity curves, with different electrode separations to 

 determine subdrift topography (after Lancaster-Jones). 



Location and depth deternaination of placer deposits is closely related to 

 the problem of determining depth to bedrock in foundation problems, 

 discussed in the next paragraph. 



3. Applications in civil engineering (foundation and highway problems). 

 Application of resistivity methods in this field is threefold: (a) determina- 

 tion of depth to bedrock; (b) determination of physical rock characteristics 

 for dams, structures, tunnels, and the like; and (c) location of construction 

 materials. 



Bedrock depth determinations by resistivity methods have been made 

 on numerous occasions. Many of these have been carried out by Schlum- 

 berger and his associates.^ An example was illustrated in Fig. 10-64. 

 In this particular survey, predictions could be verified within 1 to 2 meters 



"Leonardon and Crosby, A.LM.E. Geophys. Pros., 199-210 (1929). 



