756 



ELECTRICAL METHODS 



[Chap. 10 



1^ 

 I. 



I' 



WHw M" 



*5(r 



*i(r ^ 



0' 



-10* 



^ im 900 



^Orophifr Mr. 50'widt, dip 45" 

 localtd after eltcMtal surviy 



correction for normal ratio was applied. Observed ratios were generally- 

 converted to a potential "variation" curve, the latter being identical with 



the potential gradient referred 

 to above. Fig. 10-84 shows a 

 profile taken in the Uley graph- 

 ite area near Port Lincoln, 

 South Australia. The graphites 

 are believed to have originated 

 from a metamorphism of Pre- 

 cambrian rocks, probably con- 

 sisting originally of magnesium 

 limestones. The potential gra- 

 dient curve shows a minimum in 

 the middle of the profile, indicat- 

 ing a good conductor; the 

 small peaks on either side 

 signify a crowding of the 

 equipotential lines. The phase 

 curve has a maximum, corre- 

 sponding to out-of-phase cur- 

 rents induced in the conductor. 

 On the basis of the electrical 

 survey, a graphite body about fifty feet in width was located, dipping 

 about 45-50° to the NW. 



Application of P.D.R. surveys in oil exploration has been made to deter- 

 mine depth to key beds and to locate structure. Those known to date 

 do not involve depths exceeding several hundred feet. In some P.D.R. 

 surveys in Alberta,*^ depth interpretation was first based on peaks in the 

 reduced ratio logs and was later supplemented by an evaluation of apparent 

 resistivity curves calculated from the P.D.R. readings. ^ To what extent 

 the simple two-layer relations between the distance of the peak in the 

 P.D.R. curve and depth hold for more than one interface has not been 

 determined, although it is probable that peaks in opposite directions appear 

 near the upper and lower boundaries. 



P.D.R. methods offer a rapid means of measuring depth to bedrock in 

 foundation problems, of locating faults and shear zones, of determining 

 the general characteristics of formations, and of locating water. The 

 survey shown in Fig. 10-85 was made near the Oriental ore body at 

 Buchans in Newfoundland, where glacial drift occurs above arkose bed- 

 rock. The potential-ratio {Bj A) curves are plotted in such a manner that 



«8 Lundberg and Zuschlag, A.LM.E. Geophys. Pros., 61 (1932). 

 " Kihlstedt, op. cil., 199. 



Fig. 10-84. Potential gradient and phase 

 curve on graphite deposit at Port Lincoln, 

 South Australia (after Edge and Laby). 



