BORE-HOLE INVESTIGATIONS 



1055 



Correlation of electrical logs of wells in widely separated areas is usually 

 confined to the more important changes or breaks in sedimentation. Con- 

 tinuous correlation, bed by bed, over long distances is possible, however, if 

 the wells are spaced closely enough to indicate the presence of overlaps and 

 faults which destroy the continuity of the geologic formations. Figure 650 

 shows the correlation of a well in the Tom Ball field with one in the Cleve- 

 land field. These two wells are forty miles apart on the Conroe trend. The 

 group of sands at the top of the Marginulina zone is very distinct in both 

 wells and, in spite of the distance, certain detailed features are recognizable. 

 At greater depths, in a series of alternating beds of sands and shale of a 

 uniform distribution, a characteristic sandy horizon located just above the 

 Textularia Hockleyensis zone is apparent. This horizon is easy to identify 

 in both wells. 



SELF 

 POTEIiTIAL 



^ a 



RESISTIVITY 



Fig. 651. — Electrical connections for simultaneous 

 recording of self potential and resistivity by means of a 

 double commutator. S.P., self potential recorder; C, by- 

 pass condenser; R, resistivity recorder; A, moving current 

 electrode; M, moving potential electrode; A^ and B, sta- 

 tionary surface electrodes; /, current meter; P, power 

 supply. 



Instrumentation 



Resistivity and self potential logs may be obtained simultaneously by means of the 

 double commutator apparatus termed the pulsator, illustrated schematically in Figure 

 651. The energizing current is reversed periodically and in synchronism with the 

 resistivity potentials which are to be measured. The direct-current self potential 

 recording meter is unaffected by the periodically reversed energizing current. The 



