BORE-HOLE INVESTIGATIONS 



1093 



_ When grinding facilities are available for dressing the ends of the cores, a more 

 rapid and convenient method is that illustrated in Figure 683.t In this arrangement, 

 metal rings are pressed against the ends of the specimen and serve as the energizing 

 or current electrodes. The two potential electrodes are placed coaxially with the 

 current rings. 



Another arrangement of lower accuracy but greater convenience for irregularly 

 shaped specimens is the four-prong resistivity method illustrated in Figure 684. 



2 nnfen;,•ffLT/^''.^^^,^*"^^°^ measuring resistivities of dressed specimens. 1, current contact ring; 

 fn/.!., f- ?°"'a<:t; 3 nuts for clampmg specimens between tlie contact rings; 4, knob for tighten- 

 ing potential contact; 5, test specimens. i"B"L<;ji 



DRILLING TIME LOGS 



It has been recognized for many years that the time required for drilling 

 a unit depth of formation varies with the nature of the formation, as well as with 

 the rnechanical characteristics of the drilling equipment. Data may be plotted 

 showing depth versus drilling time per unit depth of penetration. 



Formerly, the proper correlation between time and depth was chiefly de- 

 pendent on a log made by the driller, and many errors were common. Recently, 

 It has been the practice to use mechanically actuated recorders which are geared 

 to the drill stem. This is usually accomplished by means of a flexible cable 

 connected to the swivel or the traveling block, which feeds over an idler near 

 the top of the derrick to the recording instrument.$ Movement of the cable, 

 as the drill is raised or lowered, produces corresponding movement of the re- 

 cording paper. In some cases, a ratchet device is employed to allow for con- 

 tinuous forward motion of the recording paper. In other devices, the chart is 



( riH' ^■a^^^°aH ^r^,f^^^- .Hopper "The Effect of Moisture on the Direct Current Resistivities 

 of Oil Sands and Rocks," Geophysics, Vol. 2, No. 1, January 1937 ixcMsuvuies 



/.«rLT,-JuV"i?an"d%0?'^19^3-9.''- ''''^°''' "°""'"^ '^''^^ ^^*" '" ^"'"'"^ Practice," Oil and Ga. 

 J. J. Jakosky, U. S. Patents 2,150,169, 2,153,802 and Reissue 21,102. 



