BORE-HOLE INVESTIGATIONS 



1079 



It may be seen that similar results may be obtained by making measurements of 

 resistivity, or of some other formation characteristic, instead of S.P. measurements; 

 and in some geological territories the resistivity dipmeter has definite advantages, 

 particularly in "hard rock" areas where the S.P. curve may have little feature, but 

 where satisfactory correlating points are available on the resistivity curve. The resis- 

 tivity sonde comprises a current 



electrode centered in the hole, be- Typicol Dtpmettr L«v«l 



tween three pairs of potential meas- 

 uring electrodes, positioned on three 

 sides of the hole. The potential dif- 

 ference observed between each pair 

 of measuring electrodes, due to the 

 flow of the current, is amplified and 

 transmitted to the surface where it 

 is recorded. The three curves are 

 recorded simultaneously and, as 

 with an S.P. dipmeter recording, 

 the relative depths at which corre- 

 lated anomalies occur on the three 

 curves are used to determine the dip. 



Equipment for Dipmeter Sur- 

 veys. — The hole unit assembly is 

 about 25 feet long and weighs about 

 400 pounds. The dipmeter electrode 

 unit consists of a mandrel to which 

 are attached three hard rubber arms 

 spaced 120° apart ; in the center of 

 each arm is one of the three pairs of 

 recording electrodes. For an S.P. 

 dipmeter survey, only one electrode 

 of each pair is used ; for a resistiv- 

 ity dipmeter survey, both are used. 

 The corresponding electrodes of 

 each of the three pairs are located 

 in the same plane perpendicular to 

 the axis of the instrument. 



The hard rubber arms on which 

 the electrodes are mounted are in- 

 terchangeable and come in several 

 sizes. The size is chosen so that the 

 dipmeter electrodes will be posi- 

 tioned close to the wall of the hole. 

 It is therefore necessary for the 

 operator to know in advance the 

 average size of the hole. The small- 

 est hole in which present dip- 

 meter equipment can be run is 6 

 inches in diameter. The accuracy decreases with the diameter of the bore hole. 



Above the electrode unit is positioned the photoclinometer, which determines the 

 orientation of the three S.P. or resistivity curves, and gives the drift and azimuth of 

 the drill hole. Spring guides above and below the photoclinometer electrode assembly 

 serve to keep the device centered in the hole and to prevent it from rotating. The same 

 cable and truck are used as for electrical logging. The three S.P. or resistivity dip- 

 meter curves are recorded at the surface by the standard electrical logging recorder. 



Fig. 673. — Typical S.P. dipmeter curves, showing photo- 

 clinometer pictures at top and bottom of the section. 



