386 E. A. POLYAKOV 



progressively higher as the movement towards the screen (Table 1) pro- 

 ceeds. Similar observations were made on moving the probe-bearing disc 

 towards a conductive screen. In this case, however, the influence of the 

 screen is demonstrated by the decline of the readings. From Table I it 

 follows that, in order to prevent the bore-hole walls having any influence 

 on the results of the measurements, the probe should be separated from 

 the walls by not less than 1.25 times its size. In such a case the error in the 

 results does not exceed 5 per cent. 



The length of the probe in the open resistivity -meter of the type GML 

 BGK (Oufa) equals 25 mm. The container of this resistivity -meter consists 

 of three tension shackles, which have an external diameter of about 60 mm 

 and the probe electrodes can approach the bore -hole walls up to a distance 



of 25 mm. Consequently, in this resistivity -meter — < 1.25 the influence 



of the walls is likely, which is actually recorded in practice. 



In order to remove the influence of the well walls the resistivity -meter 

 probe is commonly placed inside a flat cylinder made of a good conductor 

 or insulating material. This, however, interferes with the drilling fluid 

 circulating through the resistivity -meter, when the latter is moved along 

 the bore-hole shaft. As a consequence the usefulness of the instrument is 

 impaired . 



(3) Change of the resistivity-meter coefficient depending on the salinity 

 of the drilling fluid. 



The coefficients of the bore -hole resistivity -meters in current usage change 

 strongly depending on the degree of salinity of the fluid. This is represented 

 in Fig. 1, Avhich shows the values of the coefficients of certain serial resistiv- 

 ity-meters placed in fluids of variable salinity (curves 2-4). 



Obviously, in this case, the reason for the changes in the resistivity- 

 meter coefficient is due to a rearrangement of the electric field, produced 

 by the changes in the salinity of the ffuid. To verify this a water-filled Plexi- 

 glas tank of variable resistance was used. The bottom of the tank was cov- 

 ered with a sheet of zinc -plated iron. Observations on the electric field were 

 made with the aid of two small gradient probes, which had common current 

 electrodes (Fig. 2) and were pointing in mutually perpendicular directions ;v 

 and z. As a result the following equation was determined. 



where: Av^ is the potential difference between the electrodes M^ and N^ 

 of the probe placed along the z axis. 



