NEW TYPES OF WELL RESISTIVITY-METERS 



385 



of the sonde, changes in the shape of the electrodes and in the distance be- 

 tween them have an especially considerable influence. In order to ensure 

 the stability of the resistivity -meter coefficient the device should be sturdily 

 constructed and the junctions — and in particular the probe electrodes — 

 should not change their respective positions or size when the apparatus 

 is being used. 



(2) Influence of the surrounding medium. 



The readings of the open* resistivity-meters are sometimes influenced 

 by the surroundings such as the walls of the ^vell (metal of the cylindrical 

 casing tubes, practically non-conductive rocks of a high resistance, etc.). 



Obviously the shorter the probe the less is the influence exerted by the 

 surrounding medium and the possibility of distortion of the results of the 

 measurements. 



The following experiments were made in order to determine the length 

 of the probe so that an open resistivity -meter would not reflect the influence 

 of the well walls. A probe with its electrodes on one side of an insulating 

 disc was immersed in a relatively large tank filled with water, then the 

 electrode-bearing surface of the disc was moved towards the water surface, 

 which represented an insulating screen. 



When the disc is at some considerable distance away from the screen the 

 latter, naturally, has no influence on the measurements, but as the electrode- 

 bearing disc approaches the screen the readings begin to rise, becoming 



Table 1. Percentage deviations of the readings with a screen from the 

 readings with a distant screen. 



L — the length of the probe; H — the distance from the disc on which the electrodes are 

 placed to the screen (the screen is parallel to the disc). The insulator electrode-bearing disc 

 has a size of 10x20 cm; AM = 5 cm, MN = 5 cm, the diameter of the electrodes 1 cm; 



the screen 150x150 cm. 



* The open resistivity-meter is one in which the probe is not protected by a metallic 

 or a non-conducting screen, while the influence of the bore-hole wall is minimized by the 

 use of a short probe situated along the axis of the device. 



Applied geophysics 25 



