Chapter 7 



THEORETICAL BASES OF ELECTRICAL PROBING WITH 

 AN APPARATUS IMMERSED IN WATER 



E. I, Terekhin 



Marked successes have been achieved recently in the development of 

 electrical prospecting at sea. A method has been developed for the produc- 

 tion of continuovis two-way dipole axial probes with a distance between 

 the centres of the dipoles of up to 6 — 8 km, with sea depths of up to 50-60 m 

 (0. V. Nazarenko). 



In this method, during measurements the feeding and measuring dipoles 

 are situated at the bottom of the sea, i.e. at the lower boundary of the first 

 layer. With the same geoelectrical cross -section, the values of the apparent 

 resistance measured with the sea-bottom apparatus differ from the values 

 of apparent resistance measured with the same apparatus on the surface of 

 the water. The solution of the problem of distribution of the field of a point 

 source, within the limits of the first layer, and in particular at its lower bound- 

 ary, is therefore of practical interest. 



The problem of the distribution of potential for a point source of current 

 at the lower boundary of the first layer of a three-layer horizontally homo- 

 geneous medium was first solved in 1934 by M. Ya. Samoilov. Based on 

 this solution, in the same period in the Geophysical Section of the AzNIl 

 certain theoretical curves were calculated and published later, without 

 a derivation of formulae, in a book by S. Ya. Litvinov^"). These include 

 the two-layer curves with ^2/^1 ~ ^^ 2' ^' -^^' ^^ °°' three-layer curves 

 for the case ^3/^1 = 1.5, qJQi ~ 1' 2, 5, 10, 40, 00 and /12/^h ~ 2, 5, 

 10, 40. 



In the general case of a horizontally homogeneous stratified medium, 

 the problem of the distribution of the field of a point source at the 

 boundary of separation of the first and second layers, was solved by 

 L. L. Van'yan(3). 



In this paper two -layer curves are given for the AMNB apparatus and the 

 dipole axial apparatus and three -layer curves for the AMNB apparatus for 

 the case ^3 = 00, h^ = h-^ and various values of ^2/^1* 



