OPTICAL METHODS OF BORE-HOLE INVESTIGATION 



337 



In the case of photography of rocks in a well the resolving power must 

 be such that it is possible to discern the grain of sandstones having grain 

 dimensions of 0.2 mm; for this it is obviously necessary to have a resolving 

 power of 5 lines per mm. 



It is well known (^) that resolving power is equal to 



N = Nr, 



where if is the coefficient of reduction (loss) of contrast ; -/Vmax ^^ ^^^^ resolving 

 power for K = 1. 



We obtain the permissible coefficient of loss of contrast K by putting into 

 the formula N = 5 and N^g^^ = 30 lines per mm (the most probable value 

 of A^jnax ^^^ the Industar 22 objective). Hence we obtain 



K = 0.05 = 5% 



It is obvious that the permissible thickness of the layer of drilling fluid Amax 

 between the viewing window of the well camera and the wall of the well 

 is equal to that at which the coefficient of loss of contrast has a value less 

 than K = 0.05. The value of Amax ^^^ ^^ obtained from Fig. 5. 



Table 2. 



In evaluating the properties of clay suspensions it is best to use, instead 

 of the specific gravity, the content of solid particles in the solution, expressed 

 in weight per cent. This quantity 7], which may be called the turbidity of 

 the suspension, is connected with the specific gravity of the drilling fluid 

 by the relationship 



V 



d-1 



100%, 



(6) 



where d is the mineralogical density, which can be taken as equal to 2.65. 



As can be seen, even a thin layer or film of clay suspension excludes the 



possibility of photographing the bore-hole walls. Consequently, for photo - 



Applied geophysics 22 



