Chap. 11] GEOPHYSICAL WELL TESTING 869 



can be logged continuously by connecting a gas detector to the discharge 

 system in a suitable manner. 



G. Photoelectric Measurements 



When formation water is discharged into a well, the transparency of the 

 drilling fluid increases. This change may be measured by a photoelectric 

 detector. This unit contains a light source which projects a beam through 

 a portion of the drilling mud to a photoelectric cell connected to a pre- 

 amplifier. The resulting current fluctuations are further amplified and are 

 recorded at the surface. In operation, the well is first conditioned with a 

 light mud and then bailed out sufficiently to allow formation water to 

 enter the hole.^^ 



H. Side-Wall Sampler Bullets 



These bullets are short cylmdrical shells fastened to two retaining wires 

 and are shot electrically into the sides of an open hole. The cores thus 

 recovered are analyzed in the laboratory for porosity, permeability, 

 salinity, and content in colloids and organic matter. This procedure is 

 useful for correlation with electrical resistivity and porosity (self-potential) 

 logs; but, strictly speaking, it is not a geophysical method. Details are 

 given in the paper by E. G. Leonardon and D. C. McCann. '^ 



** Gillingham and Steward, loc. cil. 

 " Petrol. Tech., 2(2) (May, 1939). 



