Subsurface Logging Methods 493 



or plate on which the spectra of the unknown samples are photographed, 

 in order to obtain the working curve. To avoid unnecessary duplication 

 and to maintain constant development conditions, it is best, therefore, to 

 use a spectrograph that will take the maximum possible number of 

 spectrograms on one plate. 



Densitometry 



A Gaertner microdensitometer was used by Sloss and Cooke. This is 

 a subjective type of instrument, in which the density of any given line is 

 matched against the density of a calibrated neutral-density wedge. It is 

 rapid in operation, but, because a definite area of lime is required for 

 matching, the spectrum lines must be broadened by operating the spectro- 

 graph with a wide slit. Such line broadening leads to two difl&culties: 

 first, the resolving power of the spectrograph is greatly reduced, requiring 

 that the lines chosen for analysis be free from proximity to neighboring 

 lines, which may overlap when the slit is opened; and second, the back- 

 ground fog is greatly increased, which leads to the embarrassing question 

 of just how the background correction should be applied. Undoubtedly, 

 the photoelectric microphotometer, of which many types are available, is 

 best suited for measuring line density. Instruments of this type are avail- 

 able from Hilger, Bausch & Lomb, the Applied Research Laboratories, 

 and Leeds & Northrup. 



A comparatively new method of analysis ^* ^^ ^^ eliminates the photo- 

 graphic plate as an integrating device. By using combination of slits in 

 the focal plane of the spectrograph, lines of those elements the quantities 

 of which are under investigation are isolated, and the respective light 

 energies are integrated by multiplier phototubes. The method seems to 

 have been singularly successful where it has been applied, and it would 

 seem applicable to the present problem. 



Interpretation 



Limited experience with spectrochemical logging indicates that, once 

 the necessary data are accumulated, interpretation of the results in terms 

 of correlation is fairly straightforward. Graphic presentation of data is 

 necessary before they are intelligible. The writers have used multiple-bar 

 diagrams, as illustrated in figure 231, and curves simulating electric logs. 

 The latter are not entirely justified, since a sloping line connecting the 

 values for adjacent samples suggests a transitional series of values from 

 sample to sample; nevertheless, such a representation seems to be the most 

 workable, perhaps because stratigraphers are accustomed to handling 

 similar curves. In any case, when the data are plotted for a series of wells 



^ Boettner, E. A., and Brewington, G. P., The Application of Multiplier Photo-Tubes to Quantita- 

 tive Spectrochemical Analysis: Optia. Soc. America Jour., vol. 34, p. 6, 1944. 



^ Saunderson, J. L., Caldecourt, V. J., and Peterson, E. W., A Direct-Reading Spectrochemical In- 

 stallation, The Dow Chemical Co., 1945. 



^ Nashtoll, G. A., and Bryan, F. R., An Application of Multiplier Photo-Tubes to the Spectro- 

 chemical Analysis of Magnesium Alloy: Optic. Soc. America Jour., vol. 35, p. 646, 1945. 



