940 EXPLORATION GEOPHYSICS 



Gas Analysis Methods 



The gas analysis methods include those techniques which involve the 

 collection of soil air samples in situ and the analysis of these samples for 

 the minute concentrations of hydrocarbons present. The introduction and 

 development of these techniques are due principally to Laubmeyer and to 

 Sokoloff and co-workers. 



Sampling and Analysis. — Briefly, Laubmeyer's techniquef consists 

 in digging bore holes one to two meters deep, closing these holes with a 

 special seal, consisting of a lid provided with two concentric cylinders of 

 large radius and a piece of tubing to which a stopcock is attached, and 

 allowing twenty- four to forty-eight hours for the composition of the 

 enclosed air to reach equilibrium. A sample of the air is then extracted 

 for analysis. Laubmeyer points out that the hydrocarbon content of the 

 soil air sample, collected in this manner, consists principally of methane, 

 since hydrocarbons heavier than air would not tend to diffuse into the 

 bore hole. The gas is brought in contact with a heated platinum filament 

 and the hydrocarbons, in the presence of air, are burned. This reaction 

 raises the temperature of the filament which in turn increases its resistance. 

 The change in resistance is a measure of the hydrocarbon concentration. 



In the Russian technique, a hole of suitable depth (10 to 30 feet) is 

 dug by means of a hand auger so that its upper half has a larger diameter 

 than its lower half. The shoulder of the hole is then wetted, tamped, coated 

 with clay, and sealed from the atmosphere by means of a bell-shaped 

 sampling container. Next, the atmospheric gases are pumped out of the 

 enclosure by means of a vacuum pump and the soil gases are allowed to 

 seep into the enclosure and are collected in a suitable container by displace- 

 ment of water. 



Two methods of gas analysis were developed by the Russians, one for 

 use in the field and the second for use in the laboratory. In the case of 

 the former, the sample is passed through a caustic solution to remove the 

 carbon dioxide present and then through a combustion chamber which 

 oxidizes the hydrocarbons. The carbon dioxide formed on oxidation is 

 measured by allowing the gas to bubble through a barium hydroxide solu- 

 tion and noting the volume necessary to produce the first observable 

 turbidity. This volume is compared with the volume required when carbon 

 dioxide in known concentration is bubbled through the same apparatus. 

 This method gives a measure of the total hydrocarbon content of the sample 

 without regard to the types of hydrocarbons present. 



In the laboratory, a more refined apparatus is used.J: After freeing the 

 gas sample of carbon dioxide, water vapor, and basic constituents such as 



t G. Laubmeyer, "A New Geophysical Prospecting Method, Especially for Deposits of Hydro- 

 carbons," Petroleum, Vol. 29, No. 18, pp. 1-4, 1933. 



t V. A. Sokoloff, "Methods of Exploration for Natural Gas," Monograph, 1932. 

 V. A. Sokoloff, "Summary of the Experimental Work of the Gas Survey," Neftyanoye 

 Khosyaystvo, Vol. 27, No. 5, pp. 28-34, 1935. 



V. A. Sokoloff, "Methods of Interpretation of the Gas Survey," Neftyanoye Khosyaystvo, 

 Vol. 17, No. 5, pp. 18-23, 1936. 



