Miscellaneous Subsurface Methods 765 



bidity was observed. The volume of gas required to do this is compared 

 with that required when a known concentration of carbon dioxide is 

 bubbled through the same apparatus. This comparison is a measure of the 

 total hydrocarbon content of the sample without any indication of the 

 type of hydrocarbon present. 



The laboratory technique was a little more refined in that after 

 freeing the gas from carbon dioxide, water vapor, and ammonia, it was 

 circulated through a trap immersed in liquid air. All the constituents 

 other than methane and ethane were condensed in the trap and the two 

 lightest constituents were oxidized. The resulting carbon dioxide was 

 collected and its volume determined with a McLeod gauge. This is re- 

 ferred to as the light fraction. The heavy fraction which is a mixture of 

 complex hydrocarbons and their derivatives with a boiling point above 

 the temperature of liquid air was allowed to volatilize and its total volume 

 determined directly with a McLeod gauge. 



The German technique consisted of digging a shallow-bore hole and 

 suitably sealing it against the entry of gas other than soil gas for a suit- 

 able period to establish equilibrium, usually from twenty-four to forty- 

 eight hours. The gas was then pumped out of this hole across a heated 

 platinum filament in the presence of oxygen. The oxidation of the hydro- 

 carbons raised the temperature of the filament and thereby, increased its 

 resistance. The change in the resistance was used as a measure of the 

 hydrocarbon concentration. 



A modification of the gas analysis scheme was proposed and used 

 by S. J. Pirson. This process endeavors to measure the absolute rate of 

 emanation per unit area of the earth's surface for one or more of the 

 diagnostic gases. The process consists of placing over a specified, con- 

 fined surface of the sample a suitable adsorbant for the soil gas or gases. 

 This period is usually twenty-four hours. After equilibrium is attained, 

 the adsorber tubes are sealed and taken to the laboratory for degasing, 

 fractionation, distillation, identification, and quantitative measurement. 



The soil analysis techniques which were instigated largely by Rossaire 

 and Horvitz consist of taking samples of the soil at different depths, de- 

 pending upon the kind of analysis to be performed. The usual practice 

 requires samples from a depth of to 6 inches, from the interval between 

 one foot to just below the water table, and from well cores and cuttings. 

 The samples are analyzed for hydrogen, methane, hydrocarbon gases 

 higher than methane, pseudo hydrocarbon liquids and solids, and in- 

 organic ions such as sulphates, chlorides, and carbonates. 



The organic constituents are removed by suitable solvents, by low- 

 temperature degasing or by high-temperature degasing. The selection of 

 a solvent to be used for the extraction of the organic constituents depends 

 somewhat upon the nature and composition of the critical constituent. 

 Carbon tetrachloride or other chlorinated solvents are suitable if the soil 

 does not contain appreciable quantities of waxy material derived from 



