894 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



current is fixed for the desired ignition temperature of the wire. Then 

 the sample is introduced into one chamber and the (balUstic) galvanometer 

 deflection is observed. A mixture of gases may be analyzed by raising or 

 lowering, in steps, the temperature of the wire, that is, by changing the 

 bridge current. This has been demonstrated successfully for mixtures of 

 methane, carbon monoxide, and hydrogen; but it would be difficult to 

 accomplish it for a mixture of methane and ethane. Nevertheless, the 

 same apparatus is usable for a separate quantitative analysis of air- 

 methane and air-ethane mixtures if the ethane and methane have been 

 separated from each other by some other means (such as low temperature 

 fractionation, see page 900). 



Fig. 12-1 la. Gas indications (methane-ethane) measured with Lavibmeyer apparatus 

 on Pierce Junction Salt Dome. (N. Gella). 



In its application as described, the bridge apparatus proposed by Graf 

 measures the content of combustible gases in the same manner as the Laub- 

 meyer instrument, without differentiation between methane and ethane. 

 Fig. 12-lla shows the results obtained with a combustible gas detector on 

 the Pierce Junction salt dome near Houston, Texas, in units of galvanom- 

 eter scale deflections. The anomalies are most pronounced on the flanks, 

 are unsymmetrical, and are almost zero above the top of the dome. 



Another type of gas detector** measures the amount of carbon dioxide 

 liberated upon the combustion of the gaseous hydrocarbons pumped from 

 a well into the apparatus. Again, no differentiation is made between 

 methane, ethane, and heavier hydrocarbons. The hydrocarbons are 



"V. A. Sokolov, Neftianoc Hozraistvo, 27(6), 28-34 (May, 1935). 



