868 GEOPHYSICAL WELL TESTING [Chap. 11 



F. Gas Detection 



In shallow holes, gas detection methods have been used to locate gas 

 leaks in buried mains and pipes. A bar of small diameter is driven through 

 the topsoil or pavement at closely spaced points, the sampling pipe shown 

 in Fig. 11-36 is inserted, and the soil air is passed through a combustible 

 gas detector, either by an aspirator or by open flow." The detector con- 

 sists of a sampling system and a hot wire detecting circuit. The sampling 

 system is essentially a gas chamber with two platinum filaments; one of 

 these is exposed to the gas sample, and the other is sealed in air. The 

 filaments form two branches of a Wheatstone bridge that becomes un- 

 balanced when the temperature (and, therefore, the resistance of the gas 



Heiland Kesearch Corp. 



Fig. 11-36. Combustible gas detector with sampling tube and aspirator. 



filament) increases as the result of gas combustion. A triple-screen 

 explosion check surrounds the filaments inside the gas chamber. By 

 systematic reconnaissance and detail surveys it has been found possible 

 to locate leaks correctly in 90 per cent of the surveys made. 



Gas surveys for the location of subsurface oil and gas accumulations are 

 likewise made in shallow holes. These methods require detectors of much 

 greater sensitivity, as described on pages 892-898. The double filament 

 detector previously mentioned may be used in a deep well provided the 

 hole is uncased and dry. During drilling, the gas content of drilling fluid 



" P. C. Dixon, Gas-Age Record, 76(24), 517 (Dec. 14, 1935). 



