Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 905 



tions covered by overburden of uniform permeability, the depth of the 

 suboutcrop of the fault can be determined from the variation of the gas 

 concentration along the surface. Formulas for the effect of a linear source 

 of infinite strike extent have been derived by Antonov, Sokolov, Jmd 

 Pirson.^^ Those of the first author were derived under the assumption of 

 diffusion through the overburden, are equally valid for migration by per- 

 meation, and are identical in form with those given by Koenigsberger for 

 the effect of a source of radium emanation (eq. [12-3]). Very simple in 

 application is a modification of Sokolov's formulas for the gas concentra- 

 tion C at a station with the distance x from a point directly above the 

 center of a linear source of finite width and depth, h: 



C = l-<p, (12-7) 



h 



where c is a constant involving permeability, pressure at the source, and 

 viscosity of the gas, and <p the angle under which the edges of the source 

 appear from the station. 



Effusion of gas takes place through very small capillaries whose diameter 

 is less than the mean free path of the molecules ; the rate of flow is propor- 

 tional to the pressure differential and inversely proportional to the square 

 root of the molecular weight of the gas involved. This phenomenon would 

 account for the greater ease with which the lighter paraffin hydrocarbons 

 (hydrogen and methane) are carried to the surface. 



However, the greater portion of the available gases may be expected 

 to travel across the bedding planes of seemingly impervious rocks by 

 diffusion, that is^ by penetration of gas molecules through the intermolec- 

 ular space of solid substances. Diffusion of light gases (for example, 

 helium) through metals is well known in vacuum technique; hence, a 

 phenomenon observable over a short period of time must be expected to 

 assume correspondingly larger proportions in the course of millions of 

 years that elapsed since the oil deposits were laid down. The diffusion 

 laws are similar in form to those controlling permeation, and they indicate 

 that the velocity of flow is dependent on the diffused medium, the size of 

 the diffusing molecules, and the pressure gradient. Gases diffuse readily 

 through liquids; therefore, diffusion of gases through moist formations 

 should take place with comparative ease. Since the lighter gases diffuse 

 more readily, it is understandable why hydrogen (expected to be associated 

 with gaseous hydrocarbons) occurs in comparatively small quantities, 

 probably having made its escape much ahead of methane. 



In addition to migration of gases by permeation, effusion, and diffusion, 



•^ References in idem. 



