974 



EXPLORATION GEOPHYSICS 



Assuming no sources on sinks in the vicinity {q = constant), then this 

 product must be constant for the given time t. That is, 



q ^ AT 



-— = constant = k —j- 



A 7^ 



Thus, as the gradient —r~ increases with depth, the conductivity k decreases 



with depth. But the porosity Hkewise diminishes with depth. So the con- 

 ductivity diminishes with decreasing moisture content, and the thermal 

 gradient varies inversely with the conductivity. 



Fig. 596. — Depth-temperature curve and porosity curve. (C. E. Van Orstrand, 

 "Some Possible Applications of Geothermics to Geology," Bull. A.A.P.G., Vol. 18, 

 No. 1, Jan., 1934.) 



Areal Studies. — Thermal variations may be local or regional. Local, 

 or small area, variations have been found over faults, salt domes, granite 

 ridges and sand lenses. Regional, or large area, variations are of course 

 not limited to such particularized structures and their explanations are not 

 always readily apparent. In the north-central Oklahoma region both 

 regional and local variations have been encountered, and their explanation 

 may lie in a hypothesis proposed by Nevin and Sherrillf to explain the 

 origin of certain local uplifts. In this area, there is a gradual rise in 

 temperature over a diminishing depth to granite, i.e., over the basement 

 highs. The highest temperatures were found over the oil fields where the 

 granitic rocks, having higher thermal conductivities, have pushed up 

 through the basement complex. The process of uplift proposed by Nevin 

 and Sherrill may thus explain both local and regional temperature varia- 

 tions in that area. 



t C. M. Nevin and R. E. Sherrill, "The nature of uplifts in North-Central Oklahoma and their 

 local expression," Bull. A.A.P.G., Vol. 13, Jan., 1929, pp. 23-30. 



