Chap. 11] 



GEOPHYSICAL WELL TESTING 



855 



as 3.3 feet per degree F. and as large as 20-21 feet per degree F. In such 

 cases, the depth-temperature curves may consist of both extremely flat 

 and abnormally steep portions. High rock temperatures should naturally 

 be expected in areas of volcanic activity even without the presence of 

 circulating waters. There is frequent evidence of this condition in mines 

 located in volcanic areas. Underground workings in ores deposited by 

 thermal waters w;ill show high temperatures if deposition is still going on. 



.to-' 



40 



zo 



6ra\fifif Oradienf 



IE2] E22 



Terf. Cret 



Pirmion Triasic Coa/ 



Fig. 11-25. Torsion-balance and geothermal indication of fault near Winterswijk, 

 Holland (after Van den Bowhuijsen). 



3. Glaciation. As shown below (section e), any change of temperature 

 at the earth's surface is propagated downward in the form of a wave. 

 Short-periodic changes, such as the diurnal temperature variation, are 

 damped out within a few feet. Long-periodic variations, such as those 

 occurring during glacial and interglacial periods, may be expected to have 

 affected the thermal gradient down to several thousand feet of depth. 

 With sufficient data on geologic section and heat conductivities, the effect 

 of temperature changes of given periodicity on the depth-temperature 



33 A. C. Lane, Geol. Soc. Amer. Bull. No. 34, 711 (1923). 



