972 EXPLORATION GEOPHYSICS 



which gives for the weighted mean, 



,^t 



Equations 10 and 11 are the ones in general use. 



As an example, consider the following field data and calculations. 



(11) 



Applying Equation 10 to determine the arithmetic mean of the gradients (where 

 n = 10) 



0.1857 



h- 



10 



= 0.01857 F.Vft. 



Applying Equation 11 to determine the weighted mean of the gradients, 



309 

 ^=1664^ = 0-01858 F.Vft. 



Agassiz and Ingersoll have made temperature surveys in the deep 

 copper mines of the Keweenaw peninsula in northern Michigan. f The 

 results of their work gave an average gradient in that particular area of 

 1°C, in 59,5 meters or 1°F. in 108.5 feet. On the basis of measurements 

 of the diffusivity of rock specimens in the region (found to be 0.0075 

 c.g.s.), theoretical temperature-depth curves were made which indicated 

 that at least 30,000 years have elapsed since the last glacial epoch. 



Figure 595 provides a comparison of temperature gradients in oil 

 wells ; for example, the data for the California wells yield a temperature 

 gradient of l°F,/42 feet, approximately, and the data for the Louisiana 

 oil well yields a gradient of approximately 1°F./100 feet. 



t L. R. Ingersoll, "Geothermal Gradient Determinations in the Lake Superior Copper Mines," 

 Trans. Soc. Pet. Geoph., Vol. II, March, 1933. 



