982 EXPLORATION GEOPHYSICS 



. . . / . k\ 



From measurements of the lag, velocity or wave length, the diffusivity ( which is -— I 



may be readily determined. Forbesf used this method on soils near Edinburgh. 



Internal Heat Sources. — A final step in the study of earth temperatures necessi- 

 tates invoking factors for internal sources and sinks of heat. Two possible heat sources 

 will be considered briefly — radioactive and chemical. The sinks, in general, are moving 

 subterranean waters. 



Normally, the temperature of the earth at any point located below a depth subject 

 to the periodic variations will remain constant for all practical purposes. Due to the 

 great mass and heat capacity of the earth, temperature changes due to heat losses 

 alone would amount to only 1° C. over a period of from 10 to 50 million years. When 

 consideration is given to both radioactive heat sources and general heat losses, there 

 may even be an increase in temperature of perhaps 30° C. per million years, if the 

 radioactivity values at depth are of the order of the minimum values at the surface.^ 

 Though the percentage of radioactive material in surface rocks is minute, yet if such 

 material is scattered throughout the earth with the same density, then the total aggre- 

 gate would be sufficient to supply many times over the yearly loss of heat. In fact, the 

 heat generated would be so great as to necessitate the assumption that the radioactive 

 materials exist to a depth of only a few miles of the surface shell. 



The other major source of possible internal heat is chemical reaction. In mining 

 work, this source of heat may be due to oxidation or alteration of ore bodies, while in 

 petroleum exploration it may be due to oxidation of the hydrocarbons escaping from 

 the reservoir. 



To investigate the possible effects of heat from the oxidation of petroleum, Strong§ 

 made calculations based upon data obtained from geothermal field measurements. 



Experimental work has yielded a temperature of 1.65° C. at Haverhill, Kansas, 

 over a sand lens located at a depth of 762 meters. After making certain simplifying 

 assumptions, it was shown that this temperature difference can be maintained by a 

 source generating 1 • 10 "' calories/cm.Vsec. This quantity of heat was then compared 

 to that obtainable from the oxidation of petroleum in its natural state. 



Assuming that the sand lens, about 100 feet thick, contained a given quantity of 

 oil and that the heat of combustion of crude oil is about 11,094 calories/gm, it was 

 shown that to continuously generate the heat needed to maintain the 1.65° C. difference, 

 all the oil would be oxidized in 3.2 • 10" years. This result is definitely not in agreement 

 with geological facts, and so the conclusion to be drawn is that oxidation of oil, though 

 possibly a contributing factor, is not in itself the only cause of the higher temperature 

 values over oil fields. The explanation for these higher temperatures probably lies in 

 the fact that the basement complex generally is at a lesser depth over structures favor- 

 able to the accumulation of oil, and the greater transmission of heat under these con- 

 ditions supplements any heating which may be caused by oxidation of the hydrocarbons. 



Near- Surface Temperature Variations. — A homogeneous, spherical earth surface 

 would be an isothermal surface. Irregularities in the surface caused by topography, 

 cause corresponding irregularities or anomalies in the temperature distribution because 

 the isothermal surfaces are compressed underneath the valleys, and expanded over the 

 hills and mountains. In this respect, the isothermal surfaces behave almost identically 

 with the equipotential surfaces in electric potential fields. Calculations can be made 

 showing probable temperature gradients for different topographic configurations.!! 



t Forbes, Trans. Roy. Soc. Edinburgh, 16, Pt. II (1846). 



% Ingersoll and Zobel, loc. cit. 



§ M. W. Strong, "Geothermal Phenomena and Geological History with Special Reference 

 to Old Structures in Geothermal Equilibrium," Jour. Inst. Petrol. Tech. (London), ^'^ol. 16 (1930), 

 pp. 889-901. 



tt Van Orstrand, Physics. Vol. 2, No. 3, 1932. 



