978 EXPLORATION GEOPHYSICS 



over the crest of the anticline. Similar results have been obtained by 

 several investigators. f 



Periodic Heat Flow. — The radiation of the sun causes a periodic 

 change in the flow of heat through points close to the surface of the earth. 

 The periodicity of the change in temperature is a double one and is associ- 

 ated with the change in position of the sun during the day in relation to 

 the horizon and with the change in the path of the sun during the year. 

 These two periodic changes in temperature are called the diurnal and the 

 annual variation, respectively. 



The heat of the sun reaches the surface of the earth mostly by radiation. 

 The conductivity of the atmosphere is very poor.* The spectrum of 

 the sun has a peak in the yellow portion, and the absorption of this 

 radiation in the atmosphere is smaller than that of longer wave lengths. 

 During the night, the earth, which has a much lower temperature and 

 therefore a spectrum with a peak far in the infra red, loses a great 

 part of the heat received during the day; but the atmosphere absorbs 

 a great part of this radiation and therefore acts as a protective screen 

 against loss of radiation, and this factor, combined with the poor conductiv- 

 ity of the atmosphere, allows the earth in the summer to retain during the 

 night a part of the heat received from the sun. Hence, in the summer, 

 the outer portion of the earth's crust gains heat over a 24-hour period. 

 In the winter, however, the losses during the night exceed the gains during 

 the day, and the balance is negative over a 24-hour period. 



During the day the temperature is a maximum between 2 and 3 p.m. and 

 a minimum just before sunrise. The plotting of temperature against time 

 during a 24-hour period affords a means for evaluating the gains and 

 losses over the whole or part of the period. 



The diurnal temperature variations manifest themselves only to a depth 

 of a few feet below the surface. The depth to which the changes are meas- 

 urable depends on the character of the rocks close to the surface. In solid 

 rock formations the diurnal variations usually become imperceptible at a 

 depth of about 3 feet. In loose sand and alluvial fill material, the variations 

 escape measurement at a depth of 1 to 2 feet. In swamps and porous mate- 

 rials containing water and in areas where the water table is near the surface 

 of the earth, the variations become imperceptible at a depth of 3 to 5 feet. 



The annual variation for a given area is determined from data on 



t C. E. Van Orstrand, "Normal Geothermal Gradient in the United States," Bull. Amer. Assoc. 

 Pet. GeoL. Vol. 19, No. 1, Jan. 1935. 



M. W. Strong, "Significance of Underground Temperatures." Proc. World Petr. Congress, 

 Vol. 1. 1933, 124 pages, abstract. Jour. Inst. Petr. Techn.. Vol. 20, Feb. 1934, p. 63. 



E. de Golyer, "The Significance of Certain Mexican Oil Field Temperatures," Economic 

 Geology, Vol. 13, 1918, pp. 275-301. 



James Fisher, L. R. Ingersoll, and H. Vivian, "Recent Geothermal Measurements in the 

 Michigan Copper District." A.I.M.E Geophysical Prospecting. Tech. Pub. 481. 



C. E. Van Orstrand, "Some possible applications of geothermics to geology," A.A.P.G. Bull., 

 Vol. 18, No. 1, Jan., 1934. 



* The fact that the temperature of the air at higher altitude is generally lower 

 than that at lower altitude shows that conduction of heat plays a minor part in the 

 transport of the sun's heat to the surface of the earth. 



