860 GEOPHYSICAL WELL TESTING [Chap. 11 



approached asymtotically within about one month (Fig. 11-30), is between 

 40° and 65° F. The corresponding temperature variation through well 

 casing is of the order of 10°-20° F. and therefore quite readily detected. 



8. Other chemical transformations affect well and underground tempera- 

 tures when the reactions are of an exothermic nature. Well known in 

 mining are such sources of heat as: active fires of broken sulfurous ores, 

 or of coal, carbonaceous shales, and timber; the decay and oxidation of 

 timber, aided by bacteriological action; and oxidation of sulfurous ores, 

 coal, and shales (not sufficient to raise the temperature to the point of 

 ignition). An example of this kind is the pyrite deposit of Sain Bel in 

 France. Evidence of its oxidation is a zone of strong self-potentials above 

 it (see page 676). In workings 100-150 meters deep, the temperature is as 

 high as in other mines at depths of around 1000 feet. Although some 

 increase of temperature occurs in the transformation of anhydrite to gyp- 

 sum, the main reason for the high temperatures of salt domes is probably 

 the high thermal conductivity of the salt. 



E. Effect of Surface Relief and Surface Temperature 



Since the earth's surface may be considered, in very close approxima- 

 tion, an isothermal surface, and since all irregularities due to near-surface 

 distributions of conductivity and topography must be expected to be 

 equalized at a certain depth (geothermal "isostasy"), it follows that the 

 isothermal surfaces must be compressed under the depressions and ex- 

 panded under topographic highs. '^ The deflections of the isothermal sur- 

 faces due to topographic irregularities may be calculated for a medium of 

 constant diffusivity if the topographic profile and the gradients under the 

 apex of a hill and the adjacent plane are known. This calculation is 

 readily made for ridges of two-dimensional configuration.^" Fig. ll-31a 

 shows such theoretical isothermal surfaces for the topography at the Long 

 Beach field, and Fig. 11-316 represents actual rock temperatures in the 

 Moffat Tunnel near Denver. 



Temperature variations at the surface of the earth penetrate into the 

 ground to a depth which can be calculated from their periodicity. The 

 diurnal variation is relatively unimportant for well measurements; its 

 penetration is of the order of 3 to 4 feet. The annual variation, however, 

 penetrates to a depth of about 80 feet in high and 50 to 65 feet in inter- 

 mediate latitudes. For deep well measurements this variation is not im- 

 portant if observations are started below 100 feet. The variation must be 



^^ This phenomenon is analogous (and mathematically almost identical) to the 

 effect of topography on the equi potential surfaces in an electrical field (see p. 702). 

 ""Van Orstrand, Physics, 2(3), 144 (1932). 



