BORE-HOLE INVESTIGATIONS 1063 



The Vertical Investigation Characteristic. — In a similar manner, for the study 

 of beds of finite thickness, it is convenient to think of space as being divided into a 

 great number of horizontal layers of equal thickness. For this purpose, the conductivity 

 of all the medium in any layer can be assumed to be constant, the relative effect of the 

 mud and the invaded zone thus being neglected. 



Successive layers have their own geometrical factors Gz, the values of which may 

 be plotted as a function of the distance of the layer above or below the center O of the 

 coil system, as shown for a simple two-coil sonde in Figure 657. This curve is known 

 as the vertical investigation characteristic. 



If, as shown in Figure 657, there is a bed of finite thickness A between two other 

 beds D and F, the geometrical factor of each of these beds, for the indicated position 

 of the coil system, will be proportional to the respective areas, a, d, and / under the 

 vertical investigation characteristic. The contribution of each area to the signal will be 

 proportional to the product of its conductivity by its geometrical factor. 



It has been found that by proper combinations of additional coils, it is possible to 

 minimize the influence of the mud column and the invaded zone, or to minimize the 

 influence of formations adjacent the bed under study. 



The sonde assembly is about AY^ inches in diameter, 14 feet long, and weighs 250 

 pounds. Rubber centralizers, up to 8 inches in diameter, are employed for centering 

 the sonde in the bore hole. 



Temperature Measurements in Bore Holes 



As was shown in connection with Figure 595, the temperature in the 

 earth increases with depth. The rate of increase of temperature with depth 

 is known as the geothermal gradient, and varies with the locaHty and the 

 heat conductivity of the geological formations. Because of the dependency 

 of the geothermal gradient on the conductivity of the formations, measure- 

 ments of the relative temperature gradients, at various depths of wells in 

 which thermal equilibrium has been established, have been used for the 

 logging of formations. The temperature gradients are in general low in 

 formations having high heat conductivities and high if the heat conductivity 

 is low.f 



Temperature measurements offer an independent technique which fre- 

 quently provides information not available from any other source. Such 

 measurements may be used for determining the height of cement behind 

 casing, logging the position of certain formations, and locating gas or water 

 sands. In contrast to the electrical surveys, the presence of casing has 

 little effect on the temperature measurements. Hence, temperature data 

 may yield valuable information useful in the rehabilitation of declining 

 pumping wells. 



Considerable work on thermal measurements has been done by Van 

 Orstrand,J Fisher, § and others who used maximum registering thermome- 

 ters of the mercury type. Considerable work has also been done with elec- 

 trical thermometers. ff 



t M. Schlumberger, H. G. Doll, and A. A. Perebinossoff, "Temperature Measurements in Oil 

 Wells," paper presented at meeting of Institute of Petroleum Technologists, Nov. 10, 1936. 



t C. E. Van Orstrand, "On the Estimation of Temperatures at Moderate Depths," Trans. 

 Amer. Geophys. Union, 1937, pp. 21-23. 



§ J. Fisher, L. R. Ingersoll, and H. Vivian, "Recent Geothermal Measurements in Michigan 

 Copper District," A.I.M.E. Geophysical Prospecting, 1934, pp. 528-536. 



tt E. G. Leonardon, "Thermometric Measurements in Drill Holes," Geophysics, Vol. I, 1936, 

 pp. 115-126. 



