984 



EXPLORATION GEOPHYSICS 



The resistance thermometer equipmentf usually comprises some form 

 of sensitive resistance bridge, connected to a resistance thermometer. The 

 resistance thermometer should preferably be of the lead compensated type, 

 although the effects of temperature changes in the lead circuits may be 

 minimized by a high ratio of thermometer/lead resistance. One type of ther- 

 mometer bridge used for resistance measurements is a double slide-wire 

 Wheatstone bridge to measure resistance from to 200 ohms. Measure- 

 ments accurate to 0.005 ohms are necessary, and with a good resistance 

 element will read ± 0.010 to .015° C. 



Figure 601 shows the arrangement of 

 the apparatus ready for measurements. 

 In calibrating the galvanometer in 

 terms of differences in temperature, 

 the two junctions are first placed in 

 a Dewar flask filled with water or ice 

 to determine the zero of the poten- 

 tiometer and then one junction is trans- 

 ferred to a flask containing water at 

 another temperature. This is repeated 

 for a series of temperatures and a plot 

 is made of the potentials against the 

 temperature differences. 



Field Technique. — A preliminary 

 series of measurements is made at dif- 

 ferent depths in the same hole to deter- 

 mine the depth at which the diurnal 

 variations become imperceptible for the 

 particular region under investigation. 

 Anomalies in this critical depth should 

 be discovered before a great number of 

 observations are made at a depth insuf- 

 ficient to eliminate the diurnal variations. Furthermore, a determination of 

 the minimum depth to which the holes need be drilled will tend to speed up 

 the survey because the speed of drilling determines the speed of the observa- 

 tions. When the survey is made in a region where the water table is close to 

 the surface of the earth, all holes must be drilled uniformly to the water 

 table and measurements made at that depth. 



In order that thermal equilibrium may be reached, measurements 

 should not be made for a period of at least 24 hours after drilling. 



A uniform reading and thermometer-setting procedure should be used 

 to minimize the influence of diurnal temperature variations and other 

 extraneous effects. The temperature measurements are made at the same 



t J. J. Jakosky, R. M. Dreyer and C. H. Wilson, "Geophysical Investigations in the Tri-State 

 Zinc and Lead Mining District," Univ. of Kansas, Bull. 44, Dec, 1942. 



Fig. 601. — Schematic diagram of thermo- 

 pile and associated apparatus in place for 

 geothermal measurements. 1, sheath of low 

 thermal conductivity; 2j hot junction; 3, 

 cold junction; 4, potentiometer; 5, flexible 

 rubber tube sheath ; 6, cover plate of low 

 thermal conductivity; G, galvanometer. 



