844 



GEOPHYSICAL WELL TESTING 



[Chap. 11 



galvanometer deflection), it is in position 1. In the arrangement of Fig. 

 ll-18c, the resistance of the thermoelement is determined by measuring 

 the potential drop across it, with the switch to the left, and determining 

 the current in the circuit from the potential drop across the known resistor 

 r, with the switch to the right. During a run the current measurement 

 can be dispensed with, and an automatic self -balancing potentiometer can 

 be used to record the temperature. In the Schlumberger apparatus, 

 measurements are taken with an accuracy of 0.25° F., and the thermometers 

 are lowered at the rate of about 1000 feet per hour. 



^55^555^ 



j?r~. 



N;J>^^W»!^ 



(a) 



250 

 ohm 



Fig. 11-18. Resistance thermometer arrangements for well-temperature recording. 

 (a) Wheatstone bridge with Siemens lead-resistance compensation (adapted from 

 Johnston and Adams); (6) deflection bridge; (c) potentiometer method. 



For general structure correlation of deep wells, temperatures are meas- 

 ured and plotted beginning with a depth of about 100 feet. From this 

 curve a mean temperature gradient can be calculated by assumption of a 

 straight-line function ©d = 6o + bd, where e^ is the temperature at depth 

 d, % is the mean annual temperature, and b the mean slope of the curve, 

 or the temperature-depth gradient. The gradient can be measured di- 

 rectly by recording the difference in temperature between two thermome- 

 ters mounted a definite distance apart. In some cases, gradient records 

 have advantages over the direct temperature records. The reciyrocal 

 gradient 1/b is the number of feet or meters one has to advance in vertical 



