Chap. 11] GEOPHYSICAL WELL TESTING 841 



eliminated, since it affects adjoining holes alike. Thermocouples may be 

 used with the hot junction in the hole and with the cold junction in a liquid 

 kept at constant temperature. Van den Bouwhuijsen" used a Moll galva- 

 nometer of 2.7 X lO"'^ volt sensitivity with a thermocouple supplying 

 4-10~* volt e.m.f. for a temperature difference of 1° C. 



In underground workings, ordinary mercury thermometers are suitable 

 if they are properly protected. A thermometer constructed by L. R. 

 Ingersoll^^ maintained its temperature for about one minute by being 

 inclosed in a bakelite tube; the bulb was insulated with vulcanite and 

 paper. In seven-foot holes sunk from rock faces but a few days old, 

 measurements were made with sets of two or three thermometers in tan- 

 dem. These were read about every two hours for one day and the readings 

 were repeated after two or three days. This made it possible to determine 

 the cooling of the walls in drifts, tunnels, or shafts, and to calculate the 

 virgin rock temperatures. 



The mean temperature gradients in deep holes are generally determined 

 by maximum thermometers. Their use is predicated on the fact that the 

 temperature in deep holes is greater than the mean annual temperature. 

 (Maximum thermometers may also be used at shallow depths provided 

 they are chilled with ice before and after the run.) An overflow type of 

 maximum thermometer is illustrated by R. Ambronn. In it, the maxi- 

 mum temperature has to be determined by a separate testing operation 

 and by measuring the temperature at which the mercury overflows. In 

 another type of maximum thermometer, the upper 2-3 mm of mercury 

 are separated from the main stem by an air bubble. The mOst extensive 

 application has been made of the constriction type of maximum ther- 

 mometer which requires resetting in some sort of centrifugal device after 

 each use. The thermometer used by the U.S. Geological Survey has a 

 length of about 20 cm, a stem diameter of 6-7 mm, and an accuracy of 

 0.1°-0.2° F.; and it is divided in Fahrenheit degree intervals between 32" 

 and 212° over a length of 17-18 cm. Usually two or three thermometers 

 are employed for each run to increase the accuracy and to reduce errors 

 due to jarring of the assembly in hoisting and lowering. 



Fig. 11-16 shows a number of thermometer carriers^* with their con- 

 tainers adapted to various tj'^pes of wells. Fig. 11-16A' is a holder for 

 three thermometers which fits the container B' for use at the end of a rope 

 in shallow wells ; C" is a container suitable for clamping to a piano-wire line ; 

 D' is a steel container for use, at the end of a wire line, in oil or water wells. 



" Eng. and Min. J., 135(8), 342-344 (Aug., 1934). 



12 Physics, 2(3), 154-159 (March, 1932). 



1' Elements of Geophysics, p. 275. 



" C. E. Van Orstrand, Econ. GeoL, 19(3), 229-248 (1924). 



