THERMAL METHODS 983 



Field Operations 



The geothermal crew usually consists of two men, the driller who 

 drills the holes, and the instrument man who plants the thermocouples, 

 and later takes the readings. The readings at each station should not con- 

 sume more than 10 to 15 minutes. The cost of geothermal surveying is 

 approximately twice that of magnetic surveying, on a per station basis, 

 where the holes can be drilled with a hand auger. In more difficult areas, 

 the cost increases with the cost of drilling. 



The driller can usually drill the required holes with a hand drill. Any 

 well-designed rotary spiral feed post-hole digger may be employed. An 

 easily constructed hand drill for fill and surface soil may be made from a 

 rod equipped with a suitable handle, welded to a piece of pipe. The pipe 

 is cut ofif slantwise (about a 45° angle) at the end and its edge sharpened 

 and bent slightly in the form of a spoon. When the surface deposits are 

 hard, some kind of power-driven drill may be used. 



The measuring apparatus* required for this type of work may be of 

 two general types : thermocouple or resistance thermometers. The thermo- 

 couple equipment usually comprises a sensitive galvanometer, potentiom- 

 eter, and a thermopile or thermocouple of special construction. In addition, 

 a couple of ordinary thermos flasks and a thermometer, reading to 0.01 °C., 

 are needed. 



The sensitivity of the potentiometer should be such that a difiference 

 of 0.01 °C. can be measured. The best thermocouples give an E.M.F. of 

 40 microvolts per °C. Measurement of 0.01 °C. requires a voltage sensitivity 

 of the galvanometer of 4 x 10"^ volts when one couple is used. For a 

 thermopile this factor is cut down according to the number of junctions of 

 the thermopile. The thermopile or thermocouple is constructed so that the 

 hot junction can be lowered into the hole at the end of a supporting rod, 

 while the cold junction can be kept at a constant temperature in a thermos 

 flask at the surface of the earth. 



The use of a thermopile has the advantage that the sensitivity require- 

 ment of the galvanometer is less than that for a thermocouple. The use of 

 a thermocouple has the following two advantages : ( 1 ) A single thermo- 

 couple usually can be mounted to fit a hole of smaller diameter than a 

 thermopile with many junctions, and this is important because the dis- 

 turbance of the temperature equilibrium of the ground by a hole increases 

 rapidly with the diameter of the hole due to convection air currents and 

 the exposure of the hole to direct sunlight. (2) The heat capacity of a 

 single thermocouple can be made very small compared to that of a 

 thermopile, which allows the hot junction to take the temperature of the 

 rocks with which it is in contact in the shortest possible time. 



* See list of patents at end of chapter. 



