Since 1961, precise ground-temperature measurements have been made at shallow depth in a number of geological terranes and climates. The objective has been to investigate the effectiveness of shallow geothermal techniques in geological exploration. The thermal sensors used are thermistors individually calibrated to within .02°C.
It was found that shallow-earth temperatures can be related to the occurrence of ground water. Shallow, moving ground water produces low temperatures and low annual drift rates, due to damping of the annual temperature wave. Conversely, dry shallow bedrock, because of higher conductivity and lower specific heat, produces relatively higher temperatures and higher drift rates. Other permutations of observed temperature and drift rate reflect intermediate conditions of bedrock and ground-water distribution.
As a case study, the geothermal survey of Johnson Valley is described. This is a closed basin system in southern California, which shows a direct relationship between ground temperature and depth to water as measured in wells.