If a hole penetrates a hydraulically conductive fracture, then water can enter the hole at a slightly higher or lower temperature depending upon whether the water has been flowing upward or downward, respectively, in the fracture. Precision temperature probes can detect differences in temperature of just a few thousandths of a degree. If the temperature log is differentiated with respect to depth to obtain the geothermal gradient , then the depths at which water is entering or leaving the hole from a fracture zone are more easily identified. The anomalous temperatures result in abrupt changes in a plot of the temperature gradient versus depth. Under these conditions, heat is transported by convection rather than by conduction .
Convection is a much more efficient way to transfer heat.
The use of precision temperature measurements to detect water flowing into or out of fractures is the only geologic or geophysical method of identifying those fracture zones that are hydraulically conductive at depth. Daniels found that well yields increase with depth to a much greater depth than previously thought, and that well yields increase dramatically as well diameter increases. Intercrystalline porosity is absent in these rocks. Porosity is entirely a result of brittle fracturing. Groundwater flow in these rocks is localized along fractures.