Measurement of Groundwater Flow Using an In-Situ Thermal Probe
A thermal probe for the in-situ measurement of groundwater flow in a borehole was constructed and calibrated in a horizontal position. The probe is a long slender metal rod having a heat source along its entire length and a temperature sensor at its midpoint. When a constant quantity of heat is applied to the probe, the rise in temperature is inversely related to the rate of water flow passed the probe.
Full-scale calibration of the probe was considered necessary because theoretical studies oversimplify the interaction between the heated probe and the horizontal flow of groundwater. The apparatus for calibration consists of a central sand-filled chamber having a horizontal hole lined with well-screen in its center. The central chamber is hydrologically connected to two taller water-filled chambers that are used to control the rate of water flow.
Over 40 calibrations of the thermal probe were made, but most of these tests were used to perfect experimental techniques. However, 8 of the calibration runs were considered sufficiently accurate to construct preliminary Master Curves. These curves show that if temperature differences of 0.1° Centrigade can be measured at the end of a two hour test, the probe is capable of distinguishing changes in rate of water flow of 5 * 10-4 cm3/sec/cm2.