Groundwater movement in the shallow subsurface is an important variable in understanding the mass balance of water cycles in and around surface drainage areas. Moving water carries heat, which offers the possibility of detecting groundwater motion through time series measurements of ground temperature. Heat Needles are instruments for recording temperature in the top 1.1 m of the ground. Each Heat Needle contains highly sensitive (sub-millikelvin) and accurate (±3 mK) sensors at the ground surface and at 20 cm spacings from 0.1 m to 1.1 m depth. They passively record ground temperature at 15 minute intervals for up to 12 months on a single deployment.
Hot Dry Rocks and the Mexican Center for Geothermal Energy Innovation collaborated on a six-month trial deployment of six Heat Needles over the Simirao geothermal area in Central Mexico. While the ground temperature at five of the sites mimicked the changes in surface temperature over time, the temperature of the ground at the sixth site varied in a way that could not be explained by changes in surface temperature. The inferred cause of the observed temperature changes was groundwater movement in a shallow aquifer, which in turn was probably connected to a deeper and hotter reservoir.
Heat Needles are capable of detecting temperature changes far more subtle than those detected at Simirao. They represent an emerging tool for passive monitoring of groundwater movement.