Monitoring of an aquifer thermal storage system in the field scale using crosshole ERT

Heat storage in aquifer structures takes on greater significance and is therefore an important subject for risk assessment and impact analysis on groundwater resources. Geophysical methods contribute substantially to the observation of hydrogeological processes by providing information
about physical subsurface properties. In order to allow for correct process interpretation, it is essential to find and evaluate their relationship
to the corresponding rock-physical parameters. Therefore a heat injection experiment and a corresponding monitoring system have been developed
and established in a shallow aquifer environment characterized by quaternary glaciofluvial sediments. The focus is on the investigation of coherence between geophysical proxies and the temperature distribution in the near-surface. A geological subsurface model derived from geophysical and hydrological pre-investigations has been used to simulate heat distribution and resulting electrical conductivity variations in the affected area.
Tests for thermal energy storage and extraction have been conducted via Aquifer thermal energy storage (ATES) system. With time-lapse inversion
we want to detect the direct impact of changing temperature distribution in the subsurface on the related electrical resistivity when heating the
aquifer up to 80 °C. Rein et al. (2004) state that electrical conductivity of the subsurface depends to a great extent on water saturation. Heating
up the governed pore water by 1 °C results in a linear relative electrical conductivity increase of 2.5% (Dachnov, 1962). Different inhole and cross-
hole arrays at the test site assure good coverage of the heated area and pass through the monitoring routine once a day. The ongoing injection
cycles consist of a heating period of 2 weeks, a down time of 3 weeks, an extraction period of 2 weeks and another down-time of 1 week followed by
the next cycle. We prove the applicability of heat injection and extraction monitoring by combined crosshole ERT (and seismic) and correlated
the resistivity with the directly measured temperature data of the temperature sensors additionally installed in the boreholes. At the highest
observed temperature level of 75 °C the electrical conductivity increases by a factor of three. 3D inversion allows for a direct reference to the temperature distribution in the subsurface. This study provides information about the resolution capacity of crosshole ERT for heat storage systems
in shallow aquifers.
These activities have been done within the follow-on TestUM-Aquifer Project - TestUM-II ”Cyclic high temperature - Aquifer thermal energy storage (ATES) experiment” funded by the BMBF (grant 03G0898A/B).