The use of near-surface geothermal energy implying geothermal infrastructure increases significantly in Germany [1]. Therefore it becomes an essential subject for impact analysis of our groundwater resource. Rock physical properties change through the alteration of the pore fluid properties such as temperature. Those alterations happen under the influence of heat in the subsurface. Variations in geophysical proxies can provide information about the subsurface changes. Laboratory measurements by Jaya et al. [2] show that P-wave velocities decrease with increasing temperature.

Within the BMBF funded follow-on project – TestUM II a “cyclic high temperature aquifer thermal energy storage (ATES) experiment” has been conducted in the north or Germany to verify these findings. With this field experiment at a shallow aquifer environment we are able to avoid difficulties in frequency dependent upscaling procedures [3]. We investigated the coherence between geophysical proxies and the temperature distribution in the near surface with a combined hydrogeological, microbiological and geophysical monitoring system covering an area of approximately 100 m². A cyclic heat injection at a depth between 7 – 14 m was monitored over 15 months with seismic cross-hole measurements in 16 different wells to cover heat propagation and direction-dependent heterogeneities across the field. The purpose was to investigate geophysical proxy responses on the alteration of the pore fluid that is likely to change rock physical properties in the near-surface. To predict the effect of temperature on P-wave velocity we took advantage of the Jaya’s [2] modification of the Gassmann equation which accounts for the related thermophysical characteristics of the pore fluid. Unlike the findings of Jaya et al. [2], that the P-wave velocity decreases, we see the opposite, an increase in P-wave velocity in our non-closed system assuming different thermophysical characteristics of the saturating fluid. We excluded a bubble-formation since we only cover temperatures below 80°C. However, a decrease in the amplitudes due to the P-wave attenuation can be observed. According to Jaya et al. [2] this can be attributed to the decrease in water viscosity.