Gravity and 2D time lapse resistivity monitoring at Conrad Observatory for understanding local hydrology – case study of an exceptional rain event in September 2024

Since April 2024 the Conrad observatory, located 60 km SW of Vienna within a karstic area of the Eastern Alps (Austria), operates a superconducting iGrav gravimeter (iGrav050) continuing the previous more than 10 years long gravity time series of GWR C025. Their gravity residuals indicated an extremely complex local hydrology in the surroundings of the observatory. For better understanding the local hydrological processes a geoelectric profile has been installed which monitors a 2D resistivity section each day. The profile runs across the topography just above the gravimeter. The latter is an underground installation. The situation is challenging because the profile runs closely above cavities built up by the observatory’s underground labs and tunnel. In addition, the geoelectric settings are limited by the requirements of the nearby geomagnetic observatory.

In September 2024 an extraordinary rain event happened in Lower Austria with cumulative rain as large as 400 mm within a few days. First results are shown comparing the gravity signal with the time dependent resistivity sections providing insight to the water content within the terrain top layer. In addition, the gravity residual signals are compared to those originating from a snowmelt event in 2009 with similarly large water intrusion during short time as in September 2024.