Hydrological Signals in Tilt and Gravity Residuals at Conrad Observatory (Austria)

Abstract. The Superconducting Gravimeter GWR C025 monitors the vertical component of the gravity vector at the Conrad Observatory (Austria) since autumn 2007. Two tilt meters operate continuously since spring 2016: a 5.5 m long interferometric water level tilt meter and a Lippmann-type 2D pendulum tilt sensor. The co-located and co-oriented set up enables a wide range of investigations because tilts are sensitive both to geometrical solid Earth deformations and to gravity potential changes. The tide free residuals of the SG and both tilt meters clearly reflect the gravity/deformation effects associated with short- and long-term environmental processes and reveal a complex water transport process at the observatory site. Water accumulation on the terrain surface causes short-term (a few hours) effects which are clearly imaged by the SG gravity and N-S tilt residuals. Long-term (> a few days/weeks) tilt and gravity variations occur frequently after long-lasting rain, heavy rain or rapid snowmelt. Gravity and tilt residuals are associated to the same hydrological process but have different physical causes. SG gravity residuals reveal the gravitational effect of water mass transport, while modelling results exclude a purely gravitational source of the observed tilts. Tilt residuals show the response on surface loading instead. N-S tilt signals are much stronger than those of the E-W component most probably due to the well-known cavity effect of the 150 m long tunnel oriented in E-W direction.