Hydrogeological effects on superconducting gravimeter measurements at Metsahovi in Finland

The purpose of this study is to gain a better knowledge of superconducting gravimeter (SG) data at the Metsahovi Gravity station in Southern Finland. Because SG data have a high precision (1 ngal) and are very sensitive to various environmental effects, we must isolate and analyze the characteristics of these effects before we are able to consider their effects in data. As a result of extensive studies we have isolated four hydrogeological sources influencing the gravity field at the Metsahovi site. The first one, the time-varying amount of surface and soil water close to the station causes an immediate effect on gravity after rains. The second source, the water flow in the fractures of crystalline bedrock above the groundwater table below the station can be observed soon after rains. The third one, the porosity variation of the bedrock within the range of groundwater level variation is a long-term effect, which influence on the gravity field can vary over a wide range. The last source, the flow rate difference between soil and bedrock causes a detectable long-term gravity anomaly.