Short-Period Gravity Variations Induced by the Local Hydrology: Observations with Three Superconducting Gravity Sensors at Metsähovi, Finland

Abstract:

The main hydrological signal in gravity recordings by superconducting gravimeters (SGs) is typically seasonal. At the Metsähovi Geodetic Observatory of the Finnish Geodetic Institute the seasonal signal is up to 8 microgal peak-to-peak. Here we concentrate on the much smaller short-period hydrological signals in the minutes-to-hours range, due to the direct attraction of very close water masses within some hundred meters from the gravity laboratory. This period range is interesting e.g. for SG observations of the gravity signature of deep earth processes, where the signal is very small, and a possible decrease in noise level obtained from hydrological modeling would be useful.

Around the gravity laboratory at Metsähovi we have installed 12 arrays of soil moisture sensors at depths of 0.1 to 1 m below surface, three borehole wells to groundwater in the fractures of the crystalline bedrock, and 11 observation tubes into groundwater level in sediments, typically at 1 to 4 m depth below surface. Since January 2014 we record the snow water equivalent with a passive gamma ray device. However, a major component in local water mass remains inaccessible to these sensors. From earlier research we know that during summer months, even a major rainfall pulse is quickly removed from the surface layers of the soil by evapotranspiration, and frequently never reaches the depth of the soil moisture sensors; let alone the groundwater. Obviously, this also depends on the prior moisture state of the soil layers. But in any case the water mass is present for a while, and the input pulse is detectable not only by a rain gauge but also by an upward slope in the SG record.

At the laboratory we now have three superconducting gravity sensors: the SG T020 has been operating since 1994, and in February 2014 the new SG OSG-073 with two individual sensors was installed at two meters’ distance from the T020. The characteristics of the three sensors are different, and also the different locations influence slightly the gravity signal from variation in local masses. We present first results of the modeling of short-period gravity variation using all hydrological sensors, and compare them with the three SG gravity time series.