MONITORING GROUND WATER FLOW WITH ULTRA HIGH PRECISION GRAVIMETER MEASUREMENTS

The Helsinki University of Technology (TKK), the University of Helsinki and the Finnish Geodetic Institute (FGI) are jointly operated at Metsähovi Research Station, located at Kirkkonummi, Southern Finland. The main research instrument of the gravity laboratory of the FGI is superconducting gravimeter T020 used in. We studied ground water flow in order to improve the accuracy of ultra high precision gravimetric measurements (20 ngal/m).

Both short and long term noise in our gravity residuals were observed particularly after heavy rainfalls. We studied correlations between precipitation, ground water flow, and gravity residuals to figure out the sources of noise. According to aeromagnetic and aeroradiometric data and local topography there are no significant fracture zones in the study area. Moreover, the surface of bedrock is at higher level everywhere else except in the south in the study area. Thus, ground water flows (through study area) to the south. This elevates the level of ground water with delay, which is seen as an increase in the gravity field.

To find out the time shift we monitored precipitation, ground water level and gravity residuals from May 26th 1999 to July 3rd 2003 with one hour sample interval, totally 35977 samples. We removed the trend line from cumulative precipitation curve and compare it with the variations in ground water level and gravity residual. Results show that all parameters have good correlation with each other. However there is a time shift (80 days) between precipitation and other parameters, which means that most of the ground water flows from the surroundings, only a minor part infiltrate into groundwater on site.

Ultra high precision gravimetric measurements are an useful additional method in monitoring ground water flow. This is necessary improving the accuracy of ultra high precision gravimetric measurements, but in many other applications as well.