Comparison of long-period tilt measurements: results from the two clinometric stations Metsahovi and Lohja, Finland

The records of an ASKANIA borehole tiltmeter and two water tube tiltmeters are compared. The borehole tiltmeter was operating at the Geodetic Observat ory Metsahovi in a 63 m deep borehole and the long water tube tiltmeters ar e installed in a limestone mine at Lohja, both observatories are some 15 km apart west of Helsinki, Finland. Regarding the tides the comparison yields a coherence within 0.5%...3% for the diminishing factors, which proves their regional validity within the er ror bars. Air pressure induced tilts in both stations reach about 10 msec of similar signals, pointing into the same direction of about 150 degrees N. They are caused by the horizontal air pressure gradients as well as by the local air pressure variations in interaction with the coast line of the nearby sea ( Gulf of Finland). In the borehole, hydraulic induced tilt signals are reaching the amplitudes of the tides (about 20 msec), even 20 m below an aquifer in solid granite. Reproduceable short-period (minutes to hours) pore pressure induced tilts are caused by pumping ground water. These effects can be explained by poroe lastic modeling and thus, they can be partly corrected. They are correspond ing to the main directions of permeability in a system of clefts. The seaso nal drift directions in the borehole point to pore pressure variations in t he aquifer interacting with a nearby swamp, correlated with the freeze. As the physics of the hydrological and air pressure induced tilts can be ex plained, instrumental effects are excluded at both stations. Thus, continuo us tilt measurements are suitable not only for the investigation of small s cale but also for regional scale crustal dynamics. But in order to avoid lo cal disturbances the borehole tiltmeter needs to be installed deeper than n ow, in boreholes of 100...500 m depth. (C) 1998 Elsevier Science Ltd. All r ights reserved.