A number of studies have been carried out in order to determine the magnitude of effects originating from the surroundings of an observation site which influence seismometer records or deformation observations with tilt- or strainmeters. The investigations generally comprise thermal, atmospheric, topographic, and cavity effects, as well as ocean loading. Recently, these studies have been continued and extended using Finite Element (FE) modeling.The effects are investigated exemplarily using air pressure variations as loading signal. The observatory sites differ in their local surroundings. Based on this the influence on topographic and lithologic features is investigated with respect to air pressure changes. The models developed reflect different topographies around stations: wide and narrow valleys, hill flanks, and flat areas. At the foot of the hill flank, galleries are integrated into the models. The topographic and the gallery geometry influence are systematically investigated with regard to e.g. rock coverage above an observation site, changes in the hill slope, width of a valley, and lithology. The deformations are only caused by air pressure changes and not associated with gravity changes due to an advancing pressure front.For loading different air pressure scenarios are simulated with an uniform load and a pressure front which moves over the model in different directions. As an elastic rheology is considered, the effects can be scaled to actual occurring air pressure variations.Initial studies focus on the cavity effect to estimate the order of magnitude to separate the pure cavity effect from other influences. Studies of topographic effects, e.g. an increasing slope angle, yield a non-linear increase in the strain component with amplitudes up to 2 nstrain and tilt changes up to 2 nrad for a uniform air pressure load of 1 hPa for PREM parameterized models. Different lithologies can result in five times larger deformations. For moving high pressure areas, changes emerge in strain of up to 2 nstrain and in tilt of up to 4.2 nrad.The results obtained improve the interpretation of deformations observed with regard to fundamental geodynamic processes, and contribute to the selection of future observation sites.

During the past decades, the research into fluid‐controlled geodynamic processes in the upper crust of the Earth is put foreward regarding the stress accumulation, deformation, and seismicity. In a large‐scale injection experiment at the deep borehole site KTB (Kontinentale Tiefbohrung der Bundesrepublik Deutschland) in Germany, more than 84.000 m3 fresh water was injected in 4000 m depth over 10 months, and a lot of geoscientific investigations were connected to this injection test. The pore‐pressure change of more than 10 MPa yielded an induced deformation, which was detected by a tiltmeter array. The used five borehole tiltmeters of the ASKANIA type with a resolution of better than 1 nrad belong to the most sensitive tiltmeters worldwide. The poro‐elastic finite‐element modeling used for the interpretation of the observed tilts revealed an uplift of 3.1 mm above the injection point. Furthermore, it was shown that the induced stress and deformation fields depend mainly on the fluids inside the fault zones, particularly the SE2‐reflector which is the Franconian fault zone, and the local Nottersdorf fault: While the maximum pore pressure is concentrated close to the injection point at the SE2 zone, the maximum deformation of about 3 cm is located at the intersection line of both faults in 4 km depth. The area of the modeled maximum deformation is highly correlated with the region of the observed induced seismicity, thus linking seismicity and deformation. The connection of tilt observation and modeling provides a useful tool for the investigation of fluid coupled geoprocesses.

At the German Continental Deep Drilling site (KTB) the pilot borehole of a depth of 4000 m was used to inject water with a medium rate of 180 litres/minute over a period of one year. To monitor the expected surface deformation five borehole tiltmeters of the ASKANIA type Gbp10 were installed in the surrounding area of the KTB location in mid 2003. The deformation was detected at kilometre scale, together with the observations of induced seismicity in the area. We observed elastic as well as inelastic responses: changes of the rheologic properties due to pore pressure increase caused changes in the tidal parameters. First we quantified the expected additional drift for different injection scenarios at each tiltmeter site by numerical modelling. It could be demonstrated that for long term injection phases of up to four months a maximum tilt effect of about 40 nrad is modelled, which should be detectable. We expected changes of the drift curve, slow variations correlating with the injection rate as well as with changes of the rate.First results of the monitoring are presented: they reveal a slight increase of the tidal parameters (main tidal constituent 01 and M2, north-south component) and drifts associated with the long-term injection. Comprehensive numerical modelling using the Finite Element software ABAQUS is in preparation.