Monitoring fluid induced deformation of the earth's crust: A large scale experiment at the KTB location/Germany

In mid 2003, a tiltmeter array consisting of five high resolution ASKANIA borehole tiltmeters was installed in the surroundings of the German Continental Deep Drilling (KTB) location in Bavaria, Germany. Completed in 1994, two deep boreholes, the pilot borehole (4000 m) and main borehole (9101 m), are now used as crustal laboratory. The aim of the research project is to observe a presumably small deformation of the upper crust, caused by long term injection of water into the KTB pilot borehole. The deformation is expected to be detected for this kilometre scale experiment and will be interpreted by means of numerical modelling. Induced tilt signals can be understood as an additional drift, which is superimposing the always existing long- or aperiodic drift of the system borehole tiltmeter. To separate the induced deformation from the instrumental one, it is important to estimate the maximum amplitude of the drift and to determine the time window when this drift will occur at the array stations after the injection process has started. It is also necessary to eliminate local environmental effects, e.g. local pore pressure changes arising from groundwater variations. To quantify the expected additional drift for different injection scenarios at each tiltmeter site, numerical modelling using the program POEL was carried out. For injection periods of up to 4 months, a maximum tilt effect of about 40 nrad is expected, which is possibly detectable. The water injection may also influence the earth tidal tilt parameters, which depend on local and regional elastic properties of the earth's crust. Prior to the beginning of the experiment the main tidal waves have been computed by analysing the first months of recorded tilt at all stations.