Point stability at shallow depths: experience from tilt measurements in the Lower Rhine Embayment, Germany, and implications for high-resolution GPS and gravity recordings

From 1996 to 1999, we have studied ground tilts at depths of between 2 in a nd 5 m at three sites in the Lower Rhine Embayment (LRE), western Germany. The LRE is a tectonically active extensional sedimentary basin roughly 50 k m x 100 km. The purpose of the tilt measurements was (a) to provide insight into the magnitude, nature and variability of background tilts and (b) to assess possible limitations of high-resolution GPS campaigns and microgravi ty surveys due to natural ground deformation. The tilt readings, sensed by biaxial borehole tiltmeters of baselength 0.85 m, cover a frequency range f rom 10(-8) Hz to 10(-2) Hz (periods from minutes to years). Assuming that t he tilt signals represent ground displacements on a scale typically not lar ger than several times the tiltmeters' baselength, and that tilt signals at shallow depth could in a simple geometric way be related to changes in sur face elevation and gravity, we try to estimate the magnitude level of point movements and corresponding Bouguer gravity effects that is generally not surpassed. The largest tilt signals observed were some +/- 50 mu rad yr(-1) . If they were observable over a ground section of extension, e.g. 10 m, th e converted rates may correspond to about +/-0.5 nun per 10 in yr(-1) in ve rtical ground displacement, and +/-0.1 mu gal yr(-1) in Bouguer gravity eff ect, respectively. Large signals are mostly related to seasonal effects, pr obably linked to thermomechanical strain. Other causes of ground deformatio n identified include seepage effects after rainfalls (order of +/- 10 mu ra d) and diurnal strains due to thermal heating and/or fluctuations in the wa ter consumption of nearby trees (order of +/-1 mu rad). Episodic step-like tilt anomalies with amplitudes up to 22 mu rad at one of the observation si tes might reflect creep events associated to a nearby active fault. Except for short-term ground deformation caused by the passage of seismic waves fr om distant earthquakes, amplitudes of non-identified tilt signals in the st udied frequency range seem not to exceed +/-2 mu rad. As the larger tilt si gnals are close to the precision achieved with modern GPS systems and super conducting gravimeters when converted into height and gravity changes, furt her enhancement in resolution of these techniques may require simultaneous recording of local ground deformation at the observation sites.