HR: 1330h
AN: G42A-0039    [PDF]
TI: Irregularities in Pump-Induced Tilt Above Shallow Aquifers
AU: * Kuempel, H J
EM: kuempel@gga-hannover.de
AF: Leibniz Institute for Applied Geosciences, Stilleweg 2, Hannover, 30655 Germany
AU: Fabian, M
EM: fabian@geo.uni-bonn.de
AF: Geological Institute, Section Applied Geophysics, Bonn University, Nussallee 8, Bonn, 53115 Germany
AB: Near surface ground tilt induced by the extraction of fluids from subsurface reservoirs has repeatedly been used to constrain reservoir parameters. In general, pump-induced tilt is found to be proportional in strength to the pore pressure gradient created by the pumping, to be a function of poroelastic rock parameters, and to depend on the geometric configuration of the tilt sensor and the productive sections of a well. Assuming radial flow, the strike of the tilt signal should point toward the productive well. However, inversion of near surface tilt can be hampered through irregularities in the pump-induced signal and suffer from insufficient knowledge of the influence of heterogeneities in the subsoil, either within or above the reservoir. We can learn more about the impact of such heterogeneities and reduce ambiguities by analyzing case studies. New observational data from 3 test sites in Germany confirm that a variety of causes can produce irregularities in pump-induced surface deformation, namely: (1) The strike of a tilt signal can considerably deviate from the direction toward the active well. A reason could be that the steepest effective pore pressure gradient builds up in another than radial direction (thereby generating anisotropic fluid flow). Accordingly, tilt hodographs for a complete pump cycle may be elliptic rather than follow a line. (2) The normal rule of how the signal strength depends on the horizontal and the vertical distance may be violated. Structural heterogeneities in the subsurface are the most likely cause for this behaviour. (3) Recovery of the induced tilt signal following the cessation of pumping can be incomplete. This could indicate a non-reversible compaction possibly due to overexploitation of a reservoir. (4) A transient sign reversal of ground tilt may occur during the build-up phase of the signal. This feature could be an analogue to the so-called Noordbergum effect occasionally seen in the response of well levels at locations aside from the pumped well. We conclude that near surface deformation in the vicinity of pumped wells bears a wealth of information that may be useful to constrain the conditions of fluid flow at depth, and that there is often a need for model calculations to fully understand the involved phenomena.
DE: 0994 Instruments and techniques
DE: 1294 Instruments and techniques
DE: 1894 Instruments and techniques
DE: 5194 Instruments and techniques
SC: Geodesy [G]
MN: 2003 Fall Meeting