HR: 0800h
AN: T21B-0460    [Abstracts]
TI: Estimating rheological properties at geological time-scale from an analysis of in-situ natural stress profiles
AU: * Gunzburger, Y
EM: gunzburg@ipgp.jussieu.fr
AF: IPGP, 4 place Jussieu, Paris, 75252 France
AU: * Gunzburger, Y
EM: gunzburg@ipgp.jussieu.fr
AF: ANDRA, Parc de la Croix-Blanche, Chatenay, 92298 France
AU: Cornet, F H
EM: cornet@ipgp.jussieu.fr
AF: IPGP, 4 place Jussieu, Paris, 75252 France
AB: Numerous stress measurements have shown strong stress changes when nearing an important fracture zone. These may be partly reproduced by numerical modelling. But the calculations are mostly conducted with a hypothesis of elastic behaviour for the rock mass, which is not always realistic. Well-defined stress profiles yet contain a great amount of information that may help to constrain parameters of more relevant rheological laws, such as long-term visco-elastic ones, at a time scale out of reach to laboratory and in-situ testing. We present the case of an almost non-fractured region in which a detailed stress estimation was carried out by hydraulic fracturing, by shaft convergence monitoring and by systematic analyses of borehole breakouts. It provides a rather continuous stress profile through the rock mass. The orientations of the minor and major horizontal stresses (Sh and SH) are found to be in good agreement with geological considerations but their magnitudes exhibit a surprising, non-symmetric, evolution with depth: Sh is higher in the central hard-clays formation than in the surrounding limestones, even though SH is almost constant. The absence of a drop in SH within the hard-clays is associated with a horizontal differential stress (SH-Sh) of about 2MPa, which seems not to be in agreement with the viscous behaviour of the hard-clays determined from laboratory tests. An explanation to this stress state was searched in the long-term behaviour of hard-clays. An analytical calculation and a numerical modelling demonstrate that the present-day state of stress may be reproduced applying horizontal shortenings in two different directions and simulating the erosion of part of the overburden. A generalized Kelvin visco-elastic behaviour was assumed for the hard-clays. Three of its parameters (bulk modulus, short-term and long-term shear modulus) as well as the initial state of stress could be derived from inversion of the measured stress profiles, assuming that the viscous stress relief is total. The value of the viscosity coefficient can be estimated only if the age of the last perturbations (shortening, erosion) is known. A similar approach may be applied to evaluate rock mass properties in the vicinity of faults, if the age, the dimension of the slipping zone and the amount of slip are known.
DE: 5120 Plasticity, diffusion, and creep
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8160 Rheology: general (1236, 8032)
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: Fall Meeting 2005