: We have conducted a stress analysis of in situ overcoring measurements undertaken for the Prototype Repository in the Äsp ö HRL, Sweden. The inversion method used is based on a least squares criterion. Errors on all measurements are assumed to obey Gaussian distribution. The overcoring results, supported by existing hydraulic stress data, indicate that the minimum horizontal stress magnitude is close to the vertical down to approximately 500 m depth. The orientation of maximum horizontal stress is NW-SE.
1 INTRODUCTION
A large number of rock stress measurements have been conducted at the Äsp ö Hard Rock Laboratory (HRL) of the Swedish Nuclear Fuel and Waste Management Co. (Fig. 1). Knowledge of the state of stress is important for the design of the future nuclear waste repository, and different methods for measuring the state of stress have therefore been tested. In total, 80 hydraulic fracturing stress measurements
(HF), 5 hydraulic tests on pre-existing fractures (HTPF), and 109 overcoring stress measurements (OC) have been made in region (Bjarnason et al., 1989; Lee & Stillborg, 1993; Lee et al., 1994; Litter-bach et al, 1994; Ljunggren & Klasson, 1996, 1997; Nilsson et al., 1997; Ekman, 1997; Ekman et al., 1997; Ljunggren & Bergsten, 1998; Klasson et al., 2001).
