Äspö Hard Rock Laboratory; Using borehole tides to determine fracture parameters, 2 - Data analysis

Rapport, 2004

The purpose of the study was to investigate whether the results of the tidal pre-study on KAS03 section A (800 m depth) could be transferred to other boreholes around Äspö. The expectation was that fracture zone parameters could be determined sufficiently well in the other cases. The fracture zone parameters of the model are dip and strike, Poisson ratio, modulus of elasticity (E), and an along-plane thinning ratio (E beta). Of these strike and Poisson ratio are most clearly resolvable. Dip and elasticity are highly correlated and need ancillary information for unique determination. It was hoped that barometric pressure forcing would provide the missing constraint on dip. However, the effects induced by air pressure are too small and too much concentrated at low frequencies to provide sufficiently strong constraints. The most prominent finding is that almost all boreholes display very similar tidal response, and the strike angle solution for a supposed singular fracture zone is found consistently between 120 degrees and 140 degrees from north.