Seismic anisotropy in the crystalline upper crust: observations and modelling from the Outokumpu scientific borehole, Finland

Seismic velocity anisotropy measurements are made of a fractured metamorphic formation from the 2.5-km-deep International Continental Scientific Drilling Programme (ICDP) borehole in Outokumpu, Finland. Three component walk-away vertical seismic profile (VSP) measurements are made along two source-line azimuths at three receiver depths (1000, 1750 and 2500 m) and incidence angle-dependent qP- and qS-wave velocities are extracted with a tp method. The highest qP-wave anisotropy, 13.6 per cent (vfast= 6160 m s-1, vslow= 5370 m s-1), is measured between 1000- and 1750-m depth, with anisotropy of up to 9.4 per cent (vfast= 6090 m s-1, vslow= 5540 m s-1) measured between 1750 and 2500 m depth. The top similar to 1300 m of the region is composed of a homogeneous, strongly intrinsically anisotropic biotite-rich schist, and is sampled by the shallowest walk-away profile. Anisotropy of up to 11.1 per cent (vfast= 5950 m s-1, vslow= 5320 m s-1) is measured by the walk-away VSP between 50 and 1000 m depth, along with shear wave splitting averaging 5 per cent (180 m s-1). Laboratory-derived intrinsic anisotropy of the schist cannot by itself explain the degree or orientation of the anisotropy measured in the walk-away VSPs, however, a model which modifies the intrinsic stiffnesses by the inclusion of a single set of dipping, aligned cracks allows the observed in situ velocities to be reproduced. Forward modelling of the qP-wave walk-away VSP measurements from 50 to 1000 m depth is undertaken using an effective medium model to develop a 3-D velocity model of this region. An orthorhombic medium is used to represent the intrinsic anisotropy of the biotite-rich schist, and a single set of aligned cracks is added to give a bulk elastic stiffness. The resulting model predicts the schist to have an overall anisotropy of 16.8 per cent, with qP-wave velocities of up to 6315 m s-1. The accuracy of the model is assessed through its fit to the walk-away VSP measurements as well as a comparison to known geology of the region.