Seismic anisotropy in cracked crystalline rock from Outokumpu, Finland

Abstract

Lattice and shape preferred orientation of minerals, along with aligned fractures and microcracks, is expected to cause significant seismic velocity anisotropy in crystalline rocks. As seismic surveys in hard rock environments become more common, quantifying and accounting for this anisotropy in seismic processing becomes increasingly important. Outokumpu, Finland is the site of a historic base metal mine and is a classical ore province known for its Cu-Co-Zn sulphide deposits. The 2.5 km deep ICDP borehole shows the lithology in the area of the Outokumpu 2006 2D seismic survey to be primarily composed of a biotite-rich schist. Three walk-away VSP profiles were used to quantify the tilted orthorhombic in-situ anisotropy. Laboratory measurements of the qS1, qS2 and qP waves along the axial directions and in select off-axis directions at confining pressures from 10-200 MPa, and effective medium modelling were used to further inform the seismic anisotropy of the schist. Strong anisotropy is observed both in-situ and on the laboratory measurements, and a 3D velocity distribution is calculated from modelling of these results.