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

Abstract

SUMMARY 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 τ–p 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 ∼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.

Seismoacoustic signatures of fracture connectivity

“…Since in most cases the resolution of seismic data is insufficient for directly imaging fractures, most of the related research efforts focus on seismic attribute analysis [e.g., Bakulin et al, 2000]. Correspondingly, various seismic attributes, such as shear wave splitting and azimuthal variations in P wave attenuation and phase velocity, have been identified as diagnostic features that may permit to estimate some key properties of fractured media, including the density, the average length, and the orientation of the fractures [e.g., Liu et al, 2000;Maultzsch et al, 2003;Liu et al, 2007;Clark et al, 2009;Schijns et al, 2012]. Notably, seismic attenuation has been recognized as a potentially important parameter for this purpose, since different studies have shown that seismic energy loss is very significant in fractured rock masses and tends to increase with increasing fracture density [e.g., Peacock et al, 1994;Maultzsch et al, 2003;Payne et al, 2007;Clark et al, 2009;Varghese et al, 2009].…”

Experience in applying the acoustopolarization method fpr metamorphosed rock samples from Kola (SG-3), German (KTB-HB) and Finnish (OKU) investigation boreholes

“…Three component walk-away vertical seismic profile (VSP) measurements are made along two source-line azimuths at three receiver depths (Schijns et al, 2012). A summary of the results of petrophysical studies is given in the Outokumpu project report .…”

Geophysical Properties of the Near Surface Earth: Seismic Properties

“…For example, a hypothetical elliptical crack with w $ 10 À3 residing in quartz matrix closes by application of < 2 atmospheres of pressure ($0.2 MPa). One may be able to ignore this in the deep crust and the Earth's mantle, but it appears to be an important factor in seismic investigations nearer the Earth's surface (Crampin and Peacock, 2008;Schijns et al, 2012). Of course, the material would crush long before such a pressure could be reached, but these calculations serve to illustrate how easily crack-like porosity can be closed relative to more equant porosity.…”