Pressure and temperature dependence of VP and VS in rocks from the superdeep well and from surface analogues at Kola and the nature of velocity anisotropy

Abstract

P-wave velocities ( V P) and S-wave velocities ( V S) and their directional dependence (velocity anisotropy, shear wave splitting) were measured on eight cores samples from the Kola superdeep well and on eight samples collected from surface outcrops over a range of pressures up to 600 MPa at room temperature and from room temperature up to 600°C at 600 MPa. The core samples were recovered at depths of about 4670 to 11,720 m, and the outcrop samples represent lithologies similar to those of the core samples. Measurements were carried out on sample cubes of dry rocks (43 mm edge length) in a multianvil apparatus, allowing simultaneous measurements of V P and V S in the three structural directions X, Y, Z of the sample cubes and direct determination of length changes (volume change) of the sample with increasing pressure and temperature. Pronounced pressure (crack) sensitivity of P- and S-wave velocities indicates strong decompaction in the cores, due to drilling-induced damage and rapid pressure and temperature release during core retrieval. Marked velocity anisotropy and shear wave splitting is present in the foliated Proterozoic and Archean gneisses and amphibolites, due to oriented microcracks and lattice (crystallographic) preferred orientation (LPO) of the constituent mica and hornblende minerals. The experimentally determined intrinsic velocities and anisotropies compare fairly well with corresponding calculated data, based on the LPO and the single crystal properties of the major minerals. Measurements and calculations confirm strong relations of velocity anisotropy, shear wave splitting and shear wave polarisation to the structural frame of the rocks (foliation, lineation). The experimentally derived in situ velocities representing the averages of the velocities measured in the three structural directions (three P-wave and six S-wave velocities) compare fairly well with VSP data. The fit is even better when anisotropy (related to foliation and foliation dip) is included in the determination of the in situ velocities. From the relationship between experimental and calculated shear wave splitting data and the structural frame (foliation and lineation) of the Proterozoic and Archean rocks we interpret the VSP-derived shear wave polarisation at Kola ( Digranes et al., 1996) as resulting from the strong alignment of mica and hornblende, defining the foliation.