Laboratory measurements of the seismic velocities and other petrophysical properties of the Outokumpu deep drill core samples, eastern Finland

Abstract

S U M M A R YPetrophysical, in particular seismic velocity, measurements of the Outokumpu deep drill core (depth 2.5 km) have been carried out to characterize the geophysical nature of the Paleoproterozoic crustal section of eastern Finland and to find lithological and geophysical interpretations to the distinct crustal reflectors as observed in seismic surveys. The results show that different lithological units can be identified based on the petrophysical data. The density of the samples remained nearly constant throughout the drilled section. Only diopsidetremolite skarns and black schists exhibit higher densities. The samples are dominated by the paramagnetic behaviour with occasional ferromagnetic signature caused by serpentinitic rocks. Large variations in seismic velocities, both at ambient pressure and under in situ crustal conditions are observed. The porosity of the samples, which is extremely low, is either intrinsic by nature or caused by decompaction related to fracturing during the core retrieval. It is noteworthy that these microfractures have dramatically lowered the V P and V S values. From the measured velocities and density data we have calculated the seismic impedances, Young's modulus and Poisson's ratios for the lithological units of the Outokumpu section and from these data the reflection coefficients for the major lithological boundaries, evident in the surveyed section, were determined. The data show that the strong and distinct reflections visible in wide-angle seismic surveys are caused by interfaces between diopside-tremolite skarn and either serpentinites, mica schist or black schist.

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

“…The foliation plane is found to be dipping 10 • , which agrees with the evidence of a near-horizontal plane predicted by seismic records (Sorjonen-Ward 2006;Heinonen et al 2009) and core samples (Kern et al 2009;Elbra et al 2011). Although the directions differ by ∼45 • , it is possible that the direction of lineation varies with depth or with distance from the observed outcrops.…”

“…Tube waves observed in a zero-offset VSP confirm that fractures are present in the subsurface (Duo et al 2010). Elbra et al (2011) show that ultrasonic velocity measurements on the core samples are quite sensitive to microcrack porosity and crack closure, however, an increase in crack porosity towards the outer rims of the core samples indicates that drilling related stress release is an important source of microcracks in the cores. Measurements on Outokumpu core samples by Lassila et al (2010) show a steeper increase in qP-wave velocity under uniaxial compression along the core than under triaxial compression, indicating microcracks may be aligned subparallel to the relatively horizontal foliation plane.…”

“…The upper 1300 m of the Outokumpu borehole is both mineralogically (Kukkonen & Outokumpu Deep Drilling Working Group 2007) and petrophysically quite uniform, although ultrasonically measured seismic velocities are pressure sensitive due to the microcrack porosity (Kern et al 2008(Kern et al , 2009Elbra et al 2011). When ultrasonic measurements are made on core samples from the section at pressures where microcracks can be expected to be closed, measurements in the three axial directions yield consistent anisotropic qP-and qS-wave velocities (Kern et al 2009).…”

P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths

“…As the samples have been long exposed on the surface and have undergone weathering and low-temperature alteration, particularly along microcracks and grain boundaries, the basic question of whether the seismic properties of surface-derived samples are reliably extrapolated to the Earth's interior remains. Thus, it is of the utmost importance to quantify the seismic properties of unaltered rock samples from greater depths; scientifi c drilling is probably the unique tool to provide such rock samples (e.g., Berckhemer et al, 1997;Kern et al, 2001;Ji et al, 2007;Elbra et al, 2011).…”

Geophysical Properties of the Near Surface Earth: Seismic Properties

“…In the former, the anisotropy results from the stiffening of the contacts in the direction parallel to the greater compression. At the field scale, Schijns et al (2012) were able to explain the observed seismic anisotropy in a 1300 m thick section of the Outokumpu biotite gneiss on the basis of oriented microcracks and mineral CPO (Elbra et al, 2011;Kern et al, 2009;Lassila et al, 2010;Wenk et al, 2012). Nur and Simmons (1969) may have been the first to observe these phenomena although the stress concentrations in their circular sample subject to a uniaxial force were not uniform.…”