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Geophysical Journal International Geophys. J. Int. (2011) 184, 405–415 doi: 10.1111/j.1365-246X.2010.04845.x Laboratory measurements of the seismic velocities and other petrophysical properties of the Outokumpu deep drill core samples, eastern Finland Tiiu Elbra,1 Ronnie Karlqvist,2 Ilkka Lassila,2 Edward Hæggström2 and Lauri J. Pesonen1 1 Department 2 Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Finland. E-mail: [email protected] of Physics, Division of Materials Physics, University of Helsinki, Finland SUMMARY Petrophysical, 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 VP and VS 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. Keywords: Body waves; Wave propagation; Acoustic properties; Crustal structure. 1 I N T RO D U C T I O N Our knowledge of the structure of a typical Precambrian continental crust is important when one tries to understand the geological processes and evolutionary history of the shield areas. Over the past decades worldwide seismic surveys have improved our knowledge of the crustal structures (e.g. Christensen & Mooney 1995; Kukkonen et al. 2006; Brown et al. 2009; Kern et al. 2009). Even though numerous wide-angle reflection seismic transects covering the entire Fennoscandian Shield have been carried out (Kukkonen et al. 2006; Heikkinen et al. 2007; Kukkonen et al. 2007a; Schijns et al. 2007), detailed information about the distribution of rock types within the crust, as well as information about their petrophysical properties is still needed. Several attempts to build lithological models of the crust and upper mantle of the Fennoscandian Shield have been carried out (e.g. Korja et al. 2006; Kuusisto et al. 2006; Sorjonen-Ward 2006) based on surface geology, geophysical data and elastic velocity models. However, these models C 2010 The Authors C 2010 RAS Geophysical Journal International have two drawbacks. First, the knowledge of the various lithological units constituting the continental crust is poor and secondly, only a few elastic property determinations are available for the rock units exposed to real crustal pressures and temperatures. In recent years, scientific drilling has become a successful tool for providing detailed petrophysical and lithological data necessary to understand the structure, composition and physical state of the crust (e.g. Kola super deep hole: Kern et al. 2001; Chesapeake Bay impact structure: Elbra et al. 2009; Powars et al. 2009; Chicxulub impact structure: Vermeesch & Morgan 2004). For this reason, the Geological Survey of Finland (GSF) carried out the Outokumpu Deep Drilling Project in 2004–2005, in cooperation with the International Continental Scientific Drilling Program (ICDP). The project was carried out to understand the deep structure of the ore province in the Precambrian terrain of eastern Finland where the strong seismic reflectors are visible in numerous seismic reflection surveys (Heikkinen et al. 2007; Kukkonen et al. 2007a). The focus of the drilling effort (Kukkonen et al. 2007a) was to 405 GJI Seismology Accepted 2010 October 8. Received 2010 October 6; in original form 2010 January 29 406 Tiiu Elbra et al. penetrate the observed strong seismic reflectors within the upper crust. The Outokumpu (hereafter called OKU) deep drill core offers reference material to investigate the physical properties of a Paleoproterozoic crustal section as a function of depth. This core provides an opportunity to distinguish the role of burial or tectonic effects on the physical properties of the rocks. Such information could also benefit the future exploring surveys of the OKUtype mineralizations in the area by providing improved knowledge about the physical properties of the upper crustal rocks. Furthermore, the deep biosphere and the origin of the saline fluids and gases are of interest (Kukkonen et al. 2007a) and, thus, rock physical property data, notably porosity and microcracking, can provide important constraints in these studies. We emphasize however that laboratory measurements of small rock samples represent only a small part of the crustal section. Hence they don’t necessarily fully represent the same in situ unit detected by seismic surveys or even by geophysical loggings (e.g. VSP surveys; Schijns et al. 2007) for two reasons. First, the deep seismic soundings and the borehole loggings integrate much wider areas of the crust in their signals. Secondly, the surveys and loggings do not suffer fracturing caused by decompaction during core retrieval (e.g. Kern et al. 2001). In this paper, we present petrophysical, especially ultrasonic seismic velocity, density and porosity data of the Outokumpu deep drill core. From the petrophysical data we obtain the seismic impedances, Young’s modulus, Poisson’s ratios as well as the reflection coefficients for various lithological units and their boundaries. These data are used to find explanations to the distinct crustal reflectors seen in seismic reflection studies, especially the strong reflectors at depths of ∼1300–1500 m. 2 R E G I O NA L G E O LO G Y A N D GEOPHYSICS The Paleoproterozoic Outokumpu region is located in the Fennoscandian Shield, in eastern Finland, close to the ArcheanProterozoic boundary zone (Fig. 1a). It is known for its occurrences of early Proterozoic Cu–Zn–Co sulphide ore deposits (Koistinen 1981; Loukola-Ruskeeniemi 1999 and references therein). The Outokumpu ores are associated with a lithologic complex called the Outokumpu Association that consists of serpentinites, dolomites, calc-silicate rocks, siliceous rocks and metamorphosed black shales (Kukkonen & Šafanda 1996; Airo & Loukola-Ruskeeniemi 2004). This assemblance, also described as an ophiolitic complex, meanders along as a discontinuous band for more than 200 km (Papunen 1987; Kukkonen & Šafanda 1996). The rocks of the Outokumpu ophiolitic complex are embedded in mica schists and -gneisses which are the main rock types in the area (Kukkonen & Šafanda 1996). The Outokumpu basin has been extensively studied by the Outokumpu Company (numerous drillings) and the GSF, including geological, geochemical and geophysical mappings. The geophysical surveys show generally positive magnetic anomalies, which correlate with rock horizons associated with ore mineralization (Ruotoistenmäki & Tervo 2006). On the other hand, the distinct Figure 1. (a) Regional geology of Finland, showing the distribution of Archean rocks in Fennoscandian Shield (modified after Sorjonen-Ward & Luukkonen 2005) and the location of the FIRE transects, including OKU-1 line (marked in yellow) which follows the same road as one of the FIRE-3 sections (after Kukkonen et al. 2006). (b) Lithology of OKU deep drill core (drawn after Västi 2005). C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Laboratory seismic data of the Outokumpu core gravity anomalies demonstrate that complex block tectonics has taken place, reflecting either basement doming (e.g. Sotkuma) or granitic batholiths (Maarianvaara; Ruotoistenmäki & Tervo 2006). Both magnetic and gravity data reveal also fracture or shear zones. Furthermore, the central Fennoscandia is covered with numerous wide-angle reflection seismic surveys and seismic array studies (Kukkonen et al. 2008), for example, the FIRE (Finnish Reflection Experiment) project which includes profiles transecting the Outokumpu area (Fig. 1a) and reveal distinct crustal reflectors (Kukkonen et al. 2006). Despite the numerous investigations the genesis, tectonic evolution and causes for the seismic reflectors of the Outokumpu basin are still incompletely understood as the depositional basement is not met and since the assemblance itself is severely altered and tectonized (Park 1988). A review of the geology, geophysics and tectonic evolution of the Outokumpu area is published by Sorjonen-Ward (1997); Tyni et al. (1997), Airo & Loukola-Ruskeeniemi (2004) and Ruotoistenmäki & Tervo (2006). 3 OUTOKUMPU DEEP DRILLING The Outokumpu drilling was carried out in 2004–2005 by the contractor NERDA–a Russian governmental company. The drill site is located 2 km SE from the Outokumpu town (Kukkonen et al. 2009; Fig. 1a) featuring a borehole that penetrates through metasediments and ophiolitic rocks into pegmatite-granite at its deepest part (Fig. 1b). The borehole is 2516 m deep and the full sequence was cored with a core recovery of 80 per cent. Although the hole was initially vertical it soon deviated by a few degrees (up to 250 m on surface projection) towards NW and W (Kukkonen et al. 2007a; 2009). The geophysical down-hole logs and hydraulic experiments were performed throughout the hole (Kukkonen et al. 2007a; Schijns et al. 2007). The OKU borehole has been kept open as a ‘Deep Geolaboratory’ for in situ experiments and monitoring. Lithologically the topmost part of the core (33–1314 m; hereafter called upper schist series; Fig. 1b) consists of fine-grained strongly foliated mica schists containing some biotite gneiss, chlorite-sericite schist and black schist interlayers. This mineralogically most uniform area of the core (Kukkonen et al. 2007a), is made up of biotite, quartz and plagioclase minerals, exhibiting strong preferred orientation of elongated platy mica grains nearly perpendicular to the core axis (i.e. z-axis; Kern et al. 2009). The upper schist series is followed by the ophiolitic complex (1314–1515 m), consisting of serpentinites, weakly foliated diopside and tremolite skarns and black schists (Kukkonen et al. 2007a; Kern et al. 2009). Mineralogically the serpentinites consist of fine-grained platy antigorite, fibrous chrysotile and randomly distributed oxide minerals (e.g. magnetite). The diopside-skarns of the ophiolitic complex are made up of coarse-grained hornblende and clinopyroxene which are irregularly distributed in a fine-grained hornblende, diopside matrix (Kern et al. 2009). Pegmatite granites (2013–2516 m; pegmatoid series) in the lowermost part of the core underlie 500 m thick mica schist layer (1515–2013 m; lower schist series). The pegmatoids are dominated by randomly distributed coarse-grained quartz and feldspar minerals. A weak shape-preferred orientation of the minor minerals muscovite and biotite was observed only at macroscopic scale (Kern et al. 2009) with nearly horizontal foliation. The more extensive grain size distribution as well as the chemical- and modal composition of the samples from the OKU deep drill core is published by Kern et al. (2009). C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International 407 4 M E T H O D S A N D I N S T RU M E N T S Samples used in the current study come from the OKU deep drill core. Cylindrical samples of two different diameters (ø ∼ 4 cm, L ∼3–7 cm and ø ∼ 2.5 cm, L ∼2–2.5 cm) were used. The sample sets were prepared at the GSF. Basic petrophysical parameters, such as wet density (ρ), porosity (φ), magnetic susceptibility (κ) and intensity of the natural remanent magnetization (NRM) were measured for the selected samples (>350; mostly samples with ø ∼ 2.5 cm) to characterize the physical properties of the drilled section. The density and porosity were determined using the Archimedean principle based on weighting water saturated and oven dried samples in air and in water. For magnetic susceptibility RISTO-5 (1025 Hz operating frequency and 48 Am−1 field intensity) and the AGICO KLY3S (875 Hz operating frequency and 300 Am−1 field intensity) kappabridges were used. The NRM measurements were performed using a Superconducting Rock Magnetometer (Model 755, 2G Enterprises) in combination with an AF demagnetizer (Model 2G600, Applied Physics) on the ‘paleomagnetic’ samples (ø ∼ 2.5 cm). The seismic P-wave velocities (longitudinal; VP0 ) in more than 1700 watersaturated samples were measured (in ≤1 m intervals whenever the core allowed) at ambient pressure with an apparatus (93 Hz frequency) developed at the GSF. The velocity was calculated using the sample length and the P-wave travel time through the cylindrical sample. The travel time was determined using two identical transducers as transmitter and receiver, respectively (Airo et al. 2007). All VP0 measurements were conducted in water to provide good contact between the transducers and rocks. To better understand the cause of the seismic reflectors observed in seismic reflection surveys and in acoustic soundings (e.g. FIRE; Kukkonen et al. 2006) we built an ultrasonic instrument (Fig. 2) to measure the seismic P- and S-wave velocities (VP and VS , respectively) in the drill core samples under crustal pressure and temperature (Lassila et al. 2010). The system comprises two custombuilt transducers with longitudinal (1.0 MHz frequency) and shear (1.1 MHz frequency) Pz27 elements, a hydraulic cylinder (HSS 254, Hi-Force) and a load cell (Model 53, Honeywell) residing inside a metal cradle. The uniaxial compressional loading is produced with a hydraulic cylinder that is driven with a computer controlled, 3-way, load holding pump (HEP 2142S3, Hi-Force). The induced load is monitored with a load cell. The transducers are excited simultaneously (200 V, negative spike) with a pulser (5058PR, Panametrics) and the signals that have passed through the sample are sampled with an oscilloscope (9410, LeCroy). The maximum load provided by the system is limited by the load cell to 25 tons (480 MPa, which corresponds to a crustal depth of about 18 km, for a cylinder with ø ∼ 2.5 cm diameter). The change in sample height is measured with 1 μm resolution during the compression with a digimatic indicator (543–250B, Mitutoyo). All the measured ultrasonic signals, the sample temperature and height, as well as the applied load are recorded. The calibration signals are subtracted from the actual measurements to cancel out effects of transducer housing compression and heat expansion (Karlqvist 2009; Lassila et al. 2010). A brass holder supporting the sample from the side was used to avoid sample cracking under uniaxial pressure; the close-fitting limited also the sample expansion. The supporting force produces radial stress, whose magnitude depends on the Poisson’s ratio of the sample. This force decreases the stress difference between that occurring along the axial and the radial directions. The ultrasonic velocities were determined from the measured time of flight data and the sample height. 408 Tiiu Elbra et al. Figure 2. The measurement assembly. The sample (1) resides inside the sample holder (2) between the transducers (3). The distance gage (4) is attached to the base of the upper transducer while the counterpart for it is attached to the top of the lower transducer. The load cell (5) that holds the upper transducer is fixed to the piston of the hydraulic cylinder (6) that is supported from the top part of the steel frame (7). (See details in Lassila et al. 2010). We used over 100 water-saturated samples to simulate the upper crustal conditions. The pressure employed in the VP and VS measurements ranged from 3 to 40 MPa depending on actual sample depth (125–1500 m). Due to the paleoclimatic ground surface temperature history the subsurface temperature in Outokumpu is low (Kukkonen & Šafanda 1996)–at the final 2.5 km depth only 40 ◦ C (Kukkonen et al. 2007b). Tests showed no significant temperature dependence of the velocity within the interval 20–40 ◦ C; hence the VP and VS measurements were conducted in room temperature altering only the load pressure. All velocity measurements were conducted in the direction normal to the foliation plane (z-axis), that is, normal to preferred orientation as described previously. The VP and VS provide estimates of (i) seismic impedances (ZP ), VP /VS ratios, Poisson’s ratios (ν) and Young’s modulus (E) for the lithological units of the drilled section and also, (ii) the reflection coefficients (RC ) of the lithological boundaries. 5 R E S U LT S 5.1 Petrophysical characteristics at surface pressure conditions The physical properties along the core are presented in Fig. 3. The density of the core samples varied relatively little throughout the uppermost part of the core (upper schist series). Most values range between 2700 kg m−3 and 2800 kg m−3 , with a small decreasing trend with increasing depth. In the lower schist series the density continues to decrease towards the pegmatite granites which exhibit the lowest density of ∼2636 kg m−3 . Most samples throughout the core exhibited minor porosity (<1 per cent). The overall magnetic susceptibility distribution along the core indicates mostly paramagnetic behaviour (κ < 500 μSI) with a small ferromagnetic component due to small amounts of unevenly distributed magnetic fraction. The NRM was generally weak throughout the core. The interval from 1314 m to 1515 m is exceptional. This interval consists of rocks from the ophiolitic complex and shows large variations in all physical properties (e.g. ρ from 2514 kg m−3 to 3158 kg m−3 ; κ from ∼0 to 6 × 10−2 SI). While the rest of the petrophysical properties were nearly constant throughout the core (especially in the upper schist series) the VP0 exhibits large variation and a slight decrease with increasing depth (Fig. 5). The VP0 in the upper schist series varied between 3273 and 6174 m s−1 with an average VP0 of 5169 m s−1 . The ophiolitic complex exhibited higher values of VP0 (≤7350 m s−1 ; average 5508 m s−1 ). The average VP0 decreased from 5169 m s−1 (the upper schist unit) to 4512 m s−1 in the lower schist unit. Pegmatites at the bottom of the core exhibit a higher average VP0 (5120 m s−1 ) than the samples from the lower schist series but these values are still lower than the values of the rocks from the upper schist series. Based on the petrophysical data (Fig. 4; Tables 1 and 2) the different lithologies can be distinguished from each other. The metasediments, such as mica schists, chlorite-sericite schists and biotite gneisses exhibit overlapping petrophysical properties. Samples from these lithologies all show small variations in density and porosity with average values of ρ = 2739 kg m−3 (φ = 0.5 per cent), 2726 kg m−3 (0.4 per cent) and 2792 kg m−3 (0.6 per cent), respectively. The magnetic susceptibility of those samples is 70–800 μSI (one specimen showed κ = 3170 μSI). The mica schists exhibited larger variations in remanence than the biotite gneisses. While some of the physical properties of biotite gneisses and chloritesericite schists are similar, the density, and especially the VP0 in the biotite gneisses differs from those recorded for the chlorite-sericite schists. Hence these two lithologies can be distinguished from each other by means of density and VP0 (Figs 4d and e). The black schists exhibit higher density (2852 kg m−3 ) than the mica schists as well as wider variation in magnetic susceptibilities and NRM. The serpentinites, together with pegmatite granites and quartzites, exhibit on average lower density (2621 kg m−3 , 2636 kg m−3 and 2641 kg m−3 , respectively) than the metasediments. However, while the mica schists, biotite gneisses, quartzites and pegmatites show para- or diamagnetic behaviour some serpentinites show presence of unevenly distributed ferromagnetic minerals (and thus higher κ and NRM), setting them apart from the other lithologies (Fig. 4). The serpentinites were also more porous than the other samples. Pegmatite granites appear as distinct clusters of petrophysical properties compared to the other lithologies in Figs 4a–d. 5.2 Ultrasonic seismic velocities at in situ-like pressure conditions Both the VP0 results obtained in laboratory conditions and the VP and VS values with a pressure corresponding to what they face in situ, varied considerably (Fig. 5). The average VP and VS values for the upper schist series are 5509 m s−1 and 3132 m s−1 , respectively. The variation seen in the VP /VS ratio, ranging from 1.63 to 2.05, derived mainly from variation in VS rather than in VP . The average VP /VS ratio was 1.76 for this series. C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Laboratory seismic data of the Outokumpu core 409 –1 – Figure 3. Petrophysical properties (density, porosity, magnetic susceptibility and NRM) of OKU deep drill core as a function of depth. Blue arrow illustrates possible reduction trend in density and increase in porosity. There was no depth dependence observed in VS , however VP increased slightly with depth (Fig. 5). The Poisson ratio (ν) of these samples is 0.2–0.34 (0.26 on average). The seismic impedance (ZP ), calculated from the measured density and velocity data, of the upper schist series is on average 15.1 × 106 kg m−2 s−1 (14.9 × 106 kg m−2 s−1 in the upper part) whereas the corresponding Young’s modulus is 67.9 GPa. The samples from the ophiolitic complex show on average 5622 m s−1 for VP and 3076 m s−1 for VS with a VP /VS ratio of 1.85, ν = 0.29, ZP = 15.50 × 106 kg m−2 s−1 and E = 67.1 GPa, respectively. Our studies indicate that the mica schists (Table 2), the majority of the measured samples, show 5501 m s−1 for VP and 3124 m s−1 for VS with a VP /VS of 1.77 and a ν of 0.26 (ZP = 15.1 × 106 kg m−2 s−1 and E = 67.5 GPa). The chlorite-sericite schist exhibit higher VP and VS (also ZP and E) than the mica schists. For chlorite-sericite schist VP /VS = 1.63 and ν = 0.20 was obtained, whereas the biotite gneiss exhibit lower VP and VS (VP /VS = 1.83; ν = 0.29). The serpentinites differ most from the other samples C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International featuring the highest VP /VS (2.06) and Poisson’s ratio (0.35), as well as the lowest impedance (1.38 × 106 kg m−2 s−1 ). 6 DISCUSSION The upper schist series, together with the lower schist series, is mineralogically the most uniform rock unit of the drilled section (Kukkonen et al. 2007a). This is supported also by the basic petrophysical data (e.g. magnetic properties: indicating mainly paramagnetic behaviour and no stable remanence; Fig. 3), which show no distinct differences in any petrophysical properties throughout both units. The seismic velocity data, in contrast, indicates large velocity variations within these series, both in the laboratory conditions (VP0 ) and when applying in situ–like pressure conditions (VP ) to the samples. The average velocities in mica schists (Table 2) in the current study are 5501 m s−1 for VP and 3124 m s−1 for VS for in-situ pressure conditions while chlorite-sericite schist exhibit 410 Tiiu Elbra et al. – –1 –1 –1 – – Figure 4. Correlation between (a) density and porosity; (b) density and magnetic susceptibility; (c) susceptibility and NRM (including Q-ratio, representing the ratio of remanent to induced magnetization, as the line parameter); (d) density and VP0 and (e) porosity and VP0 of various lithologies. higher VP and VS (5636 and 3464 m s−1 , respectively) and biotite gneiss yield lower velocities (VP 5367 m s−1 , VS 2947 m s−1 ). The lithological differences partly explain the intrinsic velocity pattern (Fig. 5), but not the fluctuating values within the same lithology. The seismic velocities in the samples can be determined by many factors: rock forming mineral composition, fabric and structural anisotropies, density, porosity, fracturing, dislocations, as well as by the applied pressure and temperature (e.g. Kern 1993; Vajdova et al. 1999; Kern et al. 2001, 2009; Kuusisto et al. 2006). Kern et al. (2009) reported on the modal mineralogy and chemistry of the lithologies present in the OKU core. These investigations concluded that the modal composition along with the shape preferred orientation of the biotite grains partially contribute to the variation seen in the elastic wave velocities. In general, the P-wave velocity C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Laboratory seismic data of the Outokumpu core 411 Table 1. Basic petrophysical properties of various lithologies. Displayed are average values of each lithology. Lithology Mica schist Biotite gneiss Chlorite-sericite schist Black schist Diopside-tremolite skarn Serpentinite Serpentinite-tremolite-(olivine) rock Quartz Pegmatite granite n ρ (kg m−3 ) φ (per cent) κ (μSI) NRM (mAm−1 ) 269 (154/105) 26 (20/5) 6 (4/1) 7 (5/4) 2 12 (7/10) 3 2 32 (32/8) 2739 2792 2726 2852 3034 2621 2765 2641 2636 0.53 0.64 0.43 0.85 0.31 3.09 0.71 0.21 0.45 302 403 280 417 128 13790 234 −22 28 1.99 0.94 0.27 538.45 2.42 1080.48 2.47 0.05 0.27 n, Number of samples, the values in brackets indicate the number of samples used for porosity/NRM if differed from n; ρ, wet density; φ, porosity; κ, magnetic susceptibility; NRM, intensity of the natural remanent magnetization. Table 2. Elastic properties of various lithologies. Displayed are average values of each lithology. Lithology Mica schist Biotite gneiss Chlorite-sericite schist Black schist Diopside-tremolite skarn Serpentinite Serpentinite-tremolite-(olivine) rock Quartz Pegmatite granite n VP0 (m s−1 ) VP (m s−1 ) VS (m s−1 ) VP /Vs ν ZP E6 kg m−2 s E GPa 98 (1218) 4 (107) 1 (18) 1 (36) 1 (35) 1 (86) (15) (5) (256) 4919 4577 5763 5577 6267 5054 6266 5830 5396 5501 5367 5636 5431 6395 5039 - 3124 2947 3464 3134 3645 2448 - 1.77 1.83 1.63 1.73 1.75 2.06 - 0.26 0.29 0.20 0.25 0.26 0.35 - 1.51 1.47 1.54 1.56 1.71 1.38 - 67.5 62.9 78.7 66.8 90.4 44.3 - n, Number of samples, in brackets is indicated the number of samples used in VP0 measurements; VP0 , P-wave velocity measured in ambient pressure using velocity instrument in GSF; VP , P-wave velocity measured under estimated crustal in-situ pressure conditions; VS , S-wave velocity measured under in-situ pressure conditions; ν, Poisson’s ratio [calculated using formula ν = ((VP /VS )2 –2)/(2((VP /VS )2 –1))]; ZP , seismic impedance [ZP = ρVP ]; E, Young’s modulus [E = (μ(3λ+2μ)/(μ+λ), μ = ρVS 2 , λ = ρVP 2 –2μ]. of unloaded mica schists and biotite gneisses is lower than what was recorded for in-situ like pressure conditions (Table 2). The density and porosity along the drilled section (Fig. 3) is nearly constant. The density as a function of velocity (Fig. 4d) shows that the local density alone cannot explain the velocity variations. However, despite the low porosity (<1 per cent in most samples) the results suggest that the velocities are very sensitive to microfracturing. For instance VP0 decreases dramatically with increasing porosity (Fig. 4e) in all rock types. Microfracturing could therefore explain the velocity differences along the drilled crustal section. This explanation is supported by the increase in VP with increasing depth (and, thus, pressure; Fig. 5) compared to the decreasing trend in VP0 as a function of depth seen in the unloaded samples. The microcracks in the OKU drilled crustal section can occur for several reasons: (i) microcracks in in-situ rocks, (ii) drilling induced microcracks or (iii) fracturing due to the stress release in rocks during the extraction of the core. A slight increase in porosity with depth within similar lithologies (Fig. 3) suggests that the fractures are not drilling induced. The decompaction of the samples (e.g. Kern et al. 2001; Kern et al. 2009) during core retrieval may, however, explain the velocity (in ambient pressure conditions) decrease with depth. This idea is supported by the observation that the recompaction during the loading process closes the microcracks, which reduces the porosity and increases the velocity (Fig. 5). This observation is common in seismic velocity studies of the upper crust (e.g. Kuusisto et al. 2006 and the references therein; Kern et al. 2009). In fact, oriented microcracks contribute largely to the seismic anisotropies and to the reduced seismic velocities in the Kola super deep core. Such oriented microcracks allow discriminating between core samples and surface rocks (Kern et al. 2001). C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Several studies determining the seismic velocities, as well as magnetic properties, have been conducted on surface rocks. In a Finnish survey Kuusisto et al. (2006) compared the velocities adopted from wide-angle velocity models with modified laboratory sonic velocities measured in surface rocks (by Christensen & Mooney 1995; Christensen 1996). Unfortunately, their focus was on the lower crust leaving the uppermost 5 km uncovered. Our studies focused on the 2.5 km crustal section, especially on the first 1.5 km. A VP less than 5600 km s−1 , a VP /VS ratio of 1.68–1.72 (Hyvönen et al. 2007), a ν of 0.13–0.37 and an E of 37–95 GPa (Hakala et al. 2007) have been reported for Finnish mica schists and biotite gneisses. In addition to the mineralogical- and microfracture-induced velocity variations, Kern et al. (2009) reported significant foliation-related velocity anisotropy, especially at low pressures, for most of the OKU lithologies. For example, the values for VP and VS as measured down-core (normal to the foliation plane) under 35 MPa (representing 1.3 km depth) using dry samples, reveal a VP of 4915 m s−1 and a VS of 3088 m s−1 for the upper schist section which are weaker than their bulk averages. This hints to structural (foliation) anisotropy (Kern et al. 2009). A VP of 4160 m s−1 and a VS of 2275 m s−1 for serpentinites have also been reported by Kern et al. (2009). All our velocity measurements were conducted normal to the foliation plane (z-axis) and are thus minimum values. In foliated rocks the directional dependence (anisotropy) of the wave velocities at low pressures is caused by microcracks, in addition to crystallographic (CPO) and shape preferred orientation (SPO) of major minerals (muscovite and biotite) and is strongly related to the structural frame (foliation, lineation; Kern et al. 2008). The velocities recorded for the upper schist series as well as for the serpentinites agree with surface data, even though they are slightly higher (especially VP ) than the data 412 Tiiu Elbra et al. –1 –1 –1 Figure 5. Seismic velocities and their ratio, impedances, Poisson’s ratio and Young’s modulus of the upper schist series and ophiolite complex under corresponding in situ pressures (diamonds). VP0 refers to velocity measurements in laboratory pressure conditions (bar chart). Blue arrows indicate the increase of VP and decrease of VP0 with depth. reported by Kern et al. (2009). This can be due to the water saturation of our samples, which increases VP , leaving VS as unaffected (e.g. Benenson et al. 2002; Popp & Kern 1994; Schön et al. 2004). The diopside-tremolite skarns exhibit, however, lower velocities than reported by Kern et al. (2009) perhaps due to mineralogical differences or sample conditions, or due to the presence of more cracks. The VP /VS , ν and E of mica schists and serpentinites, the latter featuring the highest VP /VS (2.06) and Poisson’s ratio (0.35) as well as the lowest impedance (1.38 × 106 kg m−2 s−1 ) and Young’s modulus (44.3 GPa) of the OKU drilled section, are consistent with those reported by Christensen (1996); Grasselli & Egger (2003); Wang & Ji (2009). Furthermore, the magnetic properties indicate also the consistency to the published surface data. The susceptibilities of mica schists are generally low (0–500 μSI) while the ophiolitic rocks are reported to exhibit higher values (0–100000 μSI) depending on magnetic mineralogy and its concentration, especially on the abundance of ferromagnetic pyrrhotite (Airo & Loukola-Ruskeeniemi 2004; Ruotoistenmäki & Tervo 2006). These data agree with our results (κ < 500 μSI for mica schists and biotite gneisses and κ < 6 × 10−2 SI for serpentinites), thus support the interpretation that the magnetic anomalies occur due to the occurence of ophiolites (Ruotoistenmäki & Tervo 2006). In 2001–2005 the GSF carried out wide-angle seismic profiles along four main transects with a total length of 2104 km. The effort was done within the framework of the FIRE (Kukkonen et al. 2006). The goal of the effort was to improve the understanding of the crustal structures of the central part of the Fennoscandian Shield. These FIRE sections featured a large variety of reflectors. In addition to the uppermost surface reflectors (0 ∼ 200 m), one of the most distinct upper crustal reflectors was observed in the Outokumpu area at 1.3–1.5 km depth (Fig. 6; OKU-1; Kukkonen et al. 2006). As previously described, the OKU deep drill hole, located about 400 m W from the OKU-1 line (Heikkinen et al. 2007), cuts through an ophiolitic complex, which resides at the same depth as the observed reflectors in the Outokumpu area. To lithologically and geophysically explain the reflectors, we have calculated the reflection coefficients of various pairs of lithological units down the drill core using the velocities measured under crustal conditions as described above. According to Warner (1960) and Kern et al. (2009) reflection coefficients of RC > ±0.1 are required to cause the strong seismic reflections. The reflection coefficients obtained in the current study (Table 3; Fig. 6) show that contacts between the diopside-tremolite skarns and mica schists (RC = 0.126; contact at 1325.4 m), diopside-tremolite skarns and serpentinites C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Laboratory seismic data of the Outokumpu core 413 Figure 6. Left: Finnish seismic reflection profile (OKU-1; marked as yellow line on FIRE 3 transect in Fig. 1a) with approximate location of OKU deep drill core (adopted from Heikkinen et al. 2007). Right: reflection coefficients obtained in this study. Red diamonds denote the RC calculated from lithology means (presented also in Table 3). Table 3. Reflection coefficients for each pair of two lithological units. Lithology [1] Mica schist [2] Biotite gneiss [3] Chlorite-sericite schist [4] Black schist [5] Diopside-tremolite skarn [6] Serpentinite [7] Pegmatite granite [1] 0.003 −0.01 −0.014 – – 0.023 [2] [3] −0.003 0.01 0.012 – – – – 0.02 – – – – [4] 0.014 – – −0.112 – – [5] [6] [7] 0.126 – – 0.112 − – – −0.08 −0.19 −0.023 −0.02 – – – – 0.19 – – Coefficients are calculated from lithology means using formula RC = (ρ 2 VP2 -ρ 1 VP1 )/(ρ 2 VP2 +ρ 1 VP1 ); numbers in [] refer to rock type; ‘–’ lithological contact not met in the drill core (based on Västi 2005). (RC = ±0.19; within 1332–1493.15 m) as well as between diopsidetremolite skarns and black schists (RC = ±0.112; 1451.5–1514.3 m) can produce the strong reflections as observed in the FIRE profile. 7 C O N C LU S I O N S The results indicate that various lithologies can be distinguished by their petrophysical data. Weak depth dependence in seismic P-wave velocity, density and porosity is observed. The seismic velocity data shows large variations, whereas the density and the porosity are nearly constant along the core. The magnetic susceptibilities and remanence distinguish between the magnetically weak schist series rocks and occasionally ferromagnetic rocks in the ophiolite series. The laboratory measurements indicate that the seismic velocity is significantly affected by microfracturing, which is the main cause of the observed velocity variations. These variations are reduced when applying upper crustal pressures. Nevertheless, the velocities are still minimum values due to fabric and structural anisotropies. The measured velocities provide estimates for the reflection coefficients for major lithological boundaries. These data indicate that the strong and distinct reflections visible in the FIRE wide-angle reflection data within the 1325.4–1514.3 m interval are caused by contacts of diopside-tremolite skarn into serpentinites, into black schist or into mica schist. C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International AC K N OW L E D G M E N T S We thank the staff of the petrophysical laboratory of GSF in Espoo (especially Mrs. Satu Vuoriainen) for preparing the samples and for allowing us to use the VP0 -velocity instrument. We also acknowledge Dr. Ilmo Kukkonen for his help and Mr. Robert Klein for language checking. The authors thank the Outokumpu Oyj and the K.H. Renlund Foundations for financial support to construct the new ultrasonic device for measuring the P-wave and S-wave velocities under in situ like conditions. The reviewers, Prof. H. Kern, Dr. D. Brown and N.N. are thanked for constructive reviews, which significantly improved the paper. REFERENCES Airo, M-L. et al., 2007. Petrophysical laboratory measurements of Outokumpu Deep Drill Core samples, in Outokumpu Deep Drilling Project, pp. 35–40, ed. Kukkonen, I.T. Second International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Espoo Marine Geology and Geophysics, Report Q10.2/2007/29. Airo, M.-L. & Loukola-Ruskeeniemi, K., 2004. Characterization of sulfide deposits by airborne magnetic and gamma-ray responses in eastern Finland, in Ores and Organic Matter, pp. 67–84, ed. Coveney, R. M. & Pasava, J., Ore Geol. Rev., 24, 67–84. Benenson, W., Harris, J.W., Stcker, H. & Lutz, H., eds., 2002. Handbook of Physics, Springer, New York. 1248 pp. 414 Tiiu Elbra et al. Brown, C., Llana-Funez, S., Carbonell, R., Alvarez-Marron, J., Marti, D., & Salisbury, M., 2009. Laboratory measurements of P-wave and S-wave velocities across a surface analog of the continental crust–mantle boundary: Cabo Ortegal, Spain. Earth planet. Sci. Lett., 285, 27–38. Christensen, N.I., 1996. Poisson’s ratio and crustal seismology. J. geophys. Res., 101, 3139–3156. Christensen, N.I. & Mooney, W.D., 1995. Seismic velocity structure and composition of the continental crust: a global view, J. geophys. Res., 100, 9761–9788. Elbra, T., Kontny, A. & Pesonen, L.J., 2009. Rock-magnetic properties of the ICDP-USGS Eyreville core, Chesapeake Bay impact structure, Virginia, USA, in Deep Drilling in the Chesapeake Bay Impact Structure, pp. 119–135, eds. Gohn, G.S., Koeberl, C., Miller, K.G., & Reimold, W.U., Geol. Soc. Am. Special Paper, 458, Boulder, CO. Grasselli, G. & Egger, P., 2003. Constitutive law for the shear strength of rock joints based on three-dimensional surface parameters, Int. J. Rock Mech. Min. Sci., 40, 25–40. Hakala, M., Kuula, H. & Hudson, J.A., 2007. Estimating the transversely isotropic elastic intact rock properties for in situ stress measurement data reduction: a case study of the Olkiluoto mica gneiss, Finland, Int. J. Rock Mech. Min. Sci., 44, 14–46. Heikkinen, P.J., Koivisto, E. & Kukkonen, I.T., 2007. FIRE high resolution seismic survey in Outokumpu, in Outokumpu Deep Drilling Project, pp. 17–20, ed. Kukkonen, I.T., Second International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Marine Geology and Geophysics, Report Q10.2/2007/29. Hyvönen, T., Tiira, T., Korja, A., Heikkinen, P., Rautioaho, E. & the SVEKALAPKO Seismic Tomography Working Group, 2007. A tomographic crustal velocity model of the central Fennoscandian shield, Geophys. J. Int., 168, 1210–1226. Karlqvist, R., 2009. Probing the earth: determining longitudinal and shear ultrasound velocities in Outokumpu deep drill core and FIRE profile samples, MSc thesis, University of Helsinki, Finland. Kern, H., 1993. P- and S-wave anisotropy and shear-wave splitting at pressure and temperature in possible mantle rocks and their relation to the rock fabric. Phys. Earth Planet. Inter., 78(3–4), 245–256. Kern, H., Ivankina, T.I., Nikitin, A.N., Lokajı́ček, T. & Prosc, Z., 2008. The effect of oriented microcracks and crystallographic and shape preferred orientation on bulk elastic anisotropy of a foliated biotite gneiss from Outokumpu, Tectonophysics, 457(3–4), 143–149. Kern, H., Mengel, K., Strauss, K., Ivankina, T., Nikitin, A. & Kukkonen, I., 2009. Elastic wave velocities, chemistry and modal mineralogy of crustal rocks sampled by the Outokumpu scientific drill hole: evidence from lab measurements and modeling, Phys. Earth Planet. Inter., 175(3–4), 151–166. Kern, H., Popp, T., Gorbatsevich, F., Zharikov, A., Lobanov, K.V. & Smirnov, Yu.P., 2001. 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, Tectonophysics, 338, 113–134. Koistinen, T.J., 1981. Structural evolution of an early Proterozoic stratabound Cu-Co-Zn deposit, Outokumpu, Finland, Trans. R. Soc. Edinburgh: Earth Sci., 72, 115–158. Korja A., Lahtinen R., Heikkinen P., Kukkonen I.T. & FIRE Working Group, 2006. A geological interpretation of the upper crust along FIRE 1, Geol. Surv. Finland, Spec. Pap., 43, 45–76. Kukkonen, I.T., Heikkinen, P., Ekdahl, E., Hjelt, S.-E., Yliniemi, J., Jalkanen, E. & FIRE Working Group, 2006. Acquisition and geophysical characteristics of reflection seismic data on FIRE transects, Fennoscandian Shield, Geol. Surv. Finland Spec. Pap., 43, 13–43. Kukkonen, I.T., Kivekäs, L., Safanda, J. & Cermak, V., 2007b. Geothermal studies of the Outokumpu deep drill hole, in Outokumpu Deep Drilling Project, pp. 81–86, ed. Kukkonen, I.T., Second International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Marine Geology and Geophysics, Report Q10.2/2007/29. Kukkonen, I.T., Kuusisto, M., Lehtonen, M. & Peltonen, P., 2008. Delamination of eclogitized lower crust: control on the crust–mantle boundary in the central Fennoscandian shield, Tectonophysics, 457, 111–127. Kukkonen, I.T. & the Outokumpu Deep Drilling Working Group, 2007a. Outokumpu deep drilling project – Introduction of geology and geophysics of the deep hole and research within the project, in Outokumpu Deep Drilling Project, pp. 11–16, ed. Kukkonen, I.T., Second International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Marine Geology and Geophysics, Report Q10.2/2007/29. Kukkonen, I.T. & the Outokumpu Deep Drilling Working Group, 2009. Outokumpu deep drilling project – Introduction of geology and geophysics of the deep hole and research within the project, in Outokumpu Deep Drilling Project, pp. 11–16, ed. Kukkonen, I.T., Third International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Marine Geology and Geophysics, Report Q10.2/2009/61. Kukkonen, I. T. & Safanda, J., 1996. Palaeoclimate and structure: the most important factors controlling subsurface temperatures in crystalline rocks. A case history from Outokumpu, eastern Finland, Geophys. J. Int., 126, 101–112. Kuusisto, M., Kukkonen, I.T., Heikkinen, P. & Pesonen, L.J., 2006. Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data, Tectonophysics, 420, 283–299. Lassila, I., Karlqvist, R., Elbra, T., Gates, F., Pesonen, L.J. & Hæggström, E., 2010. Ultrasonic velocity of the upper gneiss series rocks from the Outokumpu deep drill hole – comparing uniaxial to triaxial loading. J. Appl. Geophys., in press, doi:10.1016/j.jappego.2010.09.001. Loukola-Ruskeeniemi, K., 1999. Origin of black shales and the serpentiniteassociated Cu-Zn-Co ores at Outokumpu, Finland, Econ. Geol., 94, 1007–1028. Papunen, H., 1987. Outokumpu-type ores, in Otto Trüstedt Symposium, pp. 41–50, ed. Häkli, T.A., Geological Survey of Finland Special Paper, 1, Finland. Park, A.F., 1988. Nature of the early Proterozoic Outokumpu assemblage, eastern Finland, Precamb. Res., 38(2), 131–146. Popp, T. & Kern, H., 1994. The influence of dry and water saturated cracks on seismic velocities of crustal rocks – a comparison of experimental data with theoretical model. Surv. Geophys., 15, 443–465. Powars, D.S., Catchings, R.D., Goldman, M.R., Gohn, G.S., Horton, J.W., Edwards, L.E., Rymer, M.J. & Gandhok, G., 2009. High-resolution seismic-reflection images across the ICDP-USGS Eyreville deep drilling site, Chesapeake Bay impact structure, in Deep Drilling in the Chesapeake Bay Impact Structure, pp. 209–233, eds. Gohn, G.S., Koeberl, C., Miller, K.G., & Reimold, W.U., Geol. Soc. Am. Special Paper, 458, Boulder, CO. Ruotoistenmäki, T. & Tervo, T., 2006. Geophysical characteristics of Outokumpu area, SE Finland, Geol. Surv. Finland, Report of Investigation, 162, 37 p. Schijns, H., Meillieux, D., Schmitt, D.R., Bianco, E., Welz, M., Kukkonen, I.T., Heikkinen, P., Sun, F. & Milkereit, B., 2007. Overview of a high resolution VSP survey in the international continental drilling program Outokumpu borehole, Finland: Preliminary results, in Outokumpu Deep Drilling Project, pp. 21–26, ed. Kukkonen, I.T., Second International Workshop, Programme and Extended Abstracts, Geological Survey of Finland, Southern Finland Office, Marine Geology and Geophysics, Report Q10.2/2007/29. Schön, J.H., 2004. Physical properties of rocks: Fundamentals and principles of Petrophysics, in Handbook of Geophysical Exploration. Seismic Exploration 18, 583 p, eds. Helbig, K., & Treitel, S., Elsevier, Amsterdam. Sorjonen-Ward, P., 2006. Geological and structural framework and preliminary interpretation of the FIRE 3 and FIRE 3A reflection seismic profiles, central Finland. Geol. Surv. Finland, Spec. Pap., 43, 105–159. Sorjonen-Ward, P., 1997. An overview of the geological and tectonic evolution of eastern Finland, in Research and Exploration – Where Do They Meet?, pp. 11–22, eds Loukola-Ruskeeniemi, K. & Sorjonen-Ward, P., Geological Survey of Finland, Report 42, Finland. Sorjonen-Ward, P. & Luukkonen, E.J, 2005. Archean rocks, in Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield, pp. 19–99, eds. Lehtinen, M., Nurmi, P.A. & Rämö, O.T., Elsevier Science B.V., Amsterdam. C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International Laboratory seismic data of the Outokumpu core Tyni, M., Parkkinen, J., Mäkelä, M., Pekkarinen, L., Loukola-Ruskeeniemi, K., Kuronen, E. & Tuokko, I., 1997. Outokumpu district. in Research and Exploration – Where Do They Meet?, pp. 23–41, eds LoukolaRuskeeniemi, K. & Sorjonen-Ward, P., Excursion guidebook A4: ore deposits in eastern Finland. Geological Survey of Finland, Report 42, Finland. Vajdova, V., Přikryl, R., Pros, Z. & Klı́ma, K., 1999. The effect of rock fabric on P-wave velocity distribution in amphibolites, Phys. Earth Planet. Inter., 114, 39–47. Vermeesch, P.M. & Morgan, J.V., 2004. Chicxulub central crater struc- C 2010 The Authors, GJI, 184, 405–415 C 2010 RAS Geophysical Journal International 415 ture: initial results from physical property measurements and combined velocity and gravity modeling, Met. Plan. Sci., 39(7), 1019– 1034. Västi, K., 2005. Drill core report M52.5/4222/04/R2500. Geological Survey of Finland. Wang, Q. & Ji, S., 2009. Poisson’s ratios of crystalline rocks as a function of hydrostatic confining pressure, J. geophys. Res., 114, B09202, doi:10.1029/2008JB006167. Warner, M., 1960. Absolute reflection coefficients from deep seismic reflections, Tectonophysics, 173, 15–23.