Petrophysical properties of the 9‐km‐deep crustal section at KTB

Abstract

Abstract. Petrophysical properties of drill core and drill cuttings samples from both bore holes of the German Continental Deep Drilling Program (KTB) measured at atmospheric pressure and room temperature in the field laboratory are presented, along with data of core samples measured at simulated in situ conditions by other laboratories. Most of the petrophysical properties show a bimodal frequency distribution corresponding to the two main lithologies (gneiss and metabasite), except electrical resitivity and Th/U ratio which are lithology independent (monomodal distribution). Low resistivities are mainly associated with fractures zones enriched in fluids and graphite. The most abundant ferrimagnetic mineral is monoclinic pyrrhotite. Below 8600 m, hexagonal pyrrhotite with a Curie temperature of 260øC is the stable phase. Thus the Curie isotherm of the predominant pyrrhotite was reached (bottom hole temperature about 265øC). The highest values of magnetic susceptibility are linked with magnetite. Microcracks grow due to pressure and temperature release during core uplift. This process continues after recovery and is documented by the anelastic strain relaxation and acoustic emissions. The crystalline rocks exhibit marked reversible hydration swelling. Anisotropy of electrical resistivity, permeability, P and $ wave velocity is reduced significantly by applying confining pressure, due to closing of microcracks. Fluids within the microcracks also reduce the P wave velocity anisotropy and P wave attenuation. Anisotropy and shear wave splitting observed in the field seismic experiments is caused by the foliation of rocks, as confirmed by laboratory measurements under simulated in situ conditions. The petrophysical studies provide evidence that microfracturing has an important influence on many physical rock properties in situ.

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).…”

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

“…Representative bulk anisotropic P-wave velocities of the upper crust were derived from this seismic data and compared with predictions of velocity and anisotropy based on petrophysical laboratory measurements and geological information. Petrophysical properties of the drill core and drill cuttings, samples from both bore holes of the German Continental Deep Drilling Program (KTB) measured at atmospheric pressure and room temperature in the field laboratory are presented in (Berckhemer et al, 1997). Anisotropy and shear wave splitting observed in the field seismic experiments are caused by the foliation of rocks, as confirmed by laboratory measurements under simulated in situ conditions.…”

“…The rocks underwent two metamorphic cycles that resulted in strongly anisotropic schistose rocks. The investigation results of the petrophysical properties and elastic anisotropy are shown in (Berckhemer et al, 1997;Okaya et al, 2004).…”

Why no anomaly is visible over most of the continent–ocean boundary in the global crustal magnetic field

“…The continental deep drilling program in Germany (KTB) shows a dominance of pyrrhotite ferromagnetic minerals in metabasites and gneisses at shallow depths (<7800 m) while magnetite is dominant in marble-and calcsilicate-bearing amphibolites and meta-ultramafics from the depth range 7320-7800 m (Berckhemer et al, 1997). Unfortunately, most of the KTB rocks are carriers of unstable natural remanent magnetisation (NRM) (Pohl et al, 1991;Worm and Rolf, 1994), which are of chemical or thermochemical origin (Berckhemer et al, 1997).…”

“…Unfortunately, most of the KTB rocks are carriers of unstable natural remanent magnetisation (NRM) (Pohl et al, 1991;Worm and Rolf, 1994), which are of chemical or thermochemical origin (Berckhemer et al, 1997). These remanences do not contribute significantly, compared with the induced magnetisation.…”