Combined gas permeability and P and S wave velocity measurements were carried out on rock salt samples from the Gorleben salt dome and the Morsleben salt mine under hydrostatic and triaxial loading condions, mostly at room temperature. Permeabilities in the as‐received samples vary between 10 −16 and 2×10 −20 m 2 . The initial permeability is primarily due to decompaction induced by drilling, core retrieval and sample preparation. Hydrostatic loading gives rise to a marked decrease of permeability and a coeval significant increase of P and S wave velocities due to progressive closure of grain boundary cracks, tending to approach the in situ matrix permeability (<10 −20 m 2 ). The pore space sensitivity of P and S wave velocities is used to monitor the in situ state of the microstructure. Their reversals define the boundary in the state of stresses between dilatant and compactive domains (dilatancy boundary). Dilatancy during triaxial deformation of the compacted rock salt samples is found to evolve stress dependent in various stages. The crack initiation stress increases from ∼18 MPa differential stress at 10 MPa confining pressure to ∼30 MPa at confining pressures above ∼70 MPa. Dilatancy is due to the opening of grain boundary and (100) cleavage cracks and depends on the applied confining pressure. The orientation of the open cracks is primarily controlled by the loading geometry system (compression, extension). As a consequence, permeability increases dramatically with progressive dilatancy, followed by a period of plus/minus constant permeability during strain hardening up to 10% axial strain or even more. This suggests that the evolution of permeability is not only a function of dilatancy but also of microcrack linkage. Importantly, the anisotropic crack array within the samples causes a strong directional dependence of permeability.
Granulites from the Neogene xenolith‐bearing Hannuoba alkaline basalt and from the Manjinggou‐Wayaokou exposed lower crustal section in the Archean Huai 'an terrain, which occurs within and surrounds the Hannuoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel Iherzolite xenolith as well as nine granulites and granulite‐facies metasedimentary rocks from the Archean Huai 'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600°C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single‐crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated V p at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou‐Wayaokou section and the adjacent Wutai‐Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured V p by up to 15% relative to calculated velocities, even if decompression‐induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high‐grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies.
P ‐wave velocity measurements on spherical sample ( P conf ≤ 200 MPa; T = 20°C) and P ‐ and S ‐wave velocity measurements on cube ( P conf ≤ 612 MPa; T ≤ 600°C) were carried out. It will be demonstrated that combination of both these data sets increases the accuracy in the determination of the elastic moduli of a sample. The elastic moduli can be calculated to maximum confining pressure, although the data for P conf > 200 MPa are restricted to three orthogonal directions. Even extrapolation of the data beyond the measuring range appears to be possible, if reliability of the relation used for extrapolation can be ensured.
Abstract Der Arbeitskreis AK 3.3 „Versuchstechnik Fels“ der Deutschen Gesellschaft für Geotechnik e. V. (DGGT) erarbeitet Empfehlungen für felsmechanische Labor‐ und Feldversuche sowie Messungen im Gebirge und an geotechnischen Bauwerken. Die vorliegende Empfehlung Nr. 25 behandelt die Bestimmung von mineralogisch‐petrographischen Kennwerten an Festgesteinsproben zum Zwecke der Abrasivitätsbewertung. Es werden Anforderungen an Prüfeinrichtungen und Probekörper sowie Vorgehensweisen für die Durchführung und Auswertung von Dünnschliff‐ und Pulverröntgendiffraktometeranalysen sowie für die Ableitung verschiedener Indexwerte zur Abrasivitätsbewertung festgelegt. Bei der petrographischen Analyse werden Informationen über Art und Anteile des Mineralinhalts sowie Gefügeeigenschaften eines Gesteins erhoben. Diese Informationen können entweder direkt, z. B. als Quarzgehalt, oder in Form abgeleiteter Indexwerte, z. B. Äquivalenter Quarzgehalt, Schimazek‐Verschleißindex oder Rock Abrasivity Index, zur Charakterisierung der Abrasivität herangezogen werden. In dieser Empfehlung werden der Zweck, die Begriffe, die Prüfeinrichtung, die Anforderungen an den Probekörper und die Versuchsdurchführung und ‐auswertung behandelt.
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