T. Lokajíček

Laboratory Approach to the Study of Elastic Anisotropy on Rock Samples

The experimental approach (hardware and software) to the study of the elastic anisotropy of rocks on spherical samples under hydrostatic pressure up to 400 MPa is discussed. A substantial innovation of the existing measuring system and processing methods enabled us to make a detailed investigation and evaluation of the kinematic as well as dynamic parameters of elastic waves propagating through anisotropic media. The innovation is based on digital recording of the wave pattern with a high sampling density of both time and amplitude. Several options and results obtained with the innovated laboratory equipment are presented.

Experimental weathering of marlstone from Přednı́ Kopanina (Czech Republic)—historical building stone of Prague

Abstract Romanesque builders favoured marlstones because of their easy workability. Unfortunately, this rock rapidly deteriorates when exposed to the external atmospheric conditions. Artificial weathering experiments were conducted on marlstone from Předni Kopanina quarry near Prague (Czech Republic) in order to determine: (A) which weathering factor presents major danger to this rock type, and (B) which of the test methods provide the most reliable measure of the intensity of rock disintegration. Rock specimens prepared from fresh rock were subjected to standardized freeze–thaw cycles and salt crystallization tests, and to the accelerated weathering in climatic chamber using SO 2 and a combination of freeze–thaw and SO 2 cycles. Stone properties were evaluated by non-destructive and destructive techniques including ultrasonic velocity measurements, determination of water uptake by capillary action, mechanical testing, detailed porosimetric analysis and microscopic study.

Direct measurement of 3D elastic anisotropy on rocks from the Ivrea zone (Southern Alps, NW Italy)

Abstract Lower crustal and upper mantle rocks exposed at the earths surface present direct possibility to measure their physical properties that must be, in other cases, interpreted using indirect methods. The results of these direct measurements can be then used for the corrections of models based on the indirect data. Elastic properties are among the most important parameters studied in geophysics and employed in many fields of earth sciences. In laboratory, dynamic elastic properties are commonly tested in three mutually perpendicular directions. The spatial distribution of P- and S-wave velocities are then computed using textural data, modal composition, density and elastic constants. During such computation, it is virtually impossible to involve all microfabric parameters like different types of microcracking, micropores, mineral alteration or quality of grain boundaries. In this study, complete 3D ultrasonic transmission of spherical samples in 132 independent directions at several levels of confining pressure up to 400 MPa has been employed for study of selected mafic and ultrabasic rocks sampled in and nearby Balmuccia ultrabasic massif (Ivrea zone, Southern Alps, NW Italy). This method revealed large directional variance of maximum P-wave velocity and different symmetries (orthorhombic vs. transversal isotropic) of elastic waves 3D distribution that has not been recorded on these rocks before. Moreover, one dunite sample exhibits P-wave velocity approaching to that of olivine single crystal being interpreted as influence of CPO.

Elastic Parameters of West Bohemian Granites under Hydrostatic Pressure

The West Bohemian seismoactive region is situated near the contact of the Moldanubian, Bohemian and Saxothuringian units in which a large volume is occupied by granitoid massifs. The spatial distribution of P-wave velocities and the rock fabric of five representative samples from these massifs were studied. The P-wave velocities were measured on spherical samples in 132 independent directions under hydrostatic pressure up to 400 MPa, using the pulse-transmission method. The pressure of 400 MPa corresponds to a depth of about 15 km in the area under study. The changes of P-wave velocity were correlated with the preferred orientations of the main rock fabric elements, i.e., rock forming minerals and microcracks. The values of the P-wave velocity from laboratory measurements on granite samples fit the velocity model used by seismologists in the West Bohemian seismoactive region.

Experimentally Determined P-Wave Velocity Anisotropy for Rocks Related to the Western Bohemia Seismoactive Region

The elastic properties of granites from Western Bohemia which we measured and published earlier have been supplemented with measurements of olivine nephelinite from the same region and with data on lherzolitic xenolith from the vicinity of Železný Brod. The set of velocities measured under laboratory conditions has been compared with depth profiles suggested for the purpose of locating seismic swarm events which occur in this region. P-wave velocities were measured under pressures of up to 400 MPa. This pressure corresponds to depths of about 15 km for this region. The data were extrapolated for larger depths. Comparing laboratory measurements and seismic profiles, we can conclude that the rocks under study may be constituents of crust structures.

Contact conditions on seismoactive faults

Square plate models with a diagonally located slit in compression were studied photoelastically. The resulting stress field surrounding the discontinuity (slit) is evaluated before and after seismoactive (seismic-energy-releasing) shear displacement. The mechanism of the fast shear movement (stick-slip), including its radiation properties, in interpreted. The results confirm the existence of a central locked zone in the loaded slit, the limits of which coincide with the initiation points of stick-slip movements. The zone is interpreted as the source of the seismic energy release. The complementary measurements (direct optical and ultrasonic) are presented to verify the results of the photoelastic analysis. The results obtained are discussed in regard to the conclusions that follow from the theory of elasticity.

Propagation velocity and radiation properties of induced tensile cracks

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Acoustic emission of quasi-isotropic rock samples initiated by temperature gradients

The influence of temperature gradients and heating rate on crack formation in quasi-isotropic samples of marble and sandstone is studied using acoustic emission records. The behavior of velocities and attenuation of elastic waves in the samples is also examined. The crystallographic textures of marble and sandstone are measured by the neutron diffraction method, and it is shown that temperature gradients of up to 52°C/cm do not lead to a change in the textures. It is established that the temperature dependence of acoustic activity in the sandstone samples strongly depends on the internal heating rate. The maximum acoustic activity of the sandstone samples was observed at maximum temperature gradients in the samples. The temperature dependence of acoustic activity in marble differs significantly from that in sandstone. Maximums of acoustic activity in the marble samples are observed at temperatures of 30–60 and higher than 200°C. A decrease in acoustic activity was recorded in the interval between these temperatures. Such behavior suggests a predominant role of the surface structure and the concentration of surface defects in the process of crack formation in marble. During cooling, all samples demonstrate similar behavior: maximum acoustic activity is observed at the switching-off of the heating element and the emission activities of different samples are comparable in intensity.

Non-linearity in multidirectional P-wave velocity: confining pressure behaviour based on real 3D laboratory measurements, and its mathematical approximation

Abstract Experimental laboratory measurements of P-wave velocity confirm the superposition of linearity over non-linearity by a progressive increase in confining pressure. The increase in confining pressure diminishes the influence of microcracks that are partly or totally closed. At a certain stress level, the trend of P-wave velocity with applied confining pressure approaches that of a solid without cracks, and a linear increase in elastic wave velocity occurs under high confinement. Several studies have focused on the problem of mathematical approximation of this phenomenon (Carlson & Gangi 1985; Wepfer & Christensen 1991; Greenfield & Graham 1996; Meglis et al. 1996). Although successful within a certain degree of error, they provide neither a multidirectional solution nor the comparison of results with rock fabric. In this study, an analytical relation was applied to describe the P-wave velocity-confining pressure behaviour of quasi-isotropic rocks (granites) and their anisotropic equivalents (orthogneisses). Two parameters of this relationship reflect the elastic properties of the rock matrix, and two others are related to the presence of microcracks, their density and genesis. The results of a mathematical approximation of the P-wave velocity-confining pressure behaviour show a favourable correlation to the measured data-sets. Comparison of individual fitted parameters with the rock fabric provides an improved understanding of the material’s mechanical behaviour.

Analysis of Acoustic Emission from Rock Samples Loaded to Long-Term Strength Limit

As shown in the previous papers by Rudajev,1, 2 the parameters of the statistical distribution of the acoustic emission (AE) are not random, although the origin of the acoustic signal is random in time. In the above papers it was shown that the parameters of distribution cohere with the strain stage of the loaded samples. The object of this paper is the analysis of the statistical distribution of AE, of the relation of their parameters to the process of fracturing, and mainly determining the probability of total fracture.