Large Rock Samples Research Articles

Thermoelastic instability on a frictional surface and its implication for size effect in friction experiments

Recent laboratory friction experiments on large rock samples revealed that dynamic weakening, a remarkable reduction in the friction coefficient at elevated slip rates, occurs at lower slip rates in larger samples. There is a large difference between the sizes of natural faults and those in laboratory experiments. Therefore, it is crucial to understand the effect of size on rock friction. In the field of tribology, the interaction between frictional heating and thermoelastic effect has long been investigated. It was shown that higher slip rates than the critical value {V}_{mathrm{cr}} causes growth of temperature and normal stress heterogeneity (thermoelastic instability), and {V}_{mathrm{cr}} is proportional to the wavenumber of the heterogeneity. Severely heterogeneous normal stress may cause concentration of frictional power, thus locally activating dynamic weakening and leading to macroscopic weakening. Because a larger sample hosts a perturbation of a smaller wavenumber, it is expected to weaken at a lower slip rate than a smaller sample. In this study, a new numerical method was developed for analysis of thermoelastic instability based on the definition of memory variables and numerical approximation to the integration kernel, for the 2-dimensional problem of a planar fault embedded in an infinite medium. This method was advantageous over the standard integral equation method in terms of numerical costs. Numerical solutions with the new method on sinusoidal perturbations in the normal stress were compared with previously derived steady-state solution and its stability for validation. The typical thermoelastic properties of gabbro yield {V}_{mathrm{cr}} in a range of experimentally adopted slip rates, indicating that the thermoelastic effect may play an important role in high-velocity friction experiments. Because the temperature rise and the resulting normal stress change smear out after the friction experiments, measurement of the temperature distribution in a sample during a friction experiment is important for further understanding the dynamic weakening and scale effect of rock friction.Graphical

Numerical simulation of size effect of defective rock under compression condition

The existence of various types of damage, small cracks, some large voids and the size of the sample in the rock will make the experimental results show great discreteness. In this paper, based on the results of laboratory experiments, a numerical model of large flawed rock samples is established by using particle flow software PFC2D, and the mechanical response of rocks with different length-diameter ratios and different flaw positions in uniaxial compression experiments is discussed. The results show that the specimen size has a significant effect on the crack characteristics, mechanical characteristics and energy characteristics of rock mass. From the perspective of energy and crack characteristics, the total number of cracks after the failure of the defective rock sample is slightly lower than that of the intact rock sample, resulting in a slightly lower peak strain energy during the rock failure process. From the mechanical properties of rock samples, the Poisson’s ratio of intact rock samples is slightly smaller than that of defective rock samples. The strength of the defective sample is weakened relative to the complete rock sample, and the relationship formula between the weakening range and the aspect ratio is obtained through analysis. Moreover, different defect locations lead to different crack processes and crack modes, resulting in different uniaxial compressive strength.

Investigating the potential of rock surface burial dating using IRPL and IRSL imaging

Techniques for spatially resolved measurements of infrared-stimulated luminescence (IRSL) and novel Infrared-photoluminescence (IRPL) emissions have recently been developed for applications of rock surface dating. Such spatially resolved measurements overcome the need for separating out mineral fractions, speed-up sample preparation and measurement times, and data can be quickly processed provide high-resolution luminescence-depth profiles. Here, we investigate the potential of using spatially resolved IRPL and IRSL measurements for rock surface burial dating using two large (cm-scale) rock samples with controlled exposure and surface dose histories. We use a SAR style measurement protocol, with a test-dose normalisation step to monitor sensitivity changes, a preheat to remove unstable charges and a bleaching step to reset the IRPL signal. Through establishing the response of IRPL and IRSL to dose, we are able to construct 2D maps of equivalent doses (Des) for each sample. The results here indicate that spatially resolving IRSL and IRPL from large rock samples has the potential to be used for rock surface burial dating and offers a means to investigate the spatial distribution of dose and mineral-dependent sensitivity changes through cm-scale rock samples.

Rapid in situ assessment of luminescence-bleaching depths for deriving burial and exposure chronologies of rock surfaces

Recent developments in luminescence dating offer new ways to date exposure and burial durations of rocks. The new rock surface dating methods ideally require high-resolution data, faster sample preparation and measurement times, and field screening methods to select samples with appropriate luminescence characteristics and bleaching histories. Presented here is a demonstration of an EMCCD (electron multiplying charge coupled device) based system capable of imaging high-resolution infrared stimulated luminescence (IRSL) and infrared-photoluminescence (IRPL) from rock samples. The IRPL can be detected at both 880 nm and 955 nm. Using this instrument, the entire luminescence-depth profile can be reconstructed by imaging a single surface cut perpendicular to the exposed rock face. We demonstrate the possibility of reconstructing luminescence-depth profiles suitable for rock surface dating from large (cm-scale) rock samples, without using a regeneration dose for normalisation of the natural luminescence signals. Based on the different bleaching characteristics of the IRSL and IRPL emissions at 880 nm and 955 nm, we show that it is possible to gain reliable estimates of bleaching depths from measurement of as few as two images of the IRPL signal (one for each emission), or from measurement of the IRSL decay curve. We thus by-pass laborious sample preparations and the need for a gamma source to estimate the bleaching depth, thereby extending the 2D luminescence-depth profile imaging technique to other laboratories that lack access to ionising radiation facilities. This study also makes a significant progression towards development of a field instrument for in situ relative exposure dating, and sample screening for rock surface burial dating. • Direct imaging of luminescence-depth profiles in rocks using IRPL or IRSL. • New methods for normalising spatial variations in luminescence without irradiation. • Characterising bleachability of different OSL emissions from the principal trap. • Rock surface dating for rapid field analysis or laboratories lacking irradiation facilities.

Microfacies analysis and 3D reconstruction of bioturbated sediments in the calcarenite di Gravina formation (southern Italy)

The Calcarenite di Gravina is a poorly lithified, heterogeneous bio-lithoclastic Plio-Pleistocene limestone formed in a temperate shallow-water setting. Since prehistoric times it has been used for construction, as witnessed by the “Sassi”, the old city center of Matera (southern Italy). Here, the relatively distal, fine-grained bioclastic grain-packstone is characterized by large bioturbation structures of crustaceans. The aim of this work is to understand how bioturbation controls texture, porosity, permeability, and the cementation of the calcarenite. Crustaceans such as Callianassa are known to produce vertical burrows and sort grains. A bioturbated calcarenite level, outcropping in the Parco Scultura- Cava Paradiso in La Palomba (Matera northern area), has been selected as case study. Large rock samples including fossil traces and surrounding rocks have been analysed. Optical microscope and SEM observations show a close connection between sediment sorting and cementation, and 3D analyses by means of CT scans indicate that the internal structure and the connected porosity of the samples is a function of the cementation. Moreover, the porosity network is reconstructed in 3D in order to derive the real connected porosity variations and their implications for the evolution in terms of diagenesis and alteration of the rocks. Indeed, a cementation gradient from the external lining of the burrows to the internal part of the trace fossil is observed. Cementation is less pronounced outside of the trace fossil.Cluster analysis of point-counting results allows to identify four sub-facies characterized by differences in composition, cementation and porosity. One of them is present in the interior of the burrows while the other two characterize the external lining (crust) and storage chamber of crustacean burrows. The last sub-facies occurs in the area surrounding the trace fossils and shows a weaker cementation. The presence of a cementation gradient inside the trace fossil is explained through a diagenetic model that considers the contribution of different factors. The geochemical environments allow the CaCO3 precipitation inside the burrows starting from the lining to the internal area thanks to the microbial actions, water circulation and relative oxygenation of fluids. Dissolution of aragonitic components probably represents the principal source of CaCO3. Variation in cementation and porosity are quite moderate, but capable of generating major differences in the resistance to weathering. From this study it emerges how the diagenetic processes lead to significant changes in the bioturbated fabric, making the burrows less permeable than the surrounding sediment. This is confirmed by the selective erosion that brings to light very well-preserved traces of Callianassidae even on building stones.

Shale gas permeability upscaling from the pore-scale

The effective permeability of large shale samples provides useful insights into shale gas production. However, its determination can only be achieved through upscaling, since the direct pore-scale simulation of gas flows in large rock samples is not feasible due to the high computational cost and absence of pore connectivity in sample images. Although the Brinkman formulation is widely used in the permeability upscaling of conventional rocks, how to choose the effective viscosity in this coarse-scale model is not clear. Moreover, its application in shale rocks, where the rarefaction effects are important so that the conventional Navier–Stokes equations are inadequate, is rare, and its accuracy has not been assessed. This study aims to address the above two problems, by comparing the Brinkman solutions of several two-dimensional and three-dimensional random porous media containing fractures with the fine-scale solutions of the Stokes and Boltzmann equations (for continuum and rarefied gas flows, respectively). It is found that the use of the fluid viscosity in the Brinkman model, instead of the controversial effective viscosity, leads to accurate results for the cases considered. Additionally, the macroscopic quantity in rarefied gas flows in shale rocks is well predicted by the Brinkman model for a wide range of gas rarefaction, since the error is found to be less than 7%, while neglecting the rarefaction effects leads to significant underestimation of effective permeability (up to 90% in the cases studied). Although heterogeneity and anisotropy of the porous medium increase the error of the effective permeability derived from the Brinkman model, generally speaking, the effective permeability extracted from this coarse-scale model compares favorably to its fine-scale counterpart.

Quantifid joint surface description and joint shear strength of small rock samples

Geotechnical structures in rock masses such as tunnels, underground caverns, dam foundations and rock slopes often have problems with a jointed rock mass. The shear behaviour of a jointed rock mass depends on the mechanical behaviour of the discontinuities in that particular rock mass. If we want to understand the mechanical behaviour of a jointed rock mass, it is necessary to study the deformation and strength of a single joint. One of the primary objectives of this work is to improve the understanding of the frictional behaviour of rough rock joints under shear loads with regard to the roughness of the joint surface. The main problem is how to measure and quantify the roughness of the surface joint and connect the morphological parameters into a shear strength criterion. Until now, several criteria have been developed; however, all of them used large rock samples (20×10×10 cm). It is often not possible to get large samples, especially when the rock is under a few meters thick layer of soil. In this case, samples of rock can only be acquired with investigation borehole drilling, which means that the samples of rock are small and of different shapes. The paper presents the modified criterion that is suitable for calculating the peak shear stress of small samples.

One CT imaging method of fracture intervention in rock hydraulic fracturing test

Up to now, the theory and technology of detection of sub-millimeter hydraulic fracturing cracks in large scale rock samples is the key problems of hydraulic fracturing test. According to the principle of substance X-ray absorption and attenuation, three kinds of nanometer CT imaging interventional enhancer based on the gold, mercury and bismuth were proposed. The samples of the hydraulic fracturing and the samples added in the interventional treatment were scanned individually by industrial X - ray CT. By comparing 3 kinds of interventional treatment before and after the treatment of cracks imaging effect, the following conclusions can be obtained: The imaging effect of hydraulic fracture in rock is enhanced by using the interventional enhancer, which is mainly reflected in the gray contrast and resolution of the enhanced X-ray CT imaging; the effect of nano-gold and nano-bismuth on the fracture imaging is the best; nano-bismuth intervention enhancer can detect the cracks width of 0.2 mm, and the capacity of nano-gold intervention enhancer to identify the minimum cracks width increased by 18.75%, however the resolution of the nano mercury intervention is not obvious; nano-scale high atomic number materials can be used as a cracks intervention to improve the imaging resolution of micro cracks in rock fracturing.

MEMORIAL TO MARTIN DAVID BRASIER (1947–2014)

Martin Brasier, Professor of Palaeobiology at the University of Oxford and Adjunct Professor at Memorial University, Newfoundland until his retirement in 2013, died in a car accident on 16 December 2014. He is most famous for his outstanding studies of the early biosphere, from the first appearance of life on Earth to that of essentially ‘modern’ animals and protists in the Cambrian. The enormous influence he had in this field, and the respect that he inspired, are evident from the meeting held in his honour in Oxford on his retirement in 2013 (http://www.palaeocast.com/evolution-and-early-life/), and the award of the Lyell Medal by the Geological Society of London in 2014. However, his contributions to science extended in many directions, among them the emerging discipline of astrobiology where he applied his research in Precambrian palaeobiology to the search for life on Mars. His achievements in both these fields have been explored in other memorials (e.g. Wacey, 2015; McMahon & Cockell, 2015; McLoughlin et al., 2015). Here, we will emphasise his work on modern benthic ecosystems and particularly foraminifera. This has received less attention than other parts of his scientific output, although the study of microfossils, a topic on which he published a standard textbook (Brasier, 1980a; a second edition by Armstrong & Brasier, appeared in 2005), is a theme that underpinned his research. Incidentally, the book was written while Martin was in bed recovering from a slipped disc acquired while collecting large rock samples for microfossil papers on the Cambrian of …

Real-time setup to measure radon emission during rock deformation: implications for geochemical surveillance

Abstract. Laboratory experiments can represent a valid approach to unravel the complex interplay between the geochemical behaviour of radon and rock deformation mechanisms. In light of this, we present a new real-time experimental setup for analysing in continuum the alpha-emitting 222Rn and 220Rn daughters over variable stress–strain regimes. The most innovative segment of this setup consists of the radon accumulation chamber obtained from a tough and durable material that can host large cylindrical rock samples. The accumulation chamber is connected, in a closed-loop configuration, to a gas-drying unit and to a RAD7 radon monitor. A recirculating pump moves the gas from the rock sample to a solid-state detector for alpha counting of radon and thoron progeny. The measured radon signal is enhanced by surrounding the accumulation chamber with a digitally controlled heating belt. As the temperature is increased, the number of effective collisions of radon atoms increases favouring the diffusion of radon through the material and reducing the analytical uncertainty. The accumulation chamber containing the sample is then placed into a uniaxial testing apparatus where the axial deformation is measured throughout a linear variable displacement transducer. A dedicated software allows obtaining a variety of stress–strain regimes from fast deformation rates to long-term creep tests. Experiments conducted with this new real-time setup have important ramifications for the interpretation of geochemical anomalies recorded prior to volcanic eruptions or earthquakes.

Effect of Stress-Strain Conditions on Physical Precursors and Failure Stages Development in Rock Samples

Precursory stages of failure development in large rock samples were studied and simultaneous observations of the space-time variation of several physical fields were carried out under different stress-strain states. The failure process was studied in detail. A hierarchical structure of discreet rock medium was obtained after loading. It was found that the moisture reduced the rock strength, increased the microcrack distribution and influenced the shape of the failure physical precursors. The rise in temperature up to 400 °C affected the physical precursors at the intermediate and final stages of the failure. Significant variations were detected in the acoustic and electromagnetic emissions. The coalescence criterion was slightly depending on the rock moisture and temperature effect. The possibility of identifying the precursory stage of failure at different strain conditions by means of a complex parameter derived from the convolution of physical recorded data is shown. The obtained results point out the efficiency of the laboratory modelling of seismic processes.

Three-dimensional numerical analysis of the rock-cutting behavior of a disc cutter using particle flow code

The Linear Cutting Machine (LCM) test is one of the most powerful and reliable methods for verification of the design of disc cutters and predicting the Tunnel Boring Machine (TBM) performance. It has the advantages of predicting the actual load on the disc cutter from a laboratory test on actual-sized large rock samples. However, it also has the disadvantage of requiring large rock samples to be transported and/or prepared, involving extra cost. To overcome this problem, numerical modeling can be used to expand on the result of full scale cutting tests to evaluate the potential performance of various disc cutter types and cutting geometries for optimization purposes. In this study, PFC3D has been adopted for numerical analysis of the optimum cutter spacing and failure aspects of Korean Busan tuff. The results were compared with the results of LCM testing and numerical simulations using the finite element method. The optimum cutter spacing deduced from numerical simulations is in good agreement with the results of these methods. Consequently, it was shown that PFC3D can easily represent macroscopic rock specimens containing micro cracks.

Ultra-high-resolution paleoenvironmental records via direct laser-based analysis of lipid biomarkers in sediment core samples.

Marine microorganisms adapt to their habitat by structural modification of their membrane lipids. This concept is the basis of numerous molecular proxies used for paleoenvironmental reconstruction. Archaeal tetraether lipids from ubiquitous marine planktonic archaea are particularly abundant, well preserved in the sedimentary record and used in several molecular proxies. We here introduce the direct, extraction-free analysis of these compounds in intact sediment core sections using laser desorption ionization (LDI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). LDI FTICR-MS can detect the target lipids in single submillimeter-sized spots on sediment sections, equivalent to a sample mass in the nanogram range, and could thus pave the way for biomarker-based reconstruction of past environments and ecosystems at subannual to decadal resolution. We demonstrate that ratios of selected archaeal tetraethers acquired by LDI FTICR-MS are highly correlated with values obtained by conventional liquid chromatography/MS protocols. The ratio of the major archaeal lipids, caldarchaeol and crenarchaeol, analyzed in a 6.2-cm intact section of Mediterranean sapropel S1 at 250-µm resolution (∼ 4-y temporal resolution), provides an unprecedented view of the fine-scale patchiness of sedimentary biomarker distributions and the processes involved in proxy signal formation. Temporal variations of this lipid ratio indicate a strong influence of the ∼ 200-y de Vries solar cycle on reconstructed sea surface temperatures with possible amplitudes of several degrees, and suggest signal amplification by a complex interplay of ecological and environmental factors. Laser-based biomarker analysis of geological samples has the potential to revolutionize molecular stratigraphic studies of paleoenvironments.

A novel and versatile apparatus for brittle rock deformation

This paper describes a new biaxial rock deformation apparatus within a pressure vessel, consisting of a stainless steel vessel with an internal diameter of 40cm and six main access ports for electronics (~100 pins), pore fluids (three lines), and confining oil. The apparatus has to ability to work on large rock samples, up to 20×20cm2, with horizontal and vertical forces up to 1.5MN. The maximum confining pressure of the vessel is 70MPa, and fluid flow properties and permeability of large-samples can be tested using up to 8L of fluids that can be pressurized up to 30MPa. Sliding velocity during experiments is in the range 0.1m/s – 1.0cm/s. The machine stiffness is 0.91kN/μm and 1.3kN/μm for the vertical and horizontal axes, respectively. Measurements on friction, velocity dependence of friction and healing properties of reference material like granite and talc replicate the values presented in the literature. Triaxial stress state tests with controlled confining and pore fluid pressure, and fluid flow through the samples have been performed successfully. The machine is extremely versatile, as it works as a uniaxial, triaxial or true-triaxial apparatus, on rock samples with dimensions ranging from several to tens of centimetres. Due to the broad number of potential operating conditions, we decided to name the machine BRAVA: Brittle Rock deformAtion Versatile Apparatus.

3차원 입자결합모델을 이용한 디스크 커터의 암석절삭에 관한 연구

선형절삭시험(Linear Cutting Machine)은 TBM(Tunnel Boring Machine)에서의 디스크 커터 설계와 굴진성능 예측을 위한 가장 신뢰성 있는 시험 중의 하나로서, 실대형 암석시료를 대상으로 실제의 커터 하중을 예측할 수 있는 장점을 갖고 있지만, 시료의 채취 및 운반이 용이하지 않고, 1회 시험에 많은 비용이 소요되는 단점을 갖고 있다. 최근 선형절삭시험이 갖는 이러한 경제적 시간적 제약점들을 극복할 수 있는 현실성 있는 수치모델의 개발에 관한 연구가 활발히 진행되고 있다. 본 연구에서는 3차원 입자결합모델을 이용하여 디스크 커터의 절삭에 의한 부산 응회암의 파쇄양상과 최적 커터간격을 도출하였다. 본 해석에서 분석된 비에너지는 <TEX>$14.3MJ/m^3$</TEX> 일 때 최소값을 나타냈다. 이때의 최적의 s/p 비는 약 16 으로 분석되었으며, 부산응회암의 선형절삭시험 및 유한요소법에 의한 수치해석 연구결과와 유사한 경향을 나타냈다. The LCM (Linear Cutting Machine) test is one of the most powerful and reliable methods for designing the disc cutter and for predicting the TBM (Tunnel Boring Machine) performance. It has an advantage to predict the actual load on disc cutter from the laboratory test on the real-size large rock samples, however, it also has a disadvantage to transport and/or prepare the large rock samples and to need an extra cost for experiment. In order to overcome this problem, lots of numerical studies have been performed. In this study, the PFC3D (Particle Flow Code in 3 Dimension) has been adopted for numerical analysis on optimum cutter spacing and failure aspects of Busan Tuff. The optimum cutting condition with s/p ratio of 16 and minimum specific energy of <TEX>$14MJ/m^3$</TEX> was derived from numerical analyses. The cutter spacing for Busan Tuff had the good agreements with those of LCM test and numerical analysis by finite element method.

Design, operation and validation of a new fluid-sealed direct shear apparatus capable of monitoring fault-related fluid flow to large displacements

A new type of direct shear apparatus has been developed to allow for deformation of large and intact rock samples under fluid-sealed conditions. The sealed cell was specifically designed to monitor changes to fluid flow across the evolving rupture surface to large displacements (≤120 mm), and effective stress conditions up to 36 MPa. To maintain a seal for the pore fluid during deformation, a viscous and immiscible sealant fluid was injected at the interface between the rock specimen and the inner sample cell. Finite element modelling was conducted in parallel to verify the stress distribution in the specimen, notably the effect of geometrical constraints on the fault zone development and mechanical data. Results obtained from natural Castlegate sandstone, synthetically cemented sandstone and sandstone–clay composite blocks demonstrate the suitability of this equipment to investigating coupled fluid mechanical behaviour of a wide range of rock specimens. In particular, the test configuration is demonstrated to be capable of exploring fault-related fluid flow behaviour in reservoir–seal pair analogues with application for the hydrocarbon industry and the emerging field of carbon dioxide storage assessment.

Constraints on the late Quaternary glaciations in Tibet from cosmogenic exposure ages of moraine surfaces

This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide 10Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% for ∼100 ka-old samples. We consider two scaling models for estimating the production rates of 10Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by ∼20%, we find that uncertainties related to the interpretation of the 10Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces.

An Integrated Study of the Dynamics of Electromagnetic and Acoustic Regimes During Failure of Complex Macrosystems Using Rock Blocks

The development of the failure process in complex macrosystems using large rock samples subjected to biaxial compression has been studied by means of electromagnetic radiation (EMR) and acoustic emission (AE). In order to increase the stage of macrofailure development, a special procedure of rock loading was used to reveal regularities of nucleation and evolution of electromagnetic and acoustic structures. The synchronised measurements of EMR and AE allowed the control of the stress–strain state in the rocks and the structural developments of fracturing. Non-homogeneous distribution of the rock spatial crystalline structure subject to load leads to a mosaic distribution of EMR and AE characteristics. As a result, the crack scale effect may be observed in the EMR and AE structure behaviours. The EMR and AE following the failure at different levels behave differently according to the difference in the scale and type of cracks. Intense high-frequency EMR pulses were recorded during the initial stage of microcrack generation occurring prior to major failure of material. This was not the case for AE. The nucleation and development of the macroscopic progressive failure evolution caused an alternation in energetic and frequencial properties of electromagnetic and acoustic events. It has been detected that the tensile cracks were more efficient than shear cracks in capacity of EMR generation. The analysis of self potentials allowed reaching the maximum of registered anomalous variations in the stage of microcracking interaction. This stage showed an increase in the EMR activity, which implies the nucleation of microcracks in various regions of rock interfaces. The gradual accumulation of these defects led to weakening some parts of the rock along with a disintegration of electric anomalies, increase of AE and a significant fluctuation in the rate of EMR. When crack concentration attains its critical value, which results in the formation of dangerous macroscopic failure of higher level, AE shows an intense activity as well as an EMR lower frequency. The hierarchical development of rock failure using the ratio of the average crack size and the mean distance between cracks as a statistical concentration criterion is used to control the boundary of the transition from small dispersed cracks accumulation to gradual crack merger and the formation of the main macrofailure. These results could be transferred into larger scale levels to forecast dynamic events in the earth crust.

Nonmagnetic high pressure cell for magnetic remanence measurements up to 1.5 GPa in a superconducting quantum interference device magnetometer

We describe here a compact nonmagnetic composite high pressure cell of piston-cylinder type with inner diameter of 6 mm equipped with manganin pressure sensor. This cell was developed for room temperature measurements of magnetic remanence of relatively large rock samples (up to 5.8 mm in diameter and 15 mm long cylinders) under hydrostatic pressure up to 1.5 GPa (the operating pressure limit) in the 2G Enterprises superconducting quantum interference device magnetometer. Its design was focused on minimizing the remanent magnetic moment m(r) of the cell (m(r)=3 x 10(-8) A m(2)) that allowed direct measurements of remanent magnetic moment M(r) under pressure for weakly magnetic materials-rock samples (M(r) epsilon[5 x 10(-7),10(-4)] A m(2)). The inner part of this composite cell is made of hard "Russian alloy" (Ni(57)Cr(40)Al(3)) whereas the envelope of the cell corps is made of less magnetic titanium alloy. This design solution permitted to reduce the total remanent magnetic moment of the whole cell and represents the main device feature. We describe here the choice of materials for pressure cell based on their magnetic and mechanical properties, the choice of the pressure transmitting medium (polyethilsiloxane liquid) providing perfectly hydrostatic conditions for the sample as well as the cell geometry. The cell performance is illustrated by results of pressure demagnetization experiments on rocks and minerals.

Determination of sorption properties of intact rock samples: New methods based on electromigration

Two new methods for determining sorption coefficients in large rock samples have been developed. The methods use electromigration as a means to speed up the transport process, allowing for fast equilibration between rock sample and tracer solution. An electrical potential gradient acts as a driving force for transport in addition to the concentration gradient and forces the cations through the rock sample towards the cathode. The electrical potential gradient induces both electromigration and electroosmotic flow with a resulting solute transport that is large compared to diffusive fluxes. In one of the methods, the solute is driven through the sample and collected at the outlet side. In the other, simpler method, the rock sample is equilibrated by circulating the solute through the sample. The equilibration of rock samples, up to 5 cm in length, with an aqueous solution has been accomplished within days to months. Experiments using cesium as a sorbing tracer yield results consistent with considerably more time demanding in-diffusion experiments. These methods give lower distribution coefficients than those obtained using traditional batch experiments with crushed rock.