Stress induced polarization currents and electromagnetic emission from rocks and ionic crystals, accompanying their deformation

International audienceA crucial question of the scientific community nowadays, concerns the existence of electric signals preceding earthquakes. In order to give a plausible answer to this question, we carried out two kinds of laboratory experiments of uniaxial deformation of ionic crystals and rock samples: a) In the first kind, stress induced polarization currents are detected and recorded. Our experimental results showed not only the existence of stress induced polarization currents before the fracture of the samples, but the possibility of the propagation of these signals, as well, through conductive channels, for distances much longer than the source dimensions. b) In the second, acoustic and electromagnetic signals are detected and recorded in the frequency range from 1KHz to some MHz. The mechanism of generation of these signals is shown to be different for those emitted from piezoelectric and from non-piezoelectric materials. A plausible model is also suggested, on the compatibility of our laboratory results with the processes occurring in the earth during the earthquake preparatory stage

Current challenges for preseismic electromagnetic emissions: shedding light from micro-scale plastic flow, granular packings, phase transitions and self-affinity notion of fracture process

Are there credible electromagnetic (EM) EQ precursors? This a question debated in the scientific community and there may be legitimate reasons for the critical views. The negative view concerning the existence of EM precursors is enhanced by features that accompany their observation which are considered as paradox ones, namely, these signals: (i) are not observed at the time of EQs occurrence and during the aftershock period, (ii) are not accompanied by large precursory strain changes, (iii) are not accompanied by simultaneous geodetic or seismological precursors and (v) their traceability is considered problematic. In this work, the detected candidate EM precursors are studied through a shift in thinking towards the basic science findings relative to granular packings, micron-scale plastic flow, interface depinning, fracture size effects, concepts drawn from phase transitions, self-affine notion of fracture and faulting process, universal features of fracture surfaces, recent high quality laboratory studies, theoretical models and numerical simulations. Strict criteria are established for the definition of an emerged EM anomaly as a preseismic one, while, precursory EM features, which have been considered as paradoxes, are explained. A three-stage model for EQ generation by means of preseismic fracture-induced EM emissions is proposed. The claim that the observed EM precursors may permit a real-time and step-by-step monitoring of the EQ generation is tested

Reinforcement of polypropylene with alkali-treated sugarcane bagasse fibers: Mechanism and consequences

Polypropylene composites were prepared from neat and alkali-treated sugarcane bagasse fibers. The results showed that alkali treatment leads to an increase in composite stiffness and strength. A maximum is achieved in these properties at around 5 wt% NaOH content of the treating solution. The increase in properties was assigned to the improvement in inherent fiber characteristics. Acoustic emission testing and electron microscopy showed that the two main local deformation processes related to the fibers are their fracture and debonding; the latter is accompanied by the shear yielding of the matrix. Increased inherent strength of the fibers results in an increase in the fracture initiation stress and fracture energy of the composites. Interfacial adhesion has a slight effect on stiffness, but more significant on strength and impact resistance. Changing adhesion modifies the relative importance of local deformation processes, the number of debonding events decreases, while fiber fracture increases with increasing adhesion. Increased interfacial adhesion improves stress transfer and the load bearing capacity of the fibers as well, but suppresses matrix yielding. Alkali treatment increases inherent fiber strength, which can be directly correlated with composite strength

Measurement and correlation of acoustic emissions and pressure stimulated voltages in rock using an electric potential sensor

Previous laboratory-based studies have demonstrated pressure stimulated currents and electromagnetic emissions in rock and cement mortar can be used to assess damage. There is some debate whether the current and electromagnetic emission measurement technologies used thus far are viable for field application. The results presented here provide evidence that electric potential sensors are a viable new technology for field monitoring of electrical emissions. Cylindrical specimens of rock were loaded at constant stress or strain rates until failure occurred; strain gauges, piezoelectric transducers and electric potential sensors were used to monitor the strain, acoustic emissions and pressure stimulated voltages. Pressure stimulated voltages were observed in the linear elastic and inelastic deformation regions of loading, suggesting that pressure stimulated voltages are associated with microcracking and macrocracking events. Electric potential sensors are potentially a cost effective and more advanced alternative to piezo transducers and geophones for structural health monitoring of rocks

Geophysical methods to detect tunnelling at a geological repository site : Applicability in safeguards

ABSTRACT Generating power with nuclear energy accumulates radioactive spent nuclear fuel, anticipated not to be diversified into any unknown purposes. Nuclear safeguards include bookkeeping of nuclear fuel inventories, frequent checking, and monitoring to confirm nuclear non-proliferation. Permanent isolation of radionuclides from biosphere by disposal challenges established practices, as opportunities for monitoring of individual fuel assemblies ceases. Different concepts for treatment and geological disposal of spent nuclear fuel exist. Spent nuclear fuel disposal facility is under construction in Olkiluoto in Southwest Finland. Posiva Oy has carried out multidisciplinary bedrock characterization of crystalline bedrock for siting and design of the facility. Site description involved compilation of geological models from investigations at surface level, from drillholes and from underground rock characterization facility ONKALO. Research focused on long term safety case (performance) of engineered and natural barriers in purpose to minimize risks of radionuclide release. Nuclear safeguards include several concepts. Containment and surveillance (C/S) are tracking presence of nuclear fuel through manufacturing, energy generation, cooling, transfer, and encapsulation. Continuity of knowledge (CoK) ensures traceability and non-diversion. Design information provided by the operator to the state and European Commission (Euratom), and further to IAEA describes spent nuclear fuel handling in the facility. Design information verification (DIV) using timely or unannounced inspections, provide credible assurance on absence of any ongoing undeclared activities within the disposal facility. Safeguards by design provide information applicable for the planning of safeguards measures, e.g., surveillance during operation of disposal facility. Probability of detection of an attempt to any undeclared intrusion into the repository containment needs to be high. Detection of such preparations after site closure would require long term monitoring or repeated geophysical measurements within or at proximity of the repository. Bedrock imaging (remote sensing, geophysical surveys) would serve for verifying declarations where applicable, or for characterization of surrounding rock mass to detect undeclared activities. ASTOR working group has considered ground penetrating radar (GPR) for DIV in underground constructed premises during operation. Seismic reflection survey and electrical or electromagnetic imaging may also apply. This report summarizes geophysical methods used in Olkiluoto, and some recent development, from which findings could be applied also for nuclear safeguards. In this report the geophysical source fields, involved physical properties, range of detection, resolution, survey geometries, and timing of measurements are reviewed for different survey methods. Useful interpretation of geophysical data may rely on comparison of results to declared repository layout, since independent understanding of the results may not be successful. Monitoring provided by an operator may enable alarm and localization of an undeclared activity in a cost-effective manner until closure of the site. Direct detection of constructed spaces, though possible, might require repeated effort, have difficulties to provide spatial coverage, and involve false positive alarms still requiring further inspection

Ground penetrating radar investigations in Upper Kama potash mines

An understanding of the structure and state of the rock mass surrounding underground openings in the potash mines is critically important for safe mining, planning the methods of extraction of an orebody, and preventing the influx of ground water. Continuous common offset ground penetrating radar (GPR) data were acquired in the potash mine operated by the Joint Stock Company (JSC) Silvinit (Russia) as part of an investigation of both pre-existing fractures exposed by mine workings and other anomalous geological structures. During the course of GPR investigation, the electrical properties of salt-bearing units were determined, site-specific data acquisition techniques and object-oriented data processing schemes adapted to the geological and geotechnical environment of the Upper Kama potash deposit were developed, and the methodology of 2-D and 3-D GPR data interpretation using interactive modeling was worked out. Open fractures and fault and fold features were successfully mapped using 2-D and 3-D GPR techniques. FK filtering significantly improved the reliability of fracture detection. Spatial models of mapped fractures were created using 3-D GPR imaging technique. Migration of the georadar data was required to obtain the true geometry of folded salt beds. The results of this GPR-based investigation demonstrate that the ground penetrating radar georadar method is capable of providing valuable information about deformation structures within the evaporite units of the Upper Kama potash deposit --Abstract, page iv

Experimental study on static and dynamic behavior of woven carbon fabric laminates using in-house piezoelectric sensors, acoustic emission, digital image correlation and scanning electron microscopy

This study focuses on dynamic and static failure analysis of carbon fabric reinforced polymeric composite materials. The first part signifies the production of Piezoelectric Polyvinylidene fluoride (PVDF) nanofibers-based sensor for structural health monitoring of composites. Results obtained from the characterization of electrospun PVDF nanofibers confirm that electrospinning promotes the formation of β-phase. Dynamic flexural tests are performed on woven carbon fabric composites with embedded and surface mounted PVDF sensors to study the capability of these sensors to record strain history and damage progression in composite materials. Moreover, these PVDF sensors are able to capture three distinct stages of fatigue life of composite specimen. This result is validated by the strain measurement with the video extensometer during tests. It is important to emphasize that surface mounted PVDF sensors do not show any sign of malfunctioning during the test. SEM analysis of fractured surfaces of composite specimens shows vivid delamination and fiber pullouts through the thickness, thus indicating gradual growth of damage in laminates. The second part of this study is related to static failure analysis of woven fabric carbon reinforced polymeric composites under tensile and flexural loading. To conduct a detailed investigation Acoustic Emission (AE) is used to attain damage evolution under flexural loading conditions. For the first time GAP function has been suggested to find out the optimal number of clusters for AE data, the advantage of this function is its suitability for classifying elongated data points in vectoral space of acoustic data. Three clusters of data are determined with this new approach indicating various failure types in composite laminates and it is shown that simultaneous occurrence of all failures results in a major change of material stiffness. These failures are also substantiated by Scanning Electron Microscope (SEM) studies of fracture surfaces. Further studies on tensile behavior of the same laminates are conducted with the help of SEM micrographs and 3D-digital image correlation (DIC) technique. Remarkably, it is seen that presence of the shear and transverse strain fields at the surface of the tensile specimen obtained through DIC technique can be correlated to shear dominant and high energy failure (interlaminar delamination and fiber pull outs) respectively, which are also confirmed by SEM images of same fracture regions

Evolving towards a critical point: A possible electromagnetic way in which the critical regime is reached as the rupture approaches

International audienceIn analogy to the study of critical phase transitions in statistical physics, it has been argued recently that the fracture of heterogeneous materials could be viewed as a critical phenomenon, either at laboratory or at geophysical scales. If the picture of the development of the fracture is correct one may guess that the precursors may reveal the critical approach of the main-shock. When a heterogeneous material is stretched, its evolution towards breaking is characterized by the appearance of microcracks before the final break-up. Microcracks produce both acoustic and electromagnetic(EM) emission in the frequency range from VLF to VHF. The microcracks and the associated acoustic and EM activities constitute the so-called precursors of general fracture. These precursors are detectable not only at laboratory but also at geophysical scales. VLF and VHF acoustic and EM emissions have been reported resulting from volcanic and seismic activities in various geologically distinct regions of the world. In the present work we attempt to establish the hypothesis that the evolution of the Earth's crust towards the critical point takes place not only in a mechanical but also in an electromagnetic sense. In other words, we focus on the possible electromagnetic criticality, which is reached while the catastrophic rupture in the Earth's crust approaches. Our main tool is the monitoring of micro-fractures that occur before the final breakup, by recording their radio-electromagnetic emissions. We show that the spectral power law analysis of the electromagnetic precursors reveals distinguishing signatures of underlying critical dynamics, such as: (i) the emergence of memory effects; (ii) the decrease with time of the anti-persistence behaviour; (iii) the presence of persistence properties in the tail of the sequence of the precursors; and (iv) the acceleration of the precursory electro-magnetic energy release. Moreover, the statistical analysis of the amplitudes of the electromagnetic fluctuations reveals the breaking of the symmetry as the theory predicts. Finally, we try to answer the question: how universal the observed electromagnetic critical behaviour of the failing system is