Rocks Fracture Investigations
Investigation of Formation and Development of the Fracture Nucleation Site in Rocks Aimed at Prediction of Seismic Events
Tech Area / Field
- ENV-SEM/Seismic Monitoring/Environment
- PHY-SSP/Solid State Physics/Physics
- INS-MEA/Measuring Instruments/Instrumentation
8 Project completed
Senior Project Manager
Genisaretskaya S V
VNIIEF, Russia, N. Novgorod reg., Sarov
- Russian Academy of Sciences / Physical Technical Institute, Russia, St Petersburg
The goal and importance of the Project
The financial support of the ISTC is requested for the Project aimed at elaboration of the method for predicting seismic events - earthquakes, rock bursts and other man-induced manifestations of the rock pressure. Within the framework of the Project, a team of designers of nuclear weapons from the Russian Federal Nuclear Center - All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF), in cooperation with the leading scientists of A.F.Ioffe Physico-Technical Institute of Russian Academy of Sciences, Institute of Physics of the Earth of Russian Academy of Sciences, US Geological Survey, and also researchers from the European Union and Japan is going to elaborate the systems of research and simulation of the nucleation sites for seismic events and also to design the prototypes of the systems for monitoring the seismic regime and prediction of manifestations of the rock pressure.
In terms of the Project, the basic and applied research, and also engineering works will be carried out.
The basic scientific and technical problem which should be solved is the investigation of the universal laws governing the evolution of the fracture nucleation stage of rocks.
The applied research works will be aimed at the elaboration of the efficient method for prediction of seismic events.
The goal of the engineering efforts will be the development of specialized measuring instruments for the research works.
It is highly important to study fracture of rocks because the large-scale rupture phenomena which involve the evolution of defects with the spatial sizes of hundreds of kilometers are accompanied by earthquakes which belong to natural phenomena responsible for millions of deaths and heavy damage. Rock bursts are extremely hazardous for mines.
In the 1960-70-s it was found that earthquakes are the consequence of mechanical rupture of the Earth’s crust and the elastic waves generated by this large-scale fracture reach the Earth’s surface and cause damages.
Almost simultaneously, the physical-mechanical models of rupture of the crust appeared in Russia and the USA. According to the model of the Russian scientists, the fracture occurs in a overstressed region of the crust. When a threshold crack concentration is reached, the process becomes avalanche-like. The American model is qualitatively similar to the Russian one, but strong emphasis is placed on the role of ground waters affecting the fracture process.
Because of the importance and complexity of the problem, the studies aimed at elaboration of the methods for earthquake prediction are carried out in several directions. Nevertheless, it can be believed that the most promising methods are based on the physical concept of fracture of rocks.
Different methods for the earthquake prediction are being worked out. For instance, there are approaches based on the study of seismicity variation. In a number of methods the sets of seismological parameters including weakening and intensification of seismic activity, changes in the concentration of the nucleation sites for small earthquakes and in the slope of the frequency-magnitude relation, and some other features of the seismic regime are used.
It is impossible to elaborate efficient methods for the analysis of the seismic regime without having a physical basis, that is, the theory that adequately describes the fracture of rocks. This description should rely both on the general theory of strength of solids and on the experimental data showing specific features of fracture of rocks.
Several approaches to the investigation of fracture of solids are known. One of the most widely used approaches is mechanical. It is based on the theories of critical states. These theories define the critical conditions (stresses, strains) under which the fracture occurs. This approach regards fracture as a critical event. However, a vast body of the experimental evidence accumulated by 1950-s had shown that the macroscopic fracture of a sample takes place not only when a strength limit is reached, but also if a sample is kept for some time under lower stresses. This phenomenon was explained by the kinetic concept of strength of solids suggested by Academician S.N. Zhurkov. The kinetic ideas about fracture of solids developed at Ioffe Physical Technical Institute have received ample recognition in the world.
The essence of the concept is as follows. Defects are accumulated in a sample during the major part of its lifetime. The macrofracture occurs when a critical concentration of defects is reached in the region of a future breakdown, which has been experimentally confirmed for a wide range of materials: polymers, metals, composites, and rocks. From the point of view of the kinetic concept the fracture is a process which evolves in time and space; it is not a critical event. This approach gives the possibility to predict the macrofracture.
In prediction of earthquakes, it is important to take into account not only the kinetic nature of fracture, but also the heterogeneity of rocks and their hierarchic structure. Basing on the general concept, a two-stage model of fracture independent of the fracture scale was suggested at PTI. According to this model, the fracture of a mechanically loaded heterogeneous material proceeds in two stages. At first cracks with the sizes nearly equal to those of heterogeneity (for instance, grains in granite) are formed. The accumulation of these cracks is smooth and does not lead to any significant changes in the material. When their concentration reaches a critical value, the picture becomes different. A larger crack passing through one or several clusters is formed. The cracks formed due to rupture of the volumes of the material arbitrarily distributed in space are stabilized at the borders of heterogeneity. This is the second stage of the local macrofracture. Simultaneously, an elastic wave with the energy related in a certain way to the volume of the fractured material is emitted. If the material exhibits a hierarchy in the sizes of heterogeneity as, for instance, rocks, the model assumes similar evolution of fracture successively at all scale levels, from incipient cracks to the macroscopic fracture of the object. Thus when a body is loaded, a step-by-step increase in the sizes of the formed defects, with a concomitant generation of smaller defects, occurs. The approach worked out at PTI allows the prediction of not only the place and value of the energy release, but also the time of occurrence of the seismic event.
Because of a random character of thermal fluctuations at the microlevel and stochastic distribution of local physical-mechanical properties in heterogeneous rocks, their fracture has an appreciably probabilistic character, which is confirmed by numerous experimental data. This justifies the use of statistical approaches to the investigation of fracture. In addition, this leads to the conclusion that the prediction of earthquakes and rock bursts can be only probabilistic. Therefore, an important task is to work out the strategy for prediction of seismic events which is justified both economically and socially. The project team has the experience in predicting rock bursts at the mines of “Apatity” and “Sevuralboksitruda” joint-stock companies. The strategy of prediction of both rock bursts and earthquakes is further developed through a retrospective analysis of the data base including the parameters of rock bursts in the mines mentioned above and also of the past earthquakes in the regions of lake Baikal, the Caucasus, Nurek, Gazli, and California.
In the experiments, a general relationship between the sizes of the formed defects and the frequency of the generated elastic vibrations has been found. This is the evidence of the similar mechanisms responsible for emission of elastic waves during fracture at different scale levels. The revealed similarity in the fracture behaviors of rocks at different scales gives the possibility to interpret the process in the framework of a single model and to use the same technique for controlling the process. Therefore, the studies under controllable laboratory conditions with a subsequent extrapolation to greater scales acquire particular importance.
The major obstacle to the advances in the laboratory studies and, hence, in the development of the systems for earthquake prediction is the absence of specialized sensors which can give reliable information on accumulation and evolution of defects in solids. The experimental conditions and required accuracy impose severe requirements on the measuring instruments.
For instance, to make the conditions of fracture of granites similar to the actual conditions when hydrostatic compression in the crust occurs, loading should be performed under confining pressures. The high-pressure cameras have a limited volume, which imposes rigid requirements on the sizes of pressure-resistant sensors. In the studies, spatial distribution of defects must be detected, which requires sensors with increased sensitivity and capable of selecting acoustic pulses with different polarizations. In addition to “passive” ultrasonic testing of deformed samples, valuable information about fracture can be obtained by its “active” ultrasonic testing. To realize these measurements, it is also necessary to design special ultrasonic receiving and transmitting instruments.
There are foreign developments of sensors to be used in acoustic emission studies of fracture of rocks designed, for instance, jointly by Lockheed Martin Astronautics and the University of California at Berkeley.
However, the set of specialized sensors conforming to the required specifications is not available at present.
The RFNC-VNIIEF has a wide experience in designing sensors intended for different applications. Specifically, it has experience in providing measurements of propagation of elastic waves in laboratory conditions and in the underground nuclear-weapons tests in Semipalatinsk and on Novaya Zemlya. They have also designed and employed in practice vibroacoustic and vibration transducers in the systems used to control the operation of nuclear power plants.
The team of researchers from RFNC-VNIIEF participated in projects No. 049, No. 075, and No. 738 funded by the ISTC. Owing to the ISTC support, new vibroacoustic (5 types) and vibration (3 types) sensors for the systems of control of equipment at nuclear power plants were designed, and advances were made in the design, production, and control of parameters of sensors. The design documentation of the sensors designed under the ISTC projects was approved by Gosatomnadzor of Russia, and the metrological certificates were granted by Gosstandart of Russia.
The serviceability of the designed transducers was confirmed experimentally at nuclear power plants under the conditions of high pressures, temperatures, vibrations, and powerful radiation fields.
The experience in designing and use of the test and control instruments accumulated in the weapons design works and the studies carried out in the scope of the ISTC projects gives reason to suppose that the engineering works aimed at designing the instruments for research into the earthquake prediction and the joint experiments will be successful.
The joint efforts of scientists of the Russian Academy of Sciences and Ministry of Atomic Energy supported by the ISTC will give the possibility to design the test and control instruments which meet modern requirements, further develop and refine the concepts of the physics of fracture of rocks, and elaborate the efficient method for prediction of seismic events.
Expected results and applications
· specialized acoustic emission instruments for the laboratory studies of fracture of rocks will be designed;
· a series of laboratory rock fracture experiments under different loading conditions with the aim of revealing the prediction criteria for the fracture nucleation stage of the process will be carried out;
· the retrospective analysis of the seismological data bases and the comparison of the results with the laboratory ones will be performed;
· the physical model of fracture of rocks independent of the scale of the process will be further developed;
· the computer simulation of fracture of rocks will be performed and the physical criteria for formation of the fracture nucleation stage of the process valid for wide ranges of properties of a material and loading conditions will be refined and formalized;
· the method for prediction of seismic events in a wide range of released elastic energies will be elaborated.
Conformity to the ISTC Objectives
The works in the framework of the Project will allow the former designers of weapons to be engaged in peaceful activities, which corresponds to the main objective of the ISTC. The woks to be carried out in the scope of the Project will not result in development of weapons. The participation of the scientists and specialists who were earlier engaged in the VNIIEF's defense programs will contribute to conversion of the VNIIEF's scientific and technical potential from the military to peaceful purposes and also to integration of the VNIIEF's scientists into the world scientific community. It is expected that the obtained results will be published.
Participation of Foreign Collaborator
Prof. D. Lockner will participate in supporting the Project in general, in discussing it, working out the basic decisions, and in determining the technical and output parameters of the experimental instrumentation complying with the requirements of the experiments to be carried out. He will also participate in the experiments and analysis of the obtained results.
Prof. H. Kuhnicke will participate in supporting the Project in general, in discussing it, working out the basic decisions, and in determining the technical and output parameters of the experimental instrumentation complying with the requirements of the experiments to be carried out.
Prof. A. Vinogradov will participate in supporting the Project in general, in discussing it, working out the basic decisions, and in determining the technical and output parameters of the experimental instrumentation complying with the requirements of the experiments to be carried out.
Technical Approaches and Methodology
For the implementation of the Project the rich experience of the scientists, engineers, and technicians of VNIIEF and PTI in the area of physics of strength and fracture of solids, and also in the designing and employing of the control-measuring instruments will be used.
The technical approach and methodology are worked out taking into account the specific features of the pilot production facilities of RFNC-VNIIEF. It is expected that its testing base will be efficiently used and the experienced workers earlier engaged in designing nuclear weapons will participate in the development of the systems for prediction of seismic events.
To fulfill the tasks of the Project, the experience and knowledge of experts from the leading Russian Research Centers will be used.
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