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Seismic wave-Field Structure

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Study of the Seismic Wave-Field Structure in a Regional Zone for Discrimination of Underground Explosions and Earthquakes

Tech Area / Field

  • ENV-SEM/Seismic Monitoring/Environment

Status
8 Project completed

Registration date
16.07.1996

Completion date
16.05.2001

Senior Project Manager
Malakhov Yu I

Leading Institute
MIFI, Russia, Moscow

Supporting institutes

  • VNIITF, Russia, Chelyabinsk reg., Snezhinsk\nNIIIT (Pulse Techniques), Russia, Moscow

Collaborators

  • University of Cambridge / Department of Architecture, UK, Cambridge\nInstituto Nazionale di Geofisika e Vulcanologia, Italy, Rome\nImperial College of Science, Technology and Medicine / Department of Civil Engineering, UK, London\nUniversity of Bristol / Dept. Civil Engineering, UK, Bristol\nRuhr Universität Bochum / Dept. Geophysics, Germany, Bochum\nFOA National Defence Research Establishment, Sweden, Stockholm

Project summary

The goal of the Project is to develop new effective methods for discrimination of seismic fields induced by underground nuclear explosions (TJNE) and earthquakes in the real non-uniform Earth, and to create a basis of methods for reliable detection and identification of low yield UNE at regional epicentrical distances (less than 400 – 2000 km.).

The Project is a logic continuation of the Project ISTC 001-94. The problems of the prospective Project were discussed with collaborators from leading foreign scientific centers (responses are applied).

The results of these researches will be used to control Nuclear Test Ban Treaties and non-proliferation of nuclear weapons, along with studying of an internal structure of the Earth and development of methods for seismic monitoring of earthquakes.

There are a lot of theoretical and experimental investigations of the problem on identifying the nature of registered explosive and earthquake phenomena which have found application in monitoring of nuclear tests. At the same time the low yield UNE can not be identified at teleseismic distances while the mbs criteria does not work at regional distances. Thus the estimation and subsequent taking into account the influence of seismic field propagation from source to a recording station in the real Earth requires further investigations to provide the registration of seismic source located at any point of the Earth.

To achieve the project goal it is supposed to develop the following major problems:

Development of mathematical model of rock behavior under high pressures, taking into account the stresses relaxation and the influence of the loading speed on the dynamic yield strength.

Development of a technique to simulate two-dimensional UNE seismic source function (SSF) taking into account the influence lateral inhomogenity of the Earth's upper crust.

Development of model for two-dimensional decoupling phenomenon and for decoupling in cavities filled in porous materials.

Research of peculiarities of seismic wave propagation at regional distances including the lateral inhomogenity of the Earth's upper crust. It will be based on both numerical and analytical algorithms.

Study of features of Lg-waves internal structure and quantitative relationships between UNE's SSF parameters and characteristics of Lg-waves.

Analysis of the seismic energy distribution of UNEs between various types of seismic waves in the framework of plane-layered elastic medium and its relation to magnitude criteria.

Development of automatic methods of detection and processing of seismic signals in the real-time mode at three-component seismic stations.

Development of seismic methods of UNEs and earthquakes identification at the regional distances with usage of experimental data recorded at Borovoye seismic station (East Kazakhstan) and Novaya Zemlya along with earthquakes neighboring these test sites.

Analysis of elastic non-linearity effects on propagation of seismic signal along the path from sources to receiver at the regional and teleseismic distances.

Development and tests of algorithms intended to identify industrial short-time multi-explosions. It will be based on the mining enterprises at Ural Region (Russia).

Introduction and Review

The goal of the Project is development of theoretical and experimental methods of seismic field monitoring of-UNEs and earthquakes in real non-uniform Earth, analysis of problems of their discrimination, creation of methods of reliable detection and UNE identification at regional distances (less than 400 km.).

The seismic source function computed in spherically symmetric approximation appears to be rough, it is far from the real conditions of conduction of an underground explosion. The spherical symmetry is violated by such factors as rock inhomogenity near the source-point and free surface, anisotropy of rocks and non-uniform initial stress state of rocks, gravity effects etc. Significant part of the effects enumerated can be taken into account in the framework of two-dimensional geometry (axial symmetry), that will no doubt raise a degree of adequacy of the seismic source description.

Introduction of such important factors as the stress relaxation and influence of the speed of loading on the dynamic yield strength requires further improvement of models of rock behavior.

The study of UNE decoupling is important on the point of view of controlling Nuclear Test Ban Treaties, non-proliferation of nuclear weapons and probable unsanctioned conduction of UNEs by the third countries. The question of using non-spherical cavities for UNEs conducted with decoupling has some peculiarities, requiring an additional study. These peculiarities are as follows: the essential asymmetry of radiated elastic signal, and, as a consequence, angular dependence of decoupling factor, generation of significant shear wave-component radiated by the source.

Seismic signal from explosions of low power < 1 kt can be effectively registered by an existing regional network of seismic stations with characteristic distance of the order of 500 km between stations. At the same time, because the explosions of low power radiate seismic waves with lengths not exceeding several hundreds meters, the regional peculiarities of a geological structure of the Earth render essential influence on the seismic field, resulting to its rather complex picture at the regional distances about 300-400 km.

In the Project it is offered to investigate the influence of a regional geological structure on the seismic wave field of underground explosion in frames of the following approaches:


1) Development of new effective methods of simulation of seismic fields from UNEs and earthquakes at the regional distances in environments with corrugate boundaries of Zoological structure, based on the solution of exact integral Fredholm's equations of the second kind for seismic fields and their first derivatives at layers' boundaries
2) Finite-difference solution of a problem of seismic waves propagation, exited by UNE near the corrugate boundary.
3) The processing and comparison of results, obtained by different theoretical methods (such as the ray's method, method of modal decomposition and reflections) will allow to realize clearly features of the internal structure of Lg-waves and to establish quantitative relations between SSF's parameters and registered features of Lg-waves.
4) Distribution of the seismic energy of UNE between various types of waves are supposed to be investigated in the frameworks of the plane-layered models of environment. It is known, that the most effective criterion of identification of UNE and earthquakes is based on the magnitude relation mbs, which characterizes the distribution of energy between body longitudinal waves and surface Rayleigh waves. At the regional distances from explosions of low power the main mode of a Rayleigh wave is not dominant in comparison with high modes of the same wave, and the direct P-wave from UNE is not always registered correctly. At the same time significant part of the seismic energy bears a channel Lg-wave, which can be described as a set of quasi Rayleigh waves. In this connection the new criterion of identification can appear rather sensitive to the yield and depth of a seismic source.
5) Development of automatic methods of detection and processing of seismic signals in regime of the real time at a three components station.
6) Development of seismic methods of identification of UNE and earthquakes at the regional distances from epicenter using registrated experimental data at Borovoe station (Kazakhstan), UNE's data from test sites in Kazakhstan and Novaya Zemlya. as well as near-by earthquakes data.
8) Recognition of industrial short-delayed explosions in a sequence of events, registered by the system of seismic monitoring. The given problem arises in connection with controlling Nuclear Test Ban Treaties. In general set of seismic events, registered by a network of seismic monitoring, the major part consist in events of the technical origin, in particular, industrial explosions in a series with delays of the order 20-30 ms (short-delay explosions) and large enough total yield of the order 1 kt. For the similar events not to be registrated by the monitoring system, as a doubtful, (in sense of latent nuclear test), it is expedient to create algorithm for their recognition in the real time scale. It is essential, that improvement of this algorithm is suggested to be carried out using the large working mining plants in the Russian territory, which were not used early for similar researches.

Expected Results

During the Project fulfillment, the following results will be obtained:


1) The search, accumulation and processing of an experimental material (records of UNE and earthquakes, parameters of seismic source from the seismological bulletins, data on the calibrated characteristics of recording devices) have been carried out;
2) Algorithm and computing program for simulation of synthetic seismogram of body and surface waves in regional zone of epicentral distances from UNE and earthquake (two dimensional model of environment with corrugated boundaries) have been developed;
3) A technique and a program for seismic signal detection in the on-line mode for three component station have been created;
4) A technique of identification UNE and earthquakes at regional distances from epicenter has been tested.
5) Criterion of identification of UNEs and earthquakes, based on the distribution of energy between high modes of quasi-Rayleigh waves has been developed.
6) A numerical-theoretical technique of determination of two-dimensional SSF has been created. More accomplished model of behavior of rocks, taking into account relaxation of stress and the dependence of the dynamic yielding strength on the speed of loading has been developed.
7) Parameters of the decoupling taking into account two-dimensional effects have been determined.
8) The reception of authentic initial data for creation and improvement of algorithms of recognition of industrial short-delayed explosions based on an example of the large mining enterprises in Ural.

Techniques developed in the Project, algorithms and programs for simulation of the wave field from explosions and earthquakes, detection and the identification of these phenomena can be used for monitoring the prohibition of nuclear tests and controlling the non-proliferation of nuclear weapons.

Area of Activity

The Project is devoted to development of methods of discrimination of underground explosions and earthquakes with use of data at the regional zone. The main attention is given to development of theoretical methods and comparison with available experimental data.

For achievement of the Project purposes during the Project fulfillment the solution of the following main problems is suggested.

A-l. Development of a simulation technique of two-dimensional seismic source functions (SSF) of UNE.

A-2. Development of a model of behavior of rocks, taking into account stresses relaxation and dependence of the dynamic yielding strength on the speed of loading.

A-3. Research of the decoupling phenomenon taking into account two-dimensional effects.

A-4. Research of peculiarities of the seismic waves propagation at the regional distances for lateral inhomogenity of structure of the Crust, based on numerical and analytical algorithms.

A-5. Study of peculiarities of internal structure of Lg-waves and elaboration of quantitative ratio between parameters of SSF and Lg-waves.

A-6. Research of the distribution of seismic energy of UNE between different types of waves in frameworks of plane-layered model of Environment.

A-7. Development of automatic methods of detection and processing of seismic signals in regime of the real time at the three-component seismic station.

A-8. Development of seismic methods of identification of UNE and earthquakes at the regional distances from epicenter with use of registrated experimental data of UNEs at Borovoe station (Kazakhstan), UNE at the test sites in East Kazakhstan and Novaya Zemlya as well as near-by earthquakes.

A-9. Research of non-linear effects of propagation of the seismic signal along the path of registration.

A-10. Development and testing of algorithms for recognition of industrial short-delayed explosions on example of the large mining enterprises in Ural.


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