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General description

Full Title:	Investigations of Seismo-Electromagnetic Phenomena in the Range of Extremely Low Frequencies and Creation of the Experimental Network of Amt Monitoring of Seismic Activity in Bishkek Prediction Test Site
Tech Area / Field:	ENV-SEM: Environment / Seismic Monitoring
Status:	2 / Submitted to Parties for Board Decision
Deputy Executive Director:	DED EU: Uwe Meyer
Senior Project Manager:	Shozo Hamada, tel. 7+095+7976031, fax 7+095+7976021, hamada@istc.ru
Project Officer/Assistant:	Anastasia Vladimirovna Svetlova, tel. 7+095+7976031, fax 7+095+7976021, svetlova@istc.ru
Project Manager:	Bakirov Kalys Berikovich Phone: 996+312+461813 Fax: 996+312+462942 E-mail: kis@mail.elcat.kg
Leading Institute:	Institute of Seismology, Bishkek, Kirghizstan
Supporting institutes:	St Petersburg State University / Scientific&Research Institute on the Earth Crust, St Petersburg, Russia
Collaborators:	University of Cologne, Institute of Geophysics andf Meteorology, Köln, Germany Johann Wolfgang Goethe-Universität / Institut fur Meteologie und Geophysik, Frankfurt, Germany University of Athens / Solid Earth Physics Instiurte, Athens, Greece Stanford University, Stanford, CA, USA

Project Summary

In seismic activity monitoring a special attention is paid to investigation of such precursors, as variations of rocks resistivity and ionosphere disturbances. The effectiveness of these precursors was practically confirmed in various seismically hazardous regions. A study of electromagnetic fields of artificial and natural sources in the range of extremely low frequencies from a few to thousands of hertz opens a new possible applications of the phenomena in earthquake prediction.

The objective of the project is the creation of an operating experimental network of audiomagnetotelluric (AMT) monitoring of seismic activity in Bishkek prediction test site on the basis of investigation of seismo-electromagnetic phenomena in the range of extremely low frequencies from a few to thousands of hertz (variations of rocks resistivity and ionosphere disturbances, caused by processes of earthquake preparation).

Investigation of variations of rock resistivity.

Deep electromagnetic sounding plays an important role among the methods, used in earthquake prediction. It permits to receive an information on changes of rock resistivity at rather large depths, where the influence of seasonal factors does not appear. It is possible also to expect more noticeable variations of rock resistivity because of approaching to area of the formed earthquake source.

Electromagnetic sounding with pulse sources was earlier applied in the investigations of rock resistivity at large depths most often. The use of the extremely low frequency (ELF) radio station "Zevs" is very promising in seismic activity monitoring. The ELF station is located in Kola Peninsula. It consists of a generator of harmonically varying current and an antenna with a length of about 60 km, oriented in an E-W direction and grounded at the ends. The value of the current in the antenna reaches 300 A. The ELF station can radiate signals of extremely low frequencies in the range from tens to hundreds of hertz. Its signal can be measured up to about 10,000 km away.

A method of ELF sounding, based on the use of the ELF station as a source of the electromagnetic field, provides a higher accuracy of measurements and reliability of data interpretation compare to sounding using of the electromagnetic field of pulse sources.

In 1995 the measurements of the electromagnetic field generated by the ELF station were carried out by the performers of the present project for the objectives of seismic activity monitoring in the Northern Caucasus. The distance was of 2,700 km from the source. Confident measurements of the ELF station signals using a wide band ACF-2 instrument for audiomagnetotelluric (AMT) sounding were shown to be possible. Advantages of the use of artificial electromagnetic fields in seismic monitoring compare to the natural fields were shown.

In 1996 a specialized ACF-2.2 equipment for AMT sounding and for measurements of electromagnetic fields of the ELF station in seismic monitoring was created in Earth Crust Research Institute of St. Petersburg State University. The instrument operates in a wide band mode (AMT sounding) and in a narrow band one (ELF sounding). In both mode either correlation functions or time series acquisition can be made. With the use of the new narrow band mode with the frequency resolution of 0.007 Hz the ELF signals can be measured more contrastingly over the level of the natural electromagnetic field, than in the wide band mode with the frequency resolution of 0.66 Hz. It permits to increase the accuracy of measurements of the impedance of the ELF station field for 5–10 times.

Since 1996 in the test site in Leningrad province a relation between tidal deformations of the Earth crust and variations of the ELF station electromagnetic field have been investigated. A good correlation (correlation coefficient of 0.96) between change in apparent resistivity and vertical deformations of the Earth crust was shown. While the error of measurements of the impedance being estimated of 0.3%, daily variations of the impedance, which were explained by a vertical deformation of 20 cm, were as high as 7%. Using the results the method of correction for tidal effects based on the use of correlation relationship between the measured impedance and the data of computations of deformation of the Earth crust have been suggested.

The AMT sounding method using the natural electromagnetic fields in the range of frequencies from few to thousands hertz has not practically been applied earlier in seismic activity monitoring. It is due to a low accuracy of measurements because of instability of sources, a low intensity of natural fields and an essential influence of industrial noise in the frequency range used. The new instrumental and software complex ACF provides under favorable conditions the accuracy of a few percents when making measurements of impedance using of the natural electromagnetic fields.

The ELF sounding utilizing the ELF harmonically varying field of the source with the global character of operation, made for the objectives of seismic activity monitoring, have no analogues in the world. In the monitoring the ELF sounding are expedient to be completed by AMT sounding, which enables a wider range of frequencies. A joint application of AMT and ELF soundings using differently polarized primary fields will allow increase the resolution of electromagnetic sounding. The use of the ELF sounding as reference ones will enable to increase the accuracy of measurements and reliability data of the AMT sounding.

Investigation of ionosphere disturbances.

During preparation of the earthquakes characteristic of ionosphere disturbances have been noted which appear as phase and amplitude anomalies, detected by the method of inclined sounding of ionosphere. For the study of them frequencies of 10–15 kHz and 16–70 kHz were used earlier. Numerical estimations available shows, when the frequency decreases from 10 kHz down to 1 kHz the information on the state of the ionosphere increases, therefore use of low frequencies is rather promising in the study of ionosphere effects.

The performers of the present project have carried out experimental observations of an audio effect of ground industrial explosions on propagation of the ELF station electromagnetic field as a method of simulation of processes of earthquake preparation. As a result a series of oscillations with period of about one hour and the depth of modulation in field intensity of 15–35% were measured. The oscillations of such a period in the ionosphere correspond to internal gravitational waves, which the majority of the phenomena, observable in the ionosphere prior to earthquakes, is due to.

The results obtained shows the use of the ELF station to be promising in the study of the ionosphere processes, connected to preparation of earthquakes, and the creation of a system of seismic activity monitoring on the basis of observations of phenomena of seismo-ionosphere interactions in Bishkek test site to be expedient. Besides, a study of the ionosphere effects in the ELF range is expedient when monitoring ground industrial and underground nuclear explosions in the region.

To achieve the main objective of the project it is supposed to solve the following tasks:

1. An analysis of results of prior electrical prospecting operations at Bishkek prediction test site, selection of sites for installation of the receiving equipment to create a network of AMT monitoring and investigation of sites geoelectrical structure.

2. A study of peculiarities of measurements of tidal variations of the impedance of natural and artificial electromagnetic fields and development of techniques to detect the location of pressure-sensitive zones and to choose the optimal frequencies for the monitoring.

3. A study of peculiarities of observations of ionosphere disturbances caused by ground industrial explosions.

4. Development of techniques of AMT monitoring when measuring variations of rock resistivity and ionosphere disturbances prior to earthquakes.

5. Modernization of the receiving equipment and manufacture of three ACF-4.2 instruments for the experimental AMT monitoring network.

6. Modernization of data processing software.

7. Creation of periodically operating an experimental network of AMT monitoring of seismic activity.

When solving the tasks under the project a mathematical simulation for the study of electromagnetic fields, a simulation of processes of earthquake preparation utilizing investigations of tidal variations of apparent resistivity and ionosphere phenomena due to ground industrial explosions, field experiments to study the geoelectrical structure of the test site, testing of the equipment and experimental operation of the monitoring network will be made. A modernization of the ACF equipment and creation of the four-channel instrument with the range of operating frequencies from 0.1 Hz to 12 kHz is planned. The periodically operating experimental network of AMT monitoring of seismic activity will consist of three ACF instruments.

The performance of the present project will allow create the tools for investigations of variations of rocks resistivity and ionosphere disturbances in the ELF frequency range. New data on regularities of change of geoelectrical properties of rocks due to changing their stress-strain state and on peculiarities of seismo-ionosphere interactions will be obtained. Further development of the AMT monitoring network in Bishkek test site and construction of similar systems in other seismically hazardous regions will be made possible.

When performing the project will be used also:

– results of investigations by Institute of Seismology in development and testing of geophysical methods for the earthquake prediction in Bishkek test range;
– results of investigations by Earth Crust Research Institute of St. Petersburg State University on measurements of natural and artificial extremely low frequency electromagnetic fields for geological purposes and seismic activity monitoring.

Operation under the project will be carried out in close contact with foreign collaborators. Information interchange, co-planning and carrying out experimental observations at the test range, testing of instrumental and software complexes and work seminars will be organized.