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/Seismic Monitoring/Environment
8 Project completed
Senior Project Manager
Lapidus O V
Institute of Seismology, NAS, Kyrgyzstan, Bishkek
- St Petersburg State University / Scientific&Research Institute on the Earth Crust, Russia, St Petersburg
- University of Athens / Solid Earth Physics Instiurte, Greece, Athens\nUniversity of Cologne / Institute of Geophysics and Meteorology, Germany, Köln\nWright State University, USA, FL, Daytona Beach
Project summaryIn seismic activity monitoring a special attention is paid to investigation of such precursors, as variations of rocks resistivity and electromagnetic pulses (EMP) of the source zones. The effectiveness of these precursors was practically confirmed in various seismically hazardous regions. The anomaly behaviors of the measured characteristic – apparent resistivity a were usually observed in periods from some months to some days, and EMP anomalies – from some days to some hours before earthquakes. A study of electromagnetic fields of artificial and natural sources in the range of extremely low frequencies from a few to hundreds of hertz opens a new possible applications of these phenomena in middle- and short-term earthquake prediction.
The objective of the project is the creation of periodically 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 hundreds of hertz and study the regularities of variations of rocks resistivity and EMP anomalies, caused by changes of stress-strain state of the rocks.
Investigation of variations of rocks resistivity
Deep electromagnetic soundings play an important role among the methods, used in earthquake prediction. They permit to receive an information on changes of rocks resistivity at rather big 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 forming earthquake source. Electromagnetic soundings with pulse sources were earlier applied in the investigations of rock resistivity at big depths most often. But the accuracy of such measurements were insuffi-cient for the registration of relatively small changes of apparent resistivity before the earth-quakes.
The electromagnetic monitoring based on the study of variations of rocks resistivity is necessary to develop using methods with high accuracy of the measurements ensuring confident data and promising enough in the economic sense for later wide use in seismically hazardous regions. The audiomagnetotellurics (AMT) sounding method based on the measurements of natural electromagnetic field in the range of frequencies from few to hundreds hertz meet these requirements and is planned to be used for studies of variations of resistivity in the present project. A strict and well developed theory of the method was worked out for plane wave model and frequency domain sounding, what allows the authentic interpretation of data. The extremely low frequency sounding (ELFS) using the electromag-netic fields of the high powered con-trolled sources, similar to Russian ELF radio station “Zevs”, will be also used for obtaining reference data and joint interpretation with the data of AMTS method.
The ELF radio station is lo-cated 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 sta-tion 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.
The AMT sounding method using the natural electromagnetic fields has not practically been applied earlier in seismic activity monitoring. It was 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 ACF-4M system, created by ECRI SPbSU, permits to obtain the high accuracy of the measurements of natural electromag-netic fields, quite enough for the seismic activity monitoring (no less then 1–2 % for apparent resistivity and 0.5 for phase of an impedance).
The use of natural electromagnetic fields in seismic activity monitoring is necessary to combine with the measurements of signals of the ELF radio station. The use of the ELF sounding as reference ones will enable to increase the accuracy of measurements and reliability data of the AMT sounding. The joint application of AMT and ELF soundings using differently polarized primary field will allow increase the resolution of electromagnetic soundings. The AMT and ELF soundings based on the use of harmonically varying fields and provide a higher accuracy of measurements and reliability of data interpretation compare to sounding using of the electromagnetic field of pulse sources.
When performing the project for the study of variations of apparent resistivity of the rocks the observation will be conducted using natural electromagnetic field basically. The measurements of the ELF radio station fields will be executed as additional for reception the reference data, comparison and joint interpretation with the results got on the natural fields.
Investigation of electromagnetic pulses
The investigations of EMP basically of pulse character for frequencies of unit – tens kHz before earthquakes were widely enough fulfilled worldwide. Thus measurements of the quantity of pulses or bending around amplitudes of pulse radiation were registered. The experience of such measurements shows, that the precursors are observed for some day before earthquakes, i.e. this method is directed on the short-term forecast of earthquakes. On observations at the bottom of lakes and in deep mines it was found, that certain part of EMP is connected with the deformation processes in the Earth.
In the extremely low frequency range from tens up to hundreds of hertz EMP for the purposes of earthquake prediction earlier practically have not been studied in connection with difficulties in creation of suitable equipment and presence of intensive industrial noise. At the same time, use of this frequency range for EMP measurements and monitoring of seismic activity is rather perspective. In comparison with electromagnetic fields of higher frequencies (tens of kHz and higher) in this case we have an opportunity to register radiation of deeper horizons of the Earth crust. For lower frequencies (lower than 0.5 Hz) the high level of natural electromagnetic fields of ionosphere and magnetosphere, more intensive, than radiation of the geological media is observed.
Created in ECRI SPbSU instrument ACF-4M allows to register in the time series mode EMP in the extremely low frequency range from unit up to hundreds hertz. The experience of such measurements have been obtained both in favorable and unfavorable noise conditions. The spectral analysis and processing of such records will permits to find the signals – precursors of the earthquakes in the extremely low frequency range.
To achieve the main objective of the project it is supposed to solve the following tasks:
A1: Selection of the sensitive places for installation of the receiving equipment and creation of the network of AMT monitoring in Bishkek prediction test site.
A2: Development of techniques for studying of apparent resistivity variations and EMP anomalies in extremely low frequency range.
A3: Development of antennae of ACF-4M system for AMT monitoring of seismic activity.
A4: Creation of the software for data processing and analysis.
A5: Creation of periodically operating the experimental network of AMT monitoring of seismic activity, including three ACF-4M systems with programming operation.
A6: Study of the regularities of apparent resistivity variations and EMP anomalies in the extremely low frequency range, caused by changes of stress-strain state of geological medium under influence of the Earth tidal deformations and seismic activity in Bishkek prediction test site.
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 EMP anomalies, 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 development of the antennae of ACF-4M system for study of seismoelectromagnetic phenomena is planned. The periodically operating experimental network of AMT monitoring of seismic activity will consists of three ACF-4M instruments.
The performance of the project will study the regularities of apparent resistivity variations and electromagnetic pulse anomalies in the extremely low frequency range. New data on regularities of variation of geoelectrical properties and EMP of the rocks due to changing of their stress-strain state 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 site;
– results of investigations of Earth Crust Research Institute on measurements of natural and artificial extremely low frequency electromagnetic fields for geological purposes and seismic activity monitoring.
Operations 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 scientific seminars will be organized.