AN INTRODUCTION TO THE DEVELOPMENT OF SLF/ELF ELECTROMAGNETIC RECEIVER AND ITS EXPERIMENTAL OBSERVATION

The electromagnetic method plays an important role in seismic activity monitoring and earthquake prediction and has become one of the important ways for earthquake prediction.During the past decades,many electromagnetic anomalous phenomena at available frequency bands(ELF,VLF,ULF,LF,FM etc.),especially at SLF and ELF bands were observed by scientists before earthquakes,which show that the electromagnetic effects do exist before shocks.Major earthquake may be predicted accurately if the corresponding relation between these electromagnetic anomalies and the physical process in the crust can be proved.In these studies,the objective is the natural electromagnetic field.Because its signal is characterized by instability,low precision and prone to influence of disturbance,especially at the sites with strong noise,the observational data often have low signal-tonoise ratio(SNR).Therefore,its applications are greatly limited.In order to improve the capability of identifying earthquake electromagnetic precursors,researchers must find a new source whose signal has higher stability and precision and wider coverage.In recent years,great progress has been made in the study.Russian scientists firstly applied the artificial SLF/ELF signal to probing the electrical structure of the crust in 1990s,since then,this technique has attracted much attention of geophysicists.Because the artificial SLF/ELF signal can be transmitted high precisely at each frequency under control and has the advantages of strong power,less influence of ionosphere,vast coverage,stable transmission and continuous observation,and the observed data can be analyzed by the correlation analysis technique,the SNR and precision of observed data,even in areas of strong noise,can be greatly enhanced.Measurement results show that in the same setting,the observed signal from SLF/ELF transmitter facility is much stronger than that from natural one,and the apparent resistivity obtained by the former has much higher precision than that by the latter.Therefore,this technique is very suitable for those studies requiring high-precision observational data,especially for earthquake monitoring and prediction.In our country,the study of the SLF/ELF technique started since late 1990s.In this paper,a newly developed SLF/ELF receiver's hardware system scheme,principle,main technical specifications and key technology are introduced in details.Results from a series of tests in lab and in field have shown that the SLF/ELF receiver can be used for observing the artificial SLF/ELF signal with frequency range from 0.1 to 300Hz.