Romanian Reports in Physics, Vol. 64, No. 1, P. 263–274, 2012

THE INFREP EUROPEAN VLF/LF RADIO MONITORING NETWORK – PRESENT STATUS AND PRELIMINARY

RESULTS OF THE ROMANIAN MONITORING SYSTEM*

I.A. MOLDOVAN1, A. MOLDOVAN2, P.F. BIAGI3,4, A.O. PLACINTA1, T. MAGGIPINTO3 1 National Institute for Earth Physics, PO BOX MG 2, RO-077125, Magurele, Romania,

E-mail: iren@infp.ro 2 AZEL-Designing Group S.R.L., Magurele, Romania 3Department of Physics, University of Bari, Bari, Italy

4Inter-Department Centre for the Evaluation and Mitigation of the Volcanic and Seismic Risk, University of Bari, Italy

Received November 5, 2010

Abstract. The paper presents the Romanian VLF / LF monitoring system consisting in a radio receiver – made by Elettronika S.R.L. (Italy) and provided by the Bari University – and the infrastructure that is necessary to record and transmit the collected data. This system is a part of the international initiative INFREP. Through this initiative, originated in Italy, VLF / LF radio receivers are deployed in different locations in Europe. Each one is monitoring up to ten different transmissions of radio stations across the continent. Information on electromagnetic fields’ intensities created by transmitters at each receiving site and gathered from this network are indicating the quality of the propagation along the paths between the receivers and transmitters. Studying the ionosphere influences on the electromagnetic waves’ propagation along a certain path is a method to put into evidence possible modifications of ionosphere lower structure and composition as earthquakes’ precursor.

The VLF / LF receiver installed in Romania was put into operation in February 2009 and has proved its utility in the case of Abruzzo earthquake that occurred on 6th of April 2009 (Mw = 6.3). Since then, the receiver was relocated from Bucharest to the Black-Sea shore (Dobrogea Seismologic Observatory). Changing the receiving site produced unsatisfactory monitoring data, characterized by large fluctuations of the received signals’ intensities. Trying to understand this behavior has led to the conclusion that the electric component of the electromagnetic field was possibly influenced by the local atmospheric conditions (as aerosols’ concentrations could be). Starting from this observation we have run some tests which have indicated that a loop-type antenna is more appropriate than a vertical antenna, especially for highly electric-field polluted environments. Very good results were obtained with this new configuration, even in the site located at the Black-Sea shore. Future improvements of the receiver analog front-end are still possible in order to get better monitoring data by rejecting the off-band noise induced by the aerial high-voltage lines that are surrounding the site, so that for us to accomplish the best achievable surveillance in VLF / LF bands, related to seismo-electromagnetic phenomena.

Key words: VLF / LF monitoring, seismo-electromagnetics.

* Paper presented at the National Conference on Physics, September 23–25, 2010, Iaşi, Romania.

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1. INTRODUCTION

In the last years disturbances in VLF/LF radio signals related to seismic activity have been presented [1–15]. The radio data are collected by receivers located on the ground. The ground based research needs a number of receivers able to sample -with a period of few seconds or minutes- some parameters of the radio signals, as intensity and phase are. Here, the setting up of a new network installed in Europe at the beginning of 2009 is described and the contribution of the Romanian team is detailed.

The Romanian system is a part of the international initiative INFREP. Through this initiative, originated in Italy (University of Bari), VLF / LF receivers are deployed in different locations in Europe. Each one is monitoring up to ten different transmissions of radio stations across the continent. Information on electromagnetic fields' intensities created by transmitters at each receiving site and gathered from this network is indicating the quality of the propagation along the paths between the receivers and transmitters. Studying the ionosphere's influences on the electromagnetic waves' propagation along a certain path is a method to put into evidence possible modifications of ionosphere's lower structure and composition as earthquakes' precursors.

2. THE INFREP NETWORK

On February 2002, in the framework of a scientific cooperation among Japanese, Russian and Italian teams, an OmniPal receiver, labeled IT-Ba, was put into operation in the Department of Physics of the University of Bari (south Italy) and it gave start to the development of the European network [16, 17].

During 2008 a new receiver was developed by the Italian factory Elettronika (Palo del Colle, Bari). The receiver is an equipment working in VLF and LF bands. It can monitor 10 frequencies distributed in these bands and, for each of them, saves the signal's level on a big non-volatile memory at a selectable time interval. The signal level is expressed in dBmVpp as dBmVpp=20log (Vsignal/1mVpp). The receiver has two standard XLR antenna connectors, one for each band, with four poles. On the same connector there is the differential input signal from the antenna and the dual power supply voltage for the preamplifier that is located near antenna. Its aim is to convert the high impedance of the low-frequency antennas in a low impedance to drive the cable and to amplify the captured signal level introducing very low noise. The two antennas are rectilinear ones, long 1.5 m (LF band) and 2.0 m (VLF band), respectively. The huge amount of data collected is organized in text files, one for each day. The configuration of the equipment, the check of its status and the download of the collected data are possible by a standard, powerful and ubiquitous Ethernet interface that can be connected to a PC with an Ethernet

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port. Another main feature is the internal GSM modem that can be used to control the receiver by remote by making data calls. The interface and the possibilities during remote connection are the same as the local control by Ethernet.

At the beginning of 2009 five receivers were prepared, two for Italy and by one for Greece, Turkey and Romania, respectively, and the setting up started. In 2010 another Elettronika receiver was installed in Portugal. The receivers are IT-An and IT-Tc (Italy), GRE (Greece), TUR (Turkey), ROM (Romania), POR (Portugal) and their actual location is shown in Fig. 1. A sampling period of 1 minute was fixed.

Fig. 1 – The stars in the map show the location of the VLF/LF receivers. The circles in the map indicate

the LF and VLF transmitters, the signals of which are collected by the different receivers [16].

On the basis of the best reception at the receiver and taking into account the most convenient radio paths in relation to the seismic activity, there were selected the transmitters presented in Table 1 and Fig. 1.

Some particularities of the VLF and LF transmitters and the signal of which is sampled by the different receivers, are also reported in Table 1. Their location is also shown in Fig. 1.

The list of the VLF/LF radio signals sampled by each European receiver of the network is shown in Table 2.

The setting up of the first five receivers was carried out from March to December 2009 and the last one from Portugal was finalized in May 2010. The main efforts were: a) the sampling of the same radio signals by the different receivers; b) the reception with a low noise level; c) the goodness of the on-line connection.

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Table 1

Particularities of the VLF/LF transmitters of the European network

Label Country Power (kW) Frequency (kHz) BGZ Great Britain 19.58-22.10 ICV Sardinia (Italy) 20.27 HWU France 21.75 DHO Germany 23.4 NRK Iceland 37.5 ITS Sicily (Italy) 45.9 RRO Romania 1200 153 FRI France 2000 162 TRT Turkey 1200 180 EU1 Germany 2000 183 CH1 Algeria 2000 198 MCO France 1200 216 RRU Russia 2500 261 CZE Czech republic 500 270

Table 2

List of the VLF/LF radio signals sampled by each receiver of the network

Radio Signal (transmitter) Receiver VLF LF

IT-An GBZ, HWU, DHO, NRK FRI, TRT, EU1, MCO, RRU, CZE IT-Tc GBZ, ICV, DHO, NRK, ITS RRO, TRT, EU1, MCO, CZE GRE GBZ, ICV, DHO, NRK, ITS RRO, TRT, EU1, CH1, CZE TUR GBZ, ICV, HWU, DHO, ITS RRO, TRT, EU1, RRU, CZE ROM GBZ, HWU, DHO, NRK, ITS EU1, CH1, MCO, CZE POR GBZ, HWU, DHO, NRK, ITS RRO, TRT, EU1, CH1, MCO

3. THE NEW ROMANIAN SYSTEM

The VLF / LF receiver installed in Romania was put into operation in March 2009 and has proved its utility in the case of Abruzzo earthquake that occurred on 6th of April 2009 (Mw = 6.3) [18]. In June 2010 the receiver location was relocated from the trial site from Bucharest to the Black-Sea shore (Dobruja Seismologic Observatory). The final location of the system was chosen in such way to assure the best coverage of the most important seismogenic zone from Romania (Vrancea intermediate depth source) using the transmitters from Table 1. The trial site was located in the Western side of Vrancea and, because all the transmitters were also located West, the propagation paths did not cross it.

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Changing the receiving site produced unsatisfactory monitoring data, characterized by a high level of the noise. Trying to understand this behavior has led to the conclusion that the electric component of the electromagnetic field was possibly influenced by the local atmospheric conditions (as aerosols' concentrations could be) and by the immediate vicinity of high-voltage aerial power lines surrounding the site. After some local tests we put into evidence that the noisy behavior of the receiver was due to the electric-field antennas, which weren't suitable for that site.

It has to be mentioned that there were no faulty condition in the receiver or in its front-end preamplifiers. The whole receiving chain was verified using a programmable signal generator and other specific lab-testing devices. The results of this entire check-up have proven that there was no malfunction of the receiver due to an electronic damage or something alike: the behavior was entirely produced by the local receiving conditions. The front-ends of the VLF / LF receiver were slightly modified as we have observed that the relocation of the equipment has lead to unsatisfactory data. Moving the receiver from the test site (Bucharest) to the Black-Sea shore (Dobrogea Observatory), we have noticed that the monitoring data didn't show any diurnal variation of the electric field's intensities at the chosen frequencies. Moreover, the output data were indicating a high level of the electric field's intensities at all frequencies, as there was a parasitic electric field with a large spectrum.

Assuming that this behavior could be generated either by the high-voltage aerial lines surrounding the observatory or by the specific electric properties of the local atmosphere, which is rich in salt aerosols, we have started some tests regarding the replacement of the electric-type antennas with magnetic-type ones. The magnetic-type antenna for VLF band consists in a loop of shielded cable with 6 conductors of a cross section of 0.25 mm2 each. The cable forms a coil with 12 turns that are winded in a square frame built of PVC pipe-lines of 40mm diameter and 400 mm length, coupled with 90° fittings. The cable's shield is connected with one end of the winding and together they are connected to the ground signal of the preamplifier. Figure 2 shows the old rectilinear antennas (a) and the new loop antennas (b).

The entire frame that is housing the coil is also shielded and coupled to the ground signal. The whole antenna is insulated. Five out of the six internal conductors are connected together as they are forming a coil with 12×5 = 60 turns. With these dimensions, the measured inductance of the VLF magnetic-type antenna is LVLF = 4 mH.

The LF antenna is built in the same manner, but there are only 6 turns of cable and only 3 conductors internally connected, so they form a coil with 18 turns. Its measured inductance is LLF = 0.400 mH. Both preamplifiers (VLF and LF) were modified by adding one more amplification stage with a gain of 26 dB, as front-ends. The overall gain of the preamplifiers has been raised from approximately 20 dB to 66 dB.

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Fig. 2a) – The old rectilinear electric antennas with preamplifiers installed in Dobrogea Observatory

(Black-Sea coast); b) the new loop magnetic antennas with preamplifiers into operation in the same site.

After these modifications the VLF / LF receiver started to record good data, even in the highly electric-polluted environment at the Dobrogea Observatory as shown in Fig. 3. The list of the radio signals actually sampled in this site is reported in Table 2. Figs. 3a and b present a comparison between the recorded data before and after modifying the receiver's front-end preamplifiers and antennas.

Fig. 3a) – Trend of the intensity of VLF/LF radio signals sampled in Dobrogea Observatory

(Romania) using the rectilinear electric antennas. All the signals appear disturbed and irregular; b) trend of the intensity of the same radio signals sampled in same place using the loop magnetic

antennas. The reception of the different signals appears good enough and regular.

Figure 4 compares the signal recorded with the original receiving configuration, at the frequency of 23.400 kHz (DHO38, Germany) on 12th of August 2009 and the signal from the same station, recorded on 24th of November, 2009 with the modified configuration. Despite of the fact that this station ceases its transmission between 07:00 and 08:00 UTC, this can't be seen in the upper panel of the figure. The electric field's intensity is high and presents only weak variations.

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Moreover, it can be noticed that the recording is very noisy. As it can be seen from the lower panel, the noise that was present in the signal received with electric-type antenna is missing, so that the period in which the transmitter ceases its transmission is now clearly visible. At that time, the signal strength falls to only 5 dBmVpp, which is very close to the intrinsic noise of the receiver (i.e. 3–4 dBmVpp). Natural fluctuations in the signal intensity became visible.

Fig. 4 – Example of a recording produced by VLF / LF receiver using original (electric-

type)/modified (magnetic type) antennas and preamplifiers.

Another example of the improvement of the recordings is shown in Fig. 5, where we can see a change in the trend of the HWU intensity collected by IT-An, TUR and ROM receivers starting from July 5th, 2010, when the radiated power from HWU transmitter was increased. The goodness of reception increases strongly at ROM site, where the bad signal becomes a good one, with night-time levels much higher than day-time ones. At IT-An and TUR sites there is a very visible increasing of the signal level during the day (the intensity of the ground-wave -during the day-time- increases and becomes of about the same amplitude with the intensity of the sky-wave, during the night-time). This is why the signal drops drastically around sunrises and sunsets, when the two waves (ground-wave and sky-wave) arrive at the receiving sites with opposite phases and comparable amplitudes. The same behavior (and pattern) is present within Romanian recordings before 5th of July. The different patterns recorded at the three stations are due to different propagation paths of the ground-wave.

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Fig. 5 – Trends of the intensity of the HWU radio signal collected by IT-An, TUR and ROM receivers during 1–10th June, 2010. Starting from July 5th, a change in all the trends appears. This change is due

to the increased power of the transmitter.

4. PRELIMINARY RESULTS OF THE ROMANIAN MONITORING SYSTEM

At present, the Elettronika receiver installed at Dobrogea Obsevatory is connected to Internet at a fixed IP address and is accessible from anywhere in the world. Every member of the network can access, copy and use the Romanian data. To avoid the data loss, the receiver is supplied from an uninterruptible power supply which ensures autonomy of approximately 3 hours and provides a good immunity to the power surges that may appear. The recorded data are consistent and reliable. Since it was put back into operation, on 1st of December 2009, we haven’t any interruptions, so the receiver has proven its stability and usefulness in monitoring the propagation of VLF and LF electromagnetic waves above the seismic zones across Europe.

The research of effects that are induced by earthquakes preparation processes – that take place along the propagation paths between transmitters and receivers – on the radio propagation quality is based on the spotting of anomalous variations of signal amplitude and phase. In this framework a fundamental step is the identification of possible disturbances related to causes different from the seismicity. The analysis of the data collected by the receivers of the network revealed several disturbances of this type. They appear to be related to the transmitter, to the receiver or to meteorological/geomagnetic conditions.

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The development of such a large monitoring network shows its utility not only by covering most of the European seismogenic regions, but also in discriminating different types of disturbances from real seismo-electromagnetic anomalies. The types of disturbances, the necessity of a large European network, and the utility of the Romanian data are exemplified bellow.

Disturbances due to the transmitter can appear like a drop in the intensity of the radio signal.

If the drop is recorded simultaneously on the data collected by all the receivers into operation in that period, it is evident that the disturbance is related to some interruption of the normal broadcast of the radio station and not to some phenomenon related to the precursory stage of an earthquake.

Contrarily, the possibility that a drop could be connected with the transmitter must be excluded when on the same days, the radio signal is normally recorded by other receivers of the monitoring network operating at that time and monitoring the radio signals traveling across other seismogenic zones or across aseismic regions.

This is the case of the drop of the radio signal received from MCO transmitter and recorded by the receiver from central Italy (mouth of the Amare cave, L’Aquila) 7 days prior to the 6th April 2009, Mw = 6.3, Abruzzo earthquake, when only one receiver recorded a drop of the signal. The MCO signal, at the time of the anomaly under study, was recorded in Romania and in south Italy without any drop, as it is shown in Fig. 6. Thus, the radio anomaly under study appears as a precursor of the Abruzzo earthquake [18].

Fig. 6 – MCO (216 kHz) radio signal recorded in central Italy (mouth of the Amare cave, L’Aquila),

south Italy and Romania from 28th March to 3th April 2009 [18].

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Disturbances due to the receiver can appear like a decrease of the intensity of the radio signal, on the VLF or LF band, or on both of them. If the disturbances appear only in the VLF band (for instance) and no similar effect appears on the LF radio signals collected by the same receiver at the same time and the VLF radio signals collected in the other sites of the network don’t show the same decrease, than a control of the equipment is necessary and the possibility that the drop could be connected with the receiver or antennas must be examined. In the case of Abruzzo earthquake, the possibility that the drop could be connected with the receiver was also examined but it was excluded on the basis that the other LF signals are not affected by the drop.

The disturbances due to the meteorological and geomagnetic conditions can be eliminated by complementary monitoring activities developed by each country of the European team. Simultaneously with the monitoring activities, the variations of the geomagnetic indices can reveal a large increase of the geomagnetic activity due to global effects.

5. CONCLUSIONS

The INFREP radio monitoring network has proved its utility for studies regarding the propagation of the electromagnetic waves of very low and low frequencies on different paths across Europe, including seismogenic areas. Propagation anomalies due to meteorological and geomagnetic local conditions were put into evidence and permanent improvements of the network were made.

In the case of the Romanian side, due to the highly electric-field polluted environment and to the specific, local, conditions the original VLF / LF receiver installed at Dobruja Observatory produced unsatisfactory output data. By modifying its input stage topology (antennas and preamplifiers) we have obtained an improvement of its response, resulting in better data quality. The new installed magnetic-field antennas are less susceptible to parasitic electric fields that are produced by the high-voltage aerial lines surrounding the installation site. They are also less susceptible to the electrical proprieties of the local propagation medium, than the electric-field antennas were. Future improvements regarding the rejection of the off-bands signals are still possible, in order to achieve a very good signal-to-noise ratio and reliable data.

At a superficial data examination and in the case of a single monitoring site, the increases or decreases of radio intensities could be falsely interpreted as seismic precursors, but in the case of an extended European network false alarms can be easily rejected.

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