electromagnetic monitoring of the earth’s interior in the

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ANNALS OF GEOPHYSICS, VOL. 51, N. 1, February 2008

Key words MEM Project – electromagnetic sources– Earth’s interior – geodynamical processes

1. Introduction

The main target of the MEM project is tocreate a network of permanent observatories in

Central Italy to monitor the environmental elec-tromagnetic signals. The leader partner of theproject is the Abruzzo Region; the other part-ners are the INGV (Italian Istituto Nazionale diGeofisica e Vulcanologia) Observatory ofL’Aquila, the Regional Environmental Agencyof Molise (ARPA-Molise), the University ofFerrara, the University of Tirana, Albania andthe Geomagnetic Institute of Grocka, Beograd,Serbia. The scientific objectives of the MEMProject are the characterization of the electro-magnetic background noise in the Earth-iono-sphere cavity in the band of frequency [0.001Hz-100 kHz], the realization of a model of the

Electromagnetic monitoring of the Earth’sinterior in the frame of the MEM Project

Paolo Palangio (1), Cinzia Di Lorenzo (1), Manuele Di Persio (1), Fabrizio Masci (1), Spomenko Mihajlovic (2), Lucia Santarelli (1) and Antonio Meloni (1)

(1) Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy(2) Geomagnetic Institute of Grocka, Beograd, Serbia

AbstractThe MEM Project (Magnetic and Electric fields Monitoring) has been running in the INGV Observatory ofL’Aquila since 2004. The principal purpose of the project is to create a network of observatories in Central Italyto monitor the electromagnetic signals in the frequency band [0.001 Hz-100 kHz]. This band includes naturalsignals (magnetic pulsations of magnetospheric origin, Earth-ionosphere resonance mode signals, atmosphericnoise, and so on) and artificial signals (power line emissions, VLF radio transmissions, and so on). The innova-tive characteristic of the project is the approach chosen to study the complex problem concerning the represen-tation of the spatial and temporal distributions of the electromagnetic fields in the band of interest. Both the dis-tributions can be represented by some parameters containing the locations and the characteristics of the sourcesof the electromagnetic signals. When all the stations will be in operation the wide-band interferometry will beapplied. Combining the simultaneous observations of the electromagnetic field measured in the stations of thenetwork, we will be able to obtain detailed information about the investigated electromagnetic sources. A newmeasurement system has been developed to fulfil these requirements focusing on the automation of the meas-urements. The system is designed for long term recording of the electromagnetic fields in a wide frequency band.In the frequency band [1 Hz-100 kHz] the three components of the magnetic field and the three components ofatmospheric electric field are processed in real time using DSP (Digital Signal Processing) techniques. In the fre-quency band [0.001-25] Hz the two components of the telluric field and the three components of the magneticfield are recorded as sampled (100 Hz). One of the main scientific objectives of the MEM project is the long-term monitoring of the geodynamical processes, such as the earthquakes, by the calculation of the Poynting vec-tor, and the analysis of the magnetic transfer functions and impedance tensor. In the coming years this kind ofanalysis can be useful to underline the possible correlation between the geodynamical processes and the localmagnetic field anomalies.

Mailing address: Dr. Paolo Palangio, INGV, Osservato-rio Geofisico di L’Aquila, Castello Cinquecentesco, 67100,L’Aquila, Italy; e-mail:[email protected]

Vol51,1,2008_DelNegro 16-02-2009 21:28 Pagina 225

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P. Palangio, C. Di Lorenzo, M. Di Persio, F. Masci, S. Mihajlovic, L. Santarelli and A. Meloni

electrical conductivity of the Earth’s crust, andthe study of the electromagnetic signals gener-ated in the interior of the Earth. The data col-

lected in the network stations will allow us torealize environmental electromagnetic tomog-raphy through the representation of the electro-magnetic fields distribution in the time, in thefrequency and in the space domains. When allstations will be in operation the wide-band in-terferometry technique will be applied using thesimultaneous observations of the electromag-netic field measured in the network stations(Palangio et al., 2008). This technique is a goodinvestigation tool to obtain three-dimensionalmaps of the electromagnetic signals on a re-gional scale. Tomographic maps are useful toshow the spatial distribution of electromagneticsources, and to characterize the signals bymeans of the energy, polarization and spectralcontent. The data collected in each network sta-tion will be processed to study the structure ofground electric conductivity, the electromag-netic phenomena connected with the seismicactivity, the separation of the electromagneticfields originated in the interior of the Earth, andthe study of the electromagnetic phenomenaoriginated in the magnetosphere, ionosphereand in the Earth-ionosphere cavity. Below theEarth-ionosphere cut-off frequency the interfer-ometric approach will allow us to estimate themagnetic inter-station transfer functions to ob-tain more information about the electric con-ductivity of the Earth’s crust and to locate theelectromagnetic sources by means of interfero-metric methods. The first MEM station hasbeen in operation in the area of the L’AquilaINGV Geomagnetic Observatory (42°23lN,13°19lE, 682 m a.s.l.) from May the 1st, 2005.Within the middle of 2008 two new stationswill be activated in Barete (42°30lN, 13°16lE,930 m a.s.l.) and Duronia (41°39lN, 14°28lE,910 m a.s.l.). Figure 1 shows the locations inCentral Italy of the first three MEM stations. Inthe future new stations will be added to the net-work to cover most part of Central Italy. In eachnetwork station the three components of themagnetic field, the three components of the at-mospheric electric field and the two compo-nents of the telluric field will be measured. Themagnetic field will be measured by inductioncoil magnetometers. The atmospheric electricfield will be measured by the electrometershown in fig. 3 and the telluric field will be

Fig. 1. The locations in Central Italy of the firstthree MEM stations. The faults distribution in theCentral Apennines is also shown.

Fig. 2. Three-axes search coil magnetometer forthe measurement of the magnetic field in the fre-quency band [0.001-100] Hz. The instrumental sen-sitivity is 50 . f HzT


227

Electromagnetic monitoring of the Earth’s interior in the frame of the MEM Project

measured by two orthogonal non-polarizableelectrodes placed at a distance of 140 m. Thenetwork configuration is needed to separate thenatural electromagnetic fields from the artificialones and to separate the electromagnetic fieldsoriginated in the interior of the Earth from thosegenerated by an external source. The techno-logical objectives of the project are the develop-ment of the instrumentation installed in eachnetwork station and the know-how transfer tothe industry. Figures 2 and 3 show two exam-ples of the instruments developed in the frameof the project. The industrialization of these in-struments has been realized by the Italian Geo-magnetic System industry. In the Earth-iono-sphere cavity there are a myriad of electromag-netic natural and artificial signals produced bya huge number of sources (Palangio, 1993). Thespectrum of these signals spreads twelve ordersof magnitude in the frequencies domain, andfourteen orders of magnitude in the energy do-main (Lanzerotti et al., 1990). Taking into ac-count the multiplicity of the parameters thatmust be measured, the level of instrumentalnoise needs to be so low to detect the weakestelectromagnetic natural signals. The spectraldensity of the feeblest natural signal is about [5-10] above the Earth-ionosphere cut-off frequency (1700 Hz), so the background

f HzT

noise level of the developed instrumentationmust be at least [1-2] . Moreover, tak-ing into account the great variability of the elec-tromagnetic fields involved, a 24 bit A/D con-verter is used. Using the magnetic and electricalfields measured on the surface of the Earth animpedance tensor can be estimated. The imped-ance tensor represents the apparent resistivityand phase shift between both the fields as func-tion of the frequency. In conclusion we consid-er three research areas in the electromagneticmonitoring of geodynamical processes. Thefirst is the determination of the distribution ofthe electromagnetic noise strength due to theunderground sources by means of the data setcollected in the interferometric array. The sec-ond is the long-term monitoring of the seven ro-tational invariant parameters of the impedancetensor. The third is the study of the long termmonitoring of the magnetic inter-station trans-fer functions.

2. Analysis of the signals originated in theEarth interior

The electromagnetic monitoring of the geo-dynamic processes is based on two different as-pects (Svetov et al., 1997). The first concernsthe monitoring of the time variation of theground electric conductivity due to tectonicstress variations of the rocks in the seismo-ge-netic area (Meloni et al., 1996; Teisseyre andErnst, 2002). The second is the monitoring ofthe electromagnetic fields generated by the dif-ferent mechanisms of the electro-mechanic en-ergy conversion (Ernst et al., 1997). Thesemechanisms depend on the rock characteristics(Molchanov and Hayakawa, 1995; Frid et al.,2000). The first point can be studied by thelong-term monitoring of the magnetic transferfunctions and the elements of the impedancetensor (Spitz, 1985). The knowledge of the un-derground resistivity structure is an essential re-quirement to study the electromagnetic mani-festations linked to earthquakes. Figure 4 showsthe ground electric resistivity profile calculatedfor L’Aquila station by means of the single sta-tion magnetotelluric tensor evaluation. The pro-file was obtained during May 2005 and the sig-

f HzT

Fig. 3. Three-axes electrometer for the measure-ment of the atmospheric electric field in the frequen-cy band [1 Hz-100 kHz]. The instrumental sensitivi-ty is 1 .V m Hzµ ^ h


228


nal was collected for fifteen days. To obtain theresistivity profile we used a standard Occam 1Dinversion code (Constable et al., 1987), usingfor the inversion both the apparent resistivitiesand the phases. Only the two horizontal compo-nents of magnetic field and the two horizontalcomponents of the telluric field are used in thecalculation. To estimate the transfer functionswe used the Egbert algorithm working out boththe impedances and the magnetic responsefunctions. In the elaboration, the whole dataset

was divided into intervals 4096s long. The finalresistivity profile was obtained averaging all theprofiles obtained. Concerning the second pointwe have to consider that the observed signal isthe convolution of the source-time-functionsand the Earth’s impulse response functions. Theknowledge of the response functions is a funda-mental point for this kind of investigations. An-other aspect concerns the structure of the elec-tromagnetic fields, both natural and artificial, inthe Earth-ionosphere cavity which changes in acomplex way in space and time. The measuredsignals on the surface of the Earth are given byan overlap of a myriad of waves coming to themeasurement point. The separation of the envi-ronmental noise and the signals originated inthe interior of the Earth was performed by theanalysis of the Poynting vector on the residualnon-inductive field. Some studies report mag-netic anomalies related to the seismic activity.The frequency band of these observations is ex-tremely wide, ranging from mHz to MHz(Molchanov et al., 1992). The [0.001-1000] Hzband can be related to linear processes (Mol-chanov and Hayakawa, 1995), while the [1000Hz-50 kHz] band can be related to non linearprocess in the electro-mechanic energy conver-sion (Parrot et al., 1993). In any case, taking in-to account the attenuation of the crossed medi-um on the electromagnetic signals, it is reason-able to observe electromagnetic signals in theband [0.001-1000] Hz. In this frequency band,according the conditions of (Kawate et al,1998), the amplitude of the electromagneticsignals that reach the surface of the Earth couldbe sufficient to discriminate it from the back-ground noise (Johnston, 1984). So, this fre-quency band can be considered the interval inwhich to focus the research activity. Another in-teresting band is [1-5] kHz, the so-called deadband (Dea et al., 1993). In this frequency inter-val the attenuation is high because of the char-acteristics of the crossed rocks. Moreover inthis interval the environmental noise level islow because this is the Earth-ionosphere cut-offband (Meloni et al., 1992). Figure 5 representsan example of a large electromagnetic anomalymeasured in L’Aquila on March 14, 2006 be-tween 8:00 and 10:00 UT. The figure shows thethree components of the magnetic field in a fre-

Fig. 4. 1D resistivity model structure of the subsoilin the area of L’Aquila Observatory.

Fig. 5. Large electromagnetic anomaly occurred inL’Aquila on March 14, 2006 between 8:00 and 10:00UT. The anomaly is probably due to a local electro-magnetic source. B represents each of the compo-nents (X,Y,Z) of the magnetic field.


229


quency band 6 kHz wide around 16 kHz. Thisanomaly cannot be generated or reflected by theionosphere because the field components do notfulfil the Earth’s boundary conditions. More-over, this anomaly cannot be generated in theinterior of the Earth because the skin depth isless than the thickness of the high conductivitysheet in the superficial part of the crust. Proba-bly this anomaly is due to a local electromag-netic source. On the surface of the Earth thegeneric component G, electric or magnetic, ofthe measured field is due to the superimpositionof different terms

(2.1)

where Gm is the measured field and Ge, Gi andGs are respectively the field of external origin,the field induced by the underground currentsthat Ge produces, and the field originated in theinterior of the Earth. Only the fields Ge and Gi

can be linked by the Maxwell equations, where-as Gs is completely unconnected with the previ-ous fields. In the frequency band [0.01-1] Hzonly the one mode prevails, or at least the twomodes, of the electromagnetic source. As a rulethe link between the inductive field, the fieldexternal to the interior of the Earth, and the in-duced field is outlined by means of the tensor Tthat represents the properties of the Earth’s in-terior. This tensor can be written as

(2.2)

In the eq. (2.2) the subscript i and e refer re-spectively to the induced field and to the induc-tive field; E is the electric field, H is the mag-netic field. As we know only the measuredfields Gm, the eq. (2.1) is apparently unresolved.To resolve the problem, the eq. (2.2) can besimplified considering the next five approxima-tions:

1) quasi stationary approximation: the dis-placement currents can be neglected respect tothe conductivity currents. The propagation ofthe electromagnetic fields through the Earth canbe treated as a diffusive process.

2) the vertical component Hze of the externalmagnetic field can be neglected respect to the

E E E H H H

T E E E H H H

xi yi zi xi yi zi

ij xe ye ze xe ye ze$

=l

l

G G G Gm e i s= + +

vertical component Hzi of the induced field, sothe vertical component Hzm of the measuredmagnetic field is

(2.3)

3) in the case of a subsoil with a low con-ductivity, the horizontal components Hxi and Hyi

of the induced magnetic field can be neglectedrespect to the horizontal components Hxe andHye of the external field, so the horizontal com-ponents of the measured magnetic field are

(2.4)

(2.5)

4) the external magnetic field does not havesignificant horizontal gradients on the inci-dence surface

(2.6)

5) the electric vertical field Ezi induced inthe crust of the Earth can be neglected respectto the horizontal components of the electricfield, because of the Earth’s boundary condi-tions Exe≈0 and Eye≈0.

With these assumptions, the eq. (2.2) be-comes a simplified relation between the meas-ured quantities. So, we are able to calculate thepart of the tensor T reported in the followingequation:

(2.7)

In the eq. (2.7) the inductive field is represent-ed by the horizontal components of the meas-ured magnetic field. These components of thetensor T allow us to calculate, in the time do-main, the six impulse response functions I1(t),I2(t), I3(t), I4(t), I5(t) and I6(t) (Krzysztof, 2004).These functions allow us to calculate the fieldsoriginated in the interior of the Earth by meansof the convolution of the Ii(t) functions and themeasured fields (Banks et al., 1993). The com-ponents of the inner fields can be obtained bythe difference between the measured fields and

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

E

E

H

T

T

T

T

T

T

H

H

xm

ym

zm

xm

ym

14

24

64

15

25

65

$

ω

ω

ω

ωωω

ωωω

ω

ω=

H 0e4$ .

H H H H H Hym ye yi ys ye ys.= + + +

H H H H H Hxm xe xi xs xe xs.= + + +

H H H H H Hzm zi ze zs zi zs.= + + +


230


the synthetic fields generated by the convolu-tion of the impulse response functions:

(2.8)

Figure 6 shows the vertical component of themeasured signal, the signal reconstructed froma single station data set using inductive meth-ods, and the difference between them. The dif-ference between the two signals could be due tounderground sources. The other four impulseresponse functions I7(t), I8(t), I9(t), and I10(t)canbe evaluated by the inter-station transfer func-tions (Harada et al., 2004). In this case the ap-proximation 3 is not considered. However, thedifference could also be generated on the sur-face of the Earth near the measurement point,but these proximal signals do not produce sig-nificant inductive effects. Their contribution onthe impulse response functions is null (Palangioet al., 1991). To solve this ambiguity we cancalculate the Poynting vector from the residual

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

H t H I t H I t H

H t H I t H I t H

H t H I t H I t H

E t E I t H I t H

E t E I t H I t H

xs xm xm ym

ys ym xm ym

zs zm xm ym

xs xm xm ym

ys ym xm ym

1 2

3 4

5 6

7 8

9 10

7 7

7 7

7 7

7 7

7 7

= - -= - -= - -= - -= - -

field to obtain the arrival direction of the resid-ual signals and to draw out the electromagneticfields originated in the interior of the Earth. Bythe time dependent expression of the Poyntingvector P(t)=E(t)×H(t), we obtain the three com-ponents Px Py Pz on the surface of the Earth

(2.9)

The x direction corresponds with the geograph-ic north, the y direction corresponds with thegeographic east, and the z is directed toward thecentre of the Earth. Since Ezs~0, eq. (2.9) be-comes

(2.10)

In the frequency domain, these functions arecomplex quantities. Generally we have twovectors, one real and the other imaginary, orien-

( )

P E H

P E H

P E H E H

xs ys zs

ys xs zs

zs xs ys ys xs

==-= -

( )

( )

( )

P tE

H

E

H

P tE

H

E

H

P tE

H

E

H

xs

ys

ys

zs

zs

ys

xs

xs

zs

zs

zs

xs

xs

ys

ys

=

=-

=

d

d

d

n

n

n

Fig. 6. Evaluation of the reconstructed magnetic field by the convolution of the horizontal field with the Earth’simpulse response functions. The vertical scale corresponds only to the difference ∆Z. The measured and the re-constructed vertical components are scaled for a best understanding.


231


tated in different directions. Some contributionsto the Poynting vector could be due to the re-flected signals. So, we must consider the elec-tromagnetic field component which is reflected

by the surface of the Earth. If Pref and Pinc arerespectively the flux of the reflected wave andthe flux of the incident wave, negative values ofthe transmission factor T=1−Pref /Pinc could in-

Fig. 7. Spectrogram of the horizontal magnetic field in the frequency band from [1 Hz-50 kHz].

Fig. 8. Zenithal component of the Poynting vector.


232


dicate an electromagnetic source inside theEarth. Considering a homogeneous plane waveincident on the surface of the Earth, the imped-ance of the Earth’s surface can be written as

. If Zs is isotropic anduniform, then the electric and the magneticfields are related each other by E=H Zs. Gener-ally, the impedance of the Earth’s surface is acomplex function which could be consideredconsisting by a resistive component R and a re-active component XL of equal magnitude, andcan be written as Zs=R+jXL. If σ>>ωε then

; this means that thefields E and H are 45°out of phase. In the fre-quency band [0.001Hz-5 kHz], considering theintegrated values of ρ(ω) at each frequencies, theimpedance Zs lies in the interval [0.001-4] Ωm.Concerning the signals originated in the iono-sphere, the wave impedance Zw of the signal in-cident on the surface of the Earth depends on thefrequency, on the impedance of the ionosphereand on the distance between the Earth and theionosphere. Zw represents the impedance of thewaves generated in the dynamo region of theionosphere, prior to reach the earth surface. Con-sidering a very simple model of isotropic iono-sphere, like a metallic conductor, with a conduc-tivity independent on the frequency below thelowest plasma frequency in the dynamo region,in the range of frequency [1 mHz-0.5 Hz] (strongnear field condition), the wave impedance is ap-proximately of the same order of the impedanceof the Earth’s surface. In absence of non induc-tive signals, the average flux transferred throughthe surface of the Earth is equal to Pinc−Pref, sothe electric and magnetic signals measured are

(2.11)

(2.12)

If we consider a simplified case, a polarizedfield Ey=0, Ez=0, Hx=0 and Hz=0, consequentlyPx=0, Py=0 and Pz=ExHy. Therefore

(2.13)

where Hy and Ex are the measured fields. If, Zs ≈

( )( )

PP

Z E Z HZ Z H E

inc

ref

w x

s w y x

s w2

2

= +-

H H Hm inc ref= +

E E E Z H Z Hm inc ref w inc s ref= + = -

Z j2 2s . ωρµ ωρµ+

Z j js . ωµ σ ωε+^ h

≈Zw, Ex≈ZsHy therefore Pref/Pinc≈0 and T≈1. Inthis case almost the whole energy penetrates inthe Earth. On the contrary, in the frequencyband [0.5 Hz-5 kHz] Zw>Zs, so that a reflectedsignal component is present. Generally, the sig-nals transmitted inside the Earth are

(2.14)

(2.15)

Because Zw>Zs, a fraction of the incident signalis reflected by the surface of the Earth, so thePoynting vector has a component directed awayfrom the surface of the Earth. This could be con-sidered as a signal originated in the lithosphere.In order to avoid this effect in the upper part ofthe spectrum, it is necessary to evaluate the im-pedance of the waves and the media using mod-els of the conductivities of the Earth and the ion-osphere. Figures 7, and 8 show two examplesconcerning the applications of the electromag-netic data collected in the MEM station ofL’Aquila. We stress that the data sets reportedwere chosen in the days with low atmosphericnoise. Figure 7 shows a 2D representation of thehorizontal magnetic field in the frequency andthe time domains. The plot shows stationary ar-tificial signals (50 Hz and its harmonics till fewkHz) and a lot of signals due to non stationarysources. Figure 8 shows the representation ofthe zenithal component of the Poynting vectoras function of the time and the frequency. Fromthe figure can be noted that the arrival directionis ranging mainly from 60° to 140° showing thatthe major part of the signals reaches the meas-urement point by ionospheric reflection. Re-garding the signals originated in the interior ofthe Earth we report an example of the long-termanalysis of the characteristics of the magneticinduction vectors in the lower frequency band,where the reflection problems are not present. Inorder to discriminate the anomalous variationsof the underground conductivity, this analysistends to investigate the normal behaviour of in-duction vectors parameters when no largecrustal changes due to geodynamical processesare present. From the eq. (2.7) we can estimateT64(ω) and T65(ω) which generally are complexfunctions. They are time invariant functions if

2 /E Z Z Z Et s w s inc. +] g6 @

/H Z Z Z H2t w w s incc +] g6 @


233


the electric resistivity of the ground is stable,namely when there are no large crustal changesdue to geodynamical processes. Starting fromT64(ω) and T65(ω), we can calculate both the re-al and the imaginary induction vectors modules.

(2.16a)

(2.16b)

and the real and imaginary phases

(2.17a)

(2.17b)[ ( ( ))/ ( ( ))]tan Im Ima T Timag 65 64ω ωΘ =

[ ( ( ))/ ( ( ))]tan Re Rea T Treal 65 64ω ωΘ =

[ ( ( ))] [ ( ( ))]Im ImT T Timag 642

652ω ω= +

[ ( ( ))] [ ( ( ))]Re ReT T Treal 642

652ω ω= +

Figures 9, 10 and 11 show the time variations ofthe magnetic induction vectors during 2006 forL’Aquila MEM site in three frequency bands:[0.001-0.010] Hz, [0.05-0.10] Hz and [0.15-0.20]Hz. The considered frequency intervals canbe related to the whole crust in term of skindepth. In Central Italy the major tectonic activityis concentrated in the underground depth rangeof about 5-15 km. In this area the direction of theactive faults is roughly NW-SE (see fig. 1). Infigures 10 and 11 it can be noted that the direc-tion of the real vector points toward the tectonicalignment, perpendicularly to the breakingplanes of the faults structure. The imaginary vec-tor has the same direction of the real vector. Thissuggests almost a 2D resistivity structure in themiddle part of the crust. Figure 9 shows the timevariations of the magnetic induction vectors inthe frequency band [0.001-0.010] Hz. This fre-

Fig. 9. Daily mean of the magnetic induction vectors calculated for L’Aquila data set in the frequency band[0.001-0.010]Hz. The time period cover the whole 2006.


234


Fig. 11. As fig. 12 for the frequency band [0.15-0.20] Hz.

Fig. 10. As fig. 12 for the frequency band [0.05-0.10] Hz.


235


quency interval can be related to the deeper partof the crust. The figure shows that the directionsof the real and imaginary vectors are quite differ-ent respect to the directions of the vectors in theband previously reported. This feature reflects adifferent orientation of the lateral discontinuityof the resistivity structure. In any case, the realand the imaginary vectors are parallel each oth-er; this clearly shows again a 2D resistivity struc-ture in the lower part of the crust, but different re-spect to the resistivity structure of the middlepart of the crust. The induction vectors shown infigs. 9, 10 and 11 do not show anomalous behav-iour during the reported period. In any case nosignificant seismic activity was recorded in Cen-tral Italy in this period, so no significant anom-alies in the local geomagnetic field are expected.

3. Conclusions

We have described the scientific and thetechnological objectives of the MEM projectand the methodologies proposed to study theelectromagnetic signals in the Earth-ionospherecavity. Some methodologies have been pro-posed to study the electromagnetic sources bymeans of the wide band interferometry tech-nique. We have also reported some results ob-tained from the data set of the first station of thenetwork installed in the area of the L’AquilaINGV Observatory. Some examples, such asthe Poynting vector calculation in the wholefrequency band, the energy distribution in timeand frequency domain and the annual averagedpower as function of frequency are reported.Using the single station magnetotelluric ap-proach we obtained some valuable informationon the underground resistivity structure in thearea of the measurement station. We also showan example of the reconstruction of the verticalcomponent of the magnetic field by means of asingle site impulse response functions. Con-cerning the study of the magnetic signals linkedto the tectonic activity we have reported an ex-ample of the long term behaviour of the mag-netic induction vector characteristics in thelower frequency band [0.001–0.5]Hz, showingtheir normal behaviour when no large crustalchanges due to geodynamical processes are

present. We will improve this kind of analysisfor a longer period of time in order to under-stand the nature of the relationship between theintensity of the anomalous bahaviour of thetransfer functions and the parameters of theearthquakes such as the magnitude, thehypocentral depth and the distance from themeasure site.

Acknowledgements

The author thanks the administrative staff ofthe INGV L’Aquila Observatory for the basicsupport in the activity. This work was support-ed by the MEM Project (Interreg IIIA AdriaticCross Border Programme).

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