INSTRUMENTACIÓN REVISTA MEXICANA DE FíSICA 47(4) 375-380 AGOSTO 2001

Real time analysis of electromagnetic radiation in a very wide frequency rangel.A. Peralla, P. Reyes-López, and E. Yépez

Escuela Superior de Física y MarématicQ.\', Insr;lIlto Politécnico NacionalEdificio 9, U.l~ ,Ido/fa l..ópez Matem, Zaratellco, 07738 México, D.F., Mexico

Recibido el 15 dc enero de 2001; aceptado el 7 de mayo de 200 I

In this work. wc presco! an e1eclronic appuratus that facililalcs (he moniloring .!nd analysis uf clcctromngnclic radiation in a very widcfrcquency rangc. Thc dcvicc is a combinalion of real and virtual instrumcnts. laking ad\'anlagc of ncw hardware ano software; (he measurablcrangc of frcqucncics dcpcnds 00 the spccd of.1n analogldigital convcrtcr. reachíng (cm 01'Mcgaherlz. Thc dcvicc has becn successfully uscdto monitor the cnvironrncnw.1 clcctfomagnclic radiation at very lo\\' frequency, a very useful parameter in the rescarch of electromagne(ieprecursors 01'eanhquakes. Thc appar:llus is a ncw configuration and has advantages with respcct to lhose previously used: \vhen Ihe auaehedcompulCr is fast, Fourier analysis can be done in real time. can display simultancou siy several bands, lhe digitized dala aBo•••.' a variety ofmethods of anulysis, and the apparatus is very cheap.

Keywords: Eleclromagnelic monitoring; underground electromagnetic ¡¡cid

En este tmhajo presentamos un dispositivo electrónico que facilita el registro y el análü.is de la radiación electromagn~tica en un ;;lmplio rangode frecuencia5. El aparalO es una combinación de in5trurnentos reales y virtuales, que aprovecha las capacid::.des de las nuevas interfaces yprogramas de computución; de acuerdo a la interface utilizada el rango de frecuencias medibles alcanLa decenas de Mllz. El aparato hasido exitosamente usado para registrar la radiación electromagnética ambiental de baja frecuencia, un parámelro muy útil en el estudio deprecursore5 sísmicos electromagnéticos. El dispositivo es una configuración nueva y tiene ventajas respecto a los previamente empleados:puede hacer el análisis de Fourier en tiempo real mediante una computadora rápida, puede hacer el seguimiento simultáneo de varias bandas,el registro digital permite gran versutilidad de métodos de análisis y cI dispositivo es muy bamto.

Descripto,.es: Registros electromagnéticos; c,lmpo electromagnético subterráneo

PAes: 07.57.Kp; 84.40.Ua

J. Introduction

The eleclromagnetic field plays an important role in the studyof many phenomena of the underground and lhe atmosphere.Frequenlly, that electromagnetic field appcólrs in the form ofanomalous radiation pulses, commonly in the ultra low fre-queney range (ULF, OHz <1< 311"), Ihe very low frequeneyband (VLF, 3 kll" < f < 30 kllz), and in low frequeney band(LF, 30 kll" < 1< 300 kHz). The study of eaeh one of thesephenomena requires special instrurnentalion: up to now, theinslrument is composed of two main parts, a detector and anelectronic processing devicc. The de lector could be an elec-trode, an induction coil, or an antcnna. \Vhilc the electronicpart has: a pre-amplifier, a tuner, an amplifier, recording tape,etc. Once the data are acquired, the digital process bcgins andthe data analysis is done in a convenienl way. In this kind ofstudics, frequcnt Iimitations are lhe frcqucncy range and theband wide imposed by the commercial tuner. Por eólch bandand band wide, a different experiment must be done; other-wise, multiple tuners and frequency splitters must be used,increasing the cost; besides, the analysis should be delayed.In lhe case of electromagnetic precursors of earthquakes, a101 of rescarch has been done in the ULF, VLF, and LF bands.showing the possibility of having anomalous behavior of theeJectromagnclic field in lhe temporul vicinity of an impor-tant earthquake [1-fiJ. However, in the VLF and LF bands themonitoring of the electromagnetie radiation (ER) has severalexperimental difliculties.

In Ihis \\'ork, we present an apparatus that can performIhese functions quickly and inexpensively. We use a commer-cial interface for personal computers (PC-card) that eonvertsdata from analog lo digilal eode at very high speed and forseveral channels. The programming of the interface allowsthe selection of the number of channels, the sampling spe-ed, and interruptions to make data anóllysis. The frequencyrange lo be analyzed is only restrictcd by external filters, lhesampling rate and the computer program for the analysis. Themaximum frequency that can be registered is imposed by thespeed of lhe interface; modern interfaces allow very high fre-quencies.

Bcsides [his fact, we al so used the advantage providedby the faSI Fourier transform method (Ff'T), this metho<! eanperform spectra analysis very quickly, almost in real time. Asan example of the usefulness of this apparatus. we conside-red the environmental electromagnctic radiation in the VLPband, in particular for the range 400 11" < 1< 10 kllz. Inthis band, the possibility of eleelromagnetic seismic precur-sors has been investigated extensivcly [1-6].

2. Undcrground clcctromagnctic radiation

It is well known that eleclromagnetic and acoustic anomaliesólre frequently present in the preparation process of an impor-tant earthquake [7-14}; thesc electromagnetic anomalies areassumed to be caused by Ihe imposed stress on quartz bearingrocks, very abundant in the erus!. There are many reports con-

376 lA PERALTA. P. REYES.LÓPEZ. ANO E. YEPEZ

ccrning lhe observation of anomalous electromagnetic fieldin ~evcral frcquency bands, these anomalous signals are ob-served from lens of minutes lo a few months befare an im-portant seismic event.

In the range from O-lO Hz (ULF band), there have be-en several observations of anornalous electric and magneticfield: in Japan. Greece, USA, Russia, and Mexico [7-14].In the LF band (81 kHz and IG3 kllz) there are several re-ports of anomalous ER in Japan, befare an important earth-quake [1,2], at f = 81 MHz (VHF band) the studies camefrom lhe USA [4]. Laboratory experiments 00 stressed rockscontaining quartz, have also shown burst of elcctromagneticradiation in thcse bands [l5, 1G], and lhe fracturing of smallsJl1lples of granitc produced cven ¡¡ght cmissions by lhe su-rrounding atmosphere [17}, thcse phenomena may allow lhefarecast of earthquakes.

Nitzan in the USA [15] and Sobolev in Russia [IG] sho-wed that pure quartz and quartz bearing rocks under big pres-sure produce ER in the VLF and LP bands. It seems reasona-ble to postulate that great volumes of rack under big prcssu-re in Ihe focal region of an earthquakc could produce ER inthese bands; hO\\'cver, its detection on Ihe earth surface pre-sents serious theoretical and experimental prob!ems. At thesefrequencies, the skin depth of the earth crust is only a fewhundred melers. Such lhat the deteclion of the ER on the sur-face would be impossib!e unless it would be produced verynear the surface, with a huge power, or produced by the en-tire focal region that may inelude a part of lhe surface. On-ce the radiation reaches the surface, it propagates freely inthe atmosphere and reftects partially in the ionosphere. Somepossible exp!anations for t!lis process can be found in Refs. 1and 18-20. From the experimental point of view, the detec-lion of VLF/LF electromagnetic radiation presents sorne dif-ficulties: lhe weak inlensity of the ER, ilSmain frequency andbanel wide are usually variable, the cxistence of human noisein this bands, and some other inconvenicnces. Nonetheless,several observations of anoma!ous ER in the VLF/LP rangehave been correlated with impending earthquakes [1-G].

In Japan, Oike and Ogawa [1] and Gokhberg and Go-dunov [3] observed several VLF anomalies at 1G3 kHz and81 kHz respectively, those anomalies appeared a fcw hoursbefore several important earthquakes, the anomalies appea-red in form of bursts. They also observed that the numberof pulses (above some threshold value) in a given intcrva! oftime was nearly proportional to lhe magnitude of the earth-quake. The method of Oike and Ogawa was as follows: an an- .lenna and a cornmercial radio-receptor was used lo detcct theER, those signal exceeding a lhreshold value, were digitallycounted: finally a digital-analog converter plots the numberof anomalous pulse for a given time inlerval. These authorsused a second kind of analysis, reslricting the observationsto nighttime, because the ER contaminalion by noise was 10-wer than at daytime; Ihey counted the anomalous pulses fora few hours around rnidnight. After a detailed analysis of thedata, they concluded that these kinds of anomalies could fo-recast earthquakes. The Illethod of Gokhberg and Godunov

was very similar, its main difference was that lhe para meterused to measure the anomalous behavior was the intensity ofIhe pulses, instead of the number of pulses by time interval,lheir conelusions were alike.

Warwick and his group of radio-astronomers [4]. in theUSA, reported in 1982 a subUe anomaly recorded simultane-ously in a network of radio telescopcs around the world, wor-king at 18 MHz. The anomaly was recorded in May 19GO,two days before the great Chilean earthquake; this strange re-cord was not explained in the framework of radio astronomyfor 22 years. With the increasing work on eleclromagneticprecursors of earthquakes, Warwick found lhe answer; thatanomaly was probably caused by the radiation of the fractu.ring rocks in the preparation process of the great earthquake.In this case, the fault of the earthquake was almost at thesurface, for a long distance. They also provided a theoreticalframework for the production and propagation of lhis kind ofradiation. Since then, there have been many reports of pos-sible electromagnetic precursors of earthquakes in the wholeVLF.LF frequcncy range, including satellile observations ofER coming from the crust [21-23].

3. Dala acquisition syslcm

With the purpose of studying eleetromagnetic precursors ofearthquakes, we construcled a detecting system for the e1ec-tromagnetic radiation in the VLP bando The system consis-tcd of three circular coil antennas with a diameler of 80 cmand 82 turns of copper wire (oriented in three perpendiculardireclions, in order to measure the vector field). Each coil isconnected to a commercial tuner (tuned at 81 kHz); the out-put voltage ofthe coil was amplified, rectified. and integratedfor time intervals of two seconds. The value of the integralwas stored in memory and the process repeated for hours oreven days. This detecting system was installed at two pla-ces: Acapulco and Coyuea (Guerrero, Mexico). The systemworkcd by several months, registering continuously. Howe-ver, lhe analysis of these data showed an erratic behavior ofthe radiation and diffieult interpretation. The selected bandwide was very narrow and very nois)' for severa! hours a day.Wc could not register in lowcr bands, because the minirnumfrequency a\'ailable in the tuner was 50 kHz. The study ofthese phenomena should be done in a wider range of frequen-cies, especially in lower bands \vhere the contarninating noiseis expected to be lower. In what follows, we present a cheapand powerful alternative for this study.

New analog to digital converters (PC-cards) are able topick up severa! thousands and even millions of voltage sam-pIe each second, in such a way that a computer can be usedas a virtual oseilloscope at MHz frequencies. With this ad.vantage, we developed a system to register and analyze elec-tromagnetic signals at frequencies in the range VLF/LF. Thesystem is as follows:

a) The detector is a eoil, la cm long and 1.5 cm in diame-ter w¡th a ferrile core JL -= 2.U¡LOI 110 being lhe

Re\'. Mex. Fís. 47 (4) (2001) 375-380

REAL TIME ANALYSIS OF ELECTROMAGNETlC RADlATION IN A VERY WIDE FREQUENCY RANGE 377

FIGURE 1. Characlerislic curve of the dClccling eoil from Oto 10 kHz, in the vertical axis. wc show the output voltage from(he dctccting coil measurcd by its ~~m.vohagc, {he standard uscdlo excite (he coil is dcscribcd in (he (exl.

permitivity of vacuum). and 1000 turns of copper w¡recaliber 19.

b) The induced vollage on Ihe coil was pre-amplified (in-verse amplifier IC TL071, gain factor 9 = 10 and bandwide O < f < 120 kllz). Coupled to two Bulteerworthfillers (VSCS and band wide O < f < 100 kHz) withattenuation of ..JO db/decade, connected in series in Dr.der lO eliminate frcquencies from O lo 400 Hz.

e) A second amplifier, with characlcristics identicallo those of the prcamplifier, with aptianal gains;9 = lOO, 1000; and good response from f = .100 Hzto f = 10 kllz).

d) A cornrnercial interface for a personal computer (da-ta acquisition card, National InstruITIcnts PC.LPM-16/PnP) capable of picking up ,10 000 voltage sampleper secando The acquired data can be displaycd in lhemonitor of a personal computer (pe) as in a rCOlI timeoscilloscope. by means of a simple program in graphicG-Lahview language.

The detector measures lhe induced voltagc of lhe elcc-tromagnetic field, for a particular frcqucncy thefe could be abetter possibility than this coil; howcver, this particular detec-tor is useful lo measure lhe field from400 Hz to several kHz.The efficiency of lhe system was measured by lhe followingprocedure; we constructcd the response curve of lhe systcmfor frequencies between 1 kllz and 10 kHz. As a standardradiator we used a second coil with 100 turos coilcd withcopper wire caliber 19, 12 cm in diameter and 5 cm 10ng,lo-cated at 10 cm from the detector, a signals generator (1 Voltsinusoidal output of a WAVETEK M184) in the frequencyrange O-IDO kHz fed the radiator. The response curve of thesysrem is shown in Fig. l. The efficiency of the antenna is fiatenough in the range from 3-9 kHz. The filters in the experi-mental arrangement are included in arder to eliminate signalsat fiO Hz and its harmonics. always present in the powcr line;otherwise. after the arnplifying process the resulting signalcould saturate the circuits. The interface allow the registeringof the induced voltagc up to 20 kHz, faster interfaces allowbigger frequencies. This device registers and rccords the dataaccording to the driving software, constructed explicitly rorthis purpose.

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r~~."rR1rl~'~.'-fM'rr.L016 01tl 011 018 019 080 Ilsec:)

FIGURE 2. Induccdvohage at (hedetecting coil when a small pieccof quartz is suddenly compressed. As can be sce from this figure.[he signal is nOliceablebigger (han (heenvironmenlal noisc and lastfor approxim.Jtcly0.01 s.

The fast Fourier transform method (FFr) [24,25] is avery powerful tool ror the analysis of discrete signals; itallows, in a short time, the cornputation of the Fourier trans-form and the parameters of interest in it. In the signals pro-cessing I1lcthou, several parameters are of interest: the rela-t¡ve amplitudes at each frcquency, the spectral density. theelectric power at lhe rcceiver, elC. The time consumption forthe FFT calcubtion is proportional to N log2 N. in contrastto N2 for the usual numerical integration. I'v' is the nUl1lberof salllples in the record. In modem rc's the FFf for severalthousands of samples take a fraction of a second. Once thevoltagc sample is acquired and stored, the frequency spectracan be obtained by FFr method and displayed on a personalcomputer. following lhe FFf algorithm, sorne other parame-ters can be analyzcd in real time by means of simple pro-grallls in graphic G.Labview language. In this work, we areconccrned with the absolute value ofthe Fourier components.with the numbcr of pulses excceding a threshold value, andthe analysis of Ihe spectrulll divided in several bands.

As ;'111 example of the usefulness of this apparatus, wereport lhe deleclion of the electromagnelic radiation cmittedwhcn a small piece of natural quartz is cornpressed by a sud-den hil of a hammer. With this purpose, we followed lhe pro-cedure describcd by Nitsan and Sobolev. The detecting de.vice works foc a few seconds, somewhere in the middle ofthis intcrval the comprcssion of the quarlz is done, the vol-tagc s<.lmplecoming from lhe induction coil is picked up atthe constant rate of 20 000 samples per second (see Fig. 2),lhe amplification factor in this case was 1000. As can be seenfrom this figure, the compressed quartz emitted strong pulsesof ER. In a second experiment, the ER from highly compres-sed quartz is registered by 1024 voltage samples pieked upat arate of 20 000 samples/second. For this individual vol-tage record, we calculate the frcquency spectrum by meansof the FFT method; afterwards, we obtain the absolute valueof Ihe fourier components, lhe spectrum of the absolute va-lue ofthe Fourier amplitudes is divided in five equally spacedbands betwcen O and 10 kHz. (this number is a free para meterin the driving software). Finally, the arca under lhe spectrumfor each band is calculated; these five numbers are stored inthe memory of Ihe cOlllputer instead of the rough data. Thisproccss is repeated 1\'1 times, from this set of Al areas and forcach band, we sclected lhe maximum of each band as repre-sentativc valucs of the ER foc this time interval (the average,or SOIllC other paramcter can also be used). \Vith a PC-pro-

RC\".Mcx. Fis. 47 (4) (2001) 375-380

378 lA PERALTA, P. REYES-LÓPEZ. AND E. VÉPEZ

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f'IClUI{E 3. Time cvolution 01' lhe pul~cs of clectromngnctic raJinlioll produccd by comprcssion 01' natural quanl.: in lhe vertical axi .•••wcdraw ¡he maximum arca as dcsaihcu in Ihe tcxL. Thc horizontal axis l11casurc the time as cycles ol"mcasurcmcnts, cach cyclc corrcsponds 102 seronus. Thc graphics corrcspond 10 ¡he hanos: a) 0..1-2 kilI, 11)2-1 kHz. e) 4-6 kilI.. d) 6-8 Id 11., ano e) 8-10 kHz. Thc arrow points loIhe instanl af Ihe comprcssion 01' quartz.

ces sor \\'orking al 330 MHz. the time of the calcublion isapproximately the same as the sampling lime, for faster pro-ccssors the computation time is diminishcd.

In arder 10 record the timc cvolulion of the reprcsentativevallles ofthe deteeled ER, the proeess dcseribed in the precc-ding paragraph is repcalcd n timcs, and displayed conlinuoll-sly in lhe monitor of thc complIter. In Fig. 3, \\!c show thcrcsult obtained for hundrcds of cycles when quartz is com.pressed in a short limc inlerva!. As can be seen fmm lhisfigure, the compression of quartz produces ER at all thescbands. being more intense in lhe lower bands. confirming thefael that quartz under big pressures and sudden ehanges ofprcssure, radiate in a wide range of frcquencies.

4. VLF clcctromagnctic radiation frolll thccnvironlllcnt

Thc apparatlls and the program of analysis v,'ere tested withthc VLF electromagnelic radi;;¡tion from the environmcnt, fOI"frequencies below 10 kHz. In this part of lhe spectrum. thereshould not be commercial Of civil broadcasting; howe\'cr, se-veral authors report scverc contamination in this band [1-:3),callsed by sorne unauthorizcd civil bands and old navigaliol1instrumenls. \VithoUI this human made noise, lhe VLF radia-tion with frequcncics Icsscr lhan 100 kHz would be mainlydllc to clectromagnetie phcnomcna from the ionosphere andlhe underground.

The procedure \\!e have used is as follows: The electro-magnetic field induces voltage at the detecting eoil. the out.put voltage from the coil is arnplified wilh gain factor 9 = 10,frequeneies below .,IDO Hz are filtcred. A second ampliflca-tion by a gain factor!J = 1000 is done (an over aH gainq = 10 000, higher amplification would require another kindof amplifier), then the vollage is digitizcd by a PC.Labviewinterface, the data are storcd and analyzed by a personal com-

puter. \Ve keep at our dbposal lhe sampling rate and Ihe num-ber of samples; these parameters are free in the samplingprogram and introduced in an inleraetive fashion. In arderto analyze the VLF elcctromagnetic radialion from the en.vironmcnt in the rangc betwecn .,lOO Hz to 10 kHl, we pickedup 1024 voltage samplcs at arate of 20 000 samplcs/seeond(i.e. approximately 50 ms sampling intervals). Thc FFT is ap-plied to this record; lhe spectrum of the absolute value of theFourier components are divided in 5 equally spaced bands:for each band the arca under the frequency curve is calcula.ted. after J\f cycles the maxima of these arcas are selectedas a representati\'c valuc of the ER. Thc proccss is repeatedindefinitely, in such way that we can keep a record of thetime evolulion of the intellsity of the VLF data for long pe-riods of time. without using huge amounts of mcmory. \Viththis proeedure, 24 hours of dat;;luses 1.5 Mb of mcmory (forAl = 40); inslead, tlle rollgh data would use several Gb/hour.In Fig. 4 we illustrale the procedllre, here we show a sin-gle voltage record. lhe absolute value of the Fourier amplitu-des for a this record, and lhe time evolutioll of thc ma.\imumarea of the modulus of the Fourier's amplitudes, calculatedby JI = 40 samples of voltage, the process was repcated TI

cycles.[n the study of electromagnelic precursors of earthqua.

kes, the anomalous bchavior of the electromagnelic radialionappears in the form ofvery short pulses that laSIa few millisc.conds [1-6]. It is usual to measure these anomal ¡es by meansof the excess ofthe intensity ",'ith rcspect lo the average ofthebackground noise, or sOll1earbil~ary threshold. \Vith lhe pro-cedure deseribed in lhe preccding paragraph. we can monitorthe VLF radiation and kccp memory of lhe time evolution ofthose anomalies. A wide study of electromagnetic prccursorsof earthquakes would rcquire: long periods of dala recording,the data must cover very wide range of frequencies, and theanalysis of several paramcters must be done. \Ve can do thiseasily by eXlending the program of analysis.

Re\'. Mex. Fís. ~7 (4) (2001) 375-380

REAL TIME ANALYSIS OF ELECfROMAGNETlC RADIATION IN A VERY WIDE FREQUENCY RANGE 379

P (pubttslhr)20

FIGURE 5. The daily variation of the number of pulses exceedingthe threshold value, in the band 0.4 kllz < f < 10 kHz. The timeorigin is se( at 10:00 am, the differcnees froro day.rime to night.time are notieeable.

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3

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FIGURE 4. Upper panel Voltagc data acquired al 20 000 samplc/s,mcdium panel absolutc value of lhe Fouricr amplillldes from zcrolo 10 kHz, mcasurcd by the \frms value; the vertical lines scpara-le the rcgions in which Ihe spcclrum is dividcd. time evolution ofthe ER in fivc selectcd bands sho\\'cd in lowcr panel: a) 0.4-2 kHz,b) 2-1 kHz, el 4-6 kHz. d) 6-8 kllz, and e) 8-10 kHz.

16

In arder lo test the device, we measured the environmen-tal ER for several days al our laboratory. Using the proceduredescribcd aboye, we faund that the noisc baseline is variable,for that reason. wc Illcasured the differences between consc-cutive pulses of cach area of the absolute value of the rou.rier's spectrum, and count the pulses that exceed an arbitrarylhreshold value in every cyc1e of mcasurements. The daill'behavior of the number of pulses is shown in Fig. 5, the ho-rizontal scale is the time whell the samplc is collected, thetime origin is set al 10:00 amo The vertical scale is the num.ber of anomalous pulses observed in this time interva1. Ascan be seen from this figure, lhe VLF radiation has a strongdepcndence of the lime of the dal', being very intense at wor-king hours and diminishes at nighttime. This effect could bedue to mao-made noise in these bands or to phenomena re-late to solar activity on the ionosphere. In Fig. 6, we shO\\'the time evolution of the VLF electromagnetic radiation re-gistered at our laboratory for a sampling period oft\\'o weeks.As we can see [ram the figure, the daill' behavior is qualita-ti\'cly the samc cxcept for the weekends, indicating c1earll'that this noise is produced by human activities in a very con-taminated place. From Pigs. 5 and 6, we see c1early that theVLF electromagnctic ficld at this place is uncxpectedll' verycontaminaled al frequencies fram ,100 IIz lo 10 kHz, We al-so found VLF noise al 50 kHz and above. The resulIs des-cribed in the preceding paragraph gave us confidence on thecapability of this apparatus; in such a way that new rescarchon electromagnetic precursors of earthquakes and ionosphe-rie electromagnetic phenomena can be done, covering severalbands simultaneollsly, for long periods, and inexpensively.

s, Rcsults and conc1usionsWe have developed a new apparatus eonsisting of real andvirtual instruments that can sample and analyze, in real time,

,.6

•IIlQl1dd)'. o i::I. III

FIGURE 6. Number of pulses by hour uf the VLF e1cetromagne-tic field; the number of pulses are counted as those that exeecd thethreshold value. The horizontal seale is {he time in days whilc thevenieal seale is ¡he number 01' pulses/haur. In the horizontal scalc,we draw a gross l¡nc indicating the time of high working activity,from Monday morning to Priday night; the thin horizontallines in-dieale ¡he weekends, from Friday night to Monday morning.

the electromagnetic field 3t frequencies from 400 Hz to seve-ral kHz. This instrument is very cheap beeause it avoids theuse of tuners, scanning tuners and frequency splitters. lt canbe uscd with minimum consumption of memory for the datastorage. Even more, this method can be improved by usingfaster eomputers and data acquisition eards. This device canhelp in the study of oew developing arcas on the eleetromag-netic phcnomcna of the ionosphere and the underground thatrequires special instrumentation. The cost of the instrumentis 3n essential element in the study of electromagnetic precur.sors; because a great number of monitoring stations must beinstalled, eaeh one of them must have three ¡ndependent dc-tectors. It also has greater sensitivity than acoustic-optic andelectro-aeoustic devices.

This instrumcnt has becn successfully used in the studyof the electromagnetic cmissions from compresscd naturalquartz and quartz-bearing rocks, obtaining that this radiationappears in a very wide range of frequencies (as previouslyreported). We have been able to monitor, in long periods, theVLF radiation of lhe environment 3t our ¡aboratary; the ra-

Rev. Mex. PÚ, 47 (4) (2001) 375-380

380 lA PERALTA, P. REYES-LÓPEZ, AND E. YÉPEZ

diation detected showcd a notorious contaminarian associa-ted lo working-lime hours. We al 50 knew that the detectormust he installed in a place faf fram human settlements andother saurces of electromagnetic contamination.

This device can be used lo register the anomalous electro-magnctic VLF radiarían possibly associated lO the prepationprocess of an carthquake. It can improvc these experimentsin several ways: many hands can be monitored in real timeal frcquencies reaching lens of MHz. diminishes the memoryconsumption by making the analysis of the data in real ti.

1. K, Oike and T. Owawa, J. Ceoe/elr. 3H (1986) 1031.

2. Y. Fujinawa and K. Takahashi, Natllre 347 (1990) 376.

3. M.O. Gokhhcrg and VA. Morgounov, 1. Geophys. Ress. 87( 19H2)7H24.

-l. J. W. Warwick. e. S(okcr, and T.R. Mcycr. J. Geophys. Res. 87(1982) 2851.

5. M. Hayakawa, O.A. Molchanov. T. Ondoh. and E. Kawai, 1.Phys. E,,,,"'¡'¡ (1996) 413.

6. E. Yépcz el al., Proce. oflhe IIlnremalional COllgress 0/1 At.mospheric a"d lotlospheric Eleclromagnetic Phenomena I\sso-ciated with t:arlhquakes. cdited by M. Hayakawa, (Terra Sci .•Tokyo, 1999).

7. G.A. Sobolcv. Pure Appl. Geophys. 113 (1975) 229.

8. E. Yépa et al .• Geophys. Res. Lef(. 22 (1995) 3087.

9. Y.U. Kopytcnko el al.• Phys. Earlh !'Iallet. 1111.77 (1993) 85.

10. M. HaYólkawa. R. Kuwate. O.L. ~1olchanov. and K. Yumolo,Ceo!,h)'s. Res. /.<'/1. 23 (1996) 241.

11. R. Kuwate. O.L. Molchanov. and M. Hayakawa. Phys. EarthPI,mel. /111. lOS (1998) 229.

12. A.e. Fraser-Smith el al., Geophys. Res. Lea. 17 (1990) 1465.

13. A. Bcrnardi, A.e. r=raser-Smith, P.R. McGill. and O.G. Vi-llard Jr. I'hys. Earlh Plcmel. Int. 68 (1991) 45.

me, and can caver long periods of monitoring without humanassistance. Resides the apparatus is vefY cheap compared 10

those previously used, Ihese advantages were not reachcd uptonow.

Acknowlcdgrncnls

Thanks are due lo Comisión de Operación y Fomento de lasActividades Académicas, COFAA-IPN for partia' support.

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