This article presents the design of an innovative receiver capable of identifying electric and magnetic components of electromagnetic fields. The receiver senses and records electromagnetic disturbances generated as mine tunnels collapse. It offers excellent operating specification and the ability to sense and log magnetic and electrical component strength values in real time. The paper analyzes the data obtained with the use of a system installed in a working mine and attempts to determine hazards resulting from increased rock stress levels that, cause spontaneous EM emission

Maniak, Krzysztof

Mydlikowski, Remigiusz

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

Paper Autonomous Instrumentationfor Measuring SpontaneousElectromagnetic Emissions in MiningKrzysztof Maniak1 and Remigiusz Mydlikowski21 National Institute of Telecommunications, Warsaw, Poland2 Faculty od Electronics, Wroclaw University of Science and Technology, Wroclaw, Polandhttps://doi.org/10.26636/jtit.2020.145320Abstract—This article presents the design of an innovative re-ceiver capable of identifying electric and magnetic componentsof electromagnetic fields. The receiver senses and records elec-tromagnetic disturbances generated as mine tunnels collapse.It offers excellent operating specification and the ability tosense and log magnetic and electrical component strength val-ues in real time. The paper analyzes the data obtained withthe use of a system installed in a working mine and attempts todetermine hazards resulting from increased rock stress levelsthat, cause spontaneous EM emissions.Keywords—coal mining, electromagnetic field, EM receiver,rock destruction.1. IntroductionTunnel collapses continue to be the main hazard faced inhard coal mining, posing the most serious threat to thelife and health of miners [1]. Nowadays, miners are lessthreatened by methane explosions, due to the common useof sensitive gas detectors allowing the personnel to evacuatebefore critical methane concentration are reached. In theevent of the collapse, weak seismic events occurring athard coal excavation sites may be considered their potentialprecursors. They result from a cascade-like formation ofsmall and large cracks that eventually lead to the collapse ofrock masses. The traditional method of monitoring mineevents that may potentially lead to huge rock collapses,is known as the seismic method [2]. In this method, anextensive network of geophones is used. It is operated bypersonnel responsible for safety of the mine, making thedecision on the potential evacuation of miners.An alternative method capable of forecasting hazards atsuch locations in a more effective manner consists in sens-ing electromagnetic field variations. It is based on the factthat rapid increase in the mechanical force exerted withinrock leads to weak, spontaneous EM emissions that maybe considered a precursor of a potential disaster. This phe-nomenon occurs also during landslides, providing positiveresults when correlated with the inclinometric method com-monly used in landslide testing [3].This paper aims to present the design of a compact EMfield receiver capable of sensing electric and magnetic fieldchanges, enclosed in a small housing. The receiver may alsobe used to expand the scope of research on the nature ofEM fields generated as the mine. A computer-based analy-sis of the waveforms recorded that correspond to changes infield strength makes it possible to distinguish between use-ful signals originating from deposits subject to mechanicalstress and interference signals.Many researchers reported a rapid increase in EM field in-tensity during hard coal mine collapses [4]–[6]. It is impor-tant that such a phenomenon occurs even a few hours beforethe actual incident, i.e. it offers sufficient time for successfulevacuation of the miners from the risk area. These find-ings were proved in laboratory tests concerned with EMfield emissions originating from rock samples subjected todestructive axial pressure [7], [8]. It was observed that theelectric waveforms obtained by the sensors during the testshad the form of a series of fading pulses with a continu-ous frequency spectrum [9]. The literature provides resultsof works aimed at analyzing the number of pulses withina time period, e.g. within one second, or provides descrip-tions of research concerned with DC voltage correspondingto instantaneous pulse values. This makes it possible to de-termine the amplitude of the relevant electromagnetic fieldcomponents. The frequency band of EM emissions maybe determined for individual rock samples tested using themethod from paper [10].2. EM Field Measurement Test StandIn order to measure electromagnetic signals originatingfrom rocks subjected to destructive mechanical stresses,a laboratory testbed was built, as shown in Fig. 1. Manytests of spontaneous electromagnetic emissions were per-formed on various types of rocks, such as coal, sandstone,dolomite and magnesite. The amplitudes of EM emissionsand their spectral distributions were determined duringthese studies and were recorded for further analysis.98Autonomous Instrumentation for Measuring Spontaneous Electromagnetic Emissions in MiningFig. 1. Setup for recording EM fields generated by rock samplessubjected to destructive loads: 1 – test sample, 2 – magnetic fieldsensor, 3 – electric field sensor, 4 – hydraulic press, 5 – electro-magnetic screen.Fig. 2. The recorded EM emission from coal sample duringthe break process: (a) load applied, (b) electric field value, and(c) magnetic field value.Fig. 3. Frequency spectrum of electrical (a) and magnetic(b) component of the electromagnetic wave emitted by the sample.During the tests, rocks were placed in a hydraulic presswith the maximum force of 450 kN. Two separate sensors(field intensity receivers) measuring the electric and mag-netic field components were placed near the press jaws. Assuggested in the literature, the broadband EM emissionsrecording method (up to 100 kHz) was used. To avoid in-terference, both measurement receivers, as well as the rocksample, were shielded using a flat, grounded plate.Figure 2 shows examples of EM field signals recordedwhile testing a cylindrical hard coal sample measuring30×50 mm. Spectral analysis of the signals is presentedin Fig. 3.3. Autonomous Receiver for EM FieldComponents Generated by MineRocksTests carried out in the laboratory indicated that the inte-grated EM field receiver and recorder, designed to operatein a coal mine environment, should comply with the fol-lowing specifications [10]:• minimum electric field component sensitivity ofEmin = 2 · 10−4 V/m and minimum magnetic com-ponent sensitivity of Hmin = 1.5 ·10−5 A/m,• ability to record in the 50 kHz frequency band,• ability to record real-time signals obtained from sen-sors, along with peak values,• built-in power supply for continuous, prolonged op-eration, e.g. for one month,• data protection in the event of a power failure,• no interference with other electronic devices installedin the mine,• immunity to electromagnetic interference occurringin the mine during its operation,• IP67 housing and antistatic design,• mobile enclosure that is easy to install in the mineenvironment.A block diagram of the EM field receiver is shown in Fig. 4.The device is made up of four functional components:• EM field electric signal sensor and signal condition-ing circuit,• EM field magnetic signal sensor and signal condi-tioning circuit,• digital signal and control signal processing circuit,• power supply system.99Krzysztof Maniak and Remigiusz MydlikowskiFig. 4. Block diagram of an integrated EM receiver.As detectors, two antennas were used: one for the magneticcomponent and the other for the electric component.The effective height he f of the antenna rod is 5 cm,and because he f ¿ λ the voltage induced in the antennais [11]:SEME = E ·he f , (1)where E is electric field strength in [V/m].A ferrite rod antenna coupled with an inductive coil wasused as the magnetic component sensor. The induced volt-age may be determined from [12], [13]:SEMH = Z0 ·H ·2π · z ·S ·µλ, (2)where: H – the magnetic field strength in [A/m],Z0=120 πΩ – the wave impedance of free space, z – thenumber of antenna wire turns, S – the cross section areaof antenna coil in [m2], λ – the signal wavelength in [m],µ – the magnetic permeability effective value of the an-tenna’s ferrite core in [H/m].Sensors with the antennas are located in block 1 and 2,respectively, and form analog signal tracks. In both casesthe first part of the conditioning unit has the form of a vari-able gain amplifier G. Next, the signals received via bothtracks are fed to low-pass filters with the cut-off frequencyof 50 kHz, and are then passed via buffer B to the ADCconverter in MCU. The sampled signal is used to measurethe instantaneous value, while the peak value detector givestwo additional measurements to ADC via a MUX multi-plexer. After digitalization the signals are stored on a Flashmemory card. The recorded data is directly accessible viathe RS232 interface.The digital block with a microcontroller is responsible forsignal interpretation and for storing it on a Flash mem-ory card. The MCU used is characterized by low powerconsumption and allows to update firmware without disas-sembling the device.All electronic systems are powered from a 3.3 – 3 V powersupply. Therefore, a 3.6 V lithium-ion battery was used.As a DC-DC converter and an LDO regulator are used, theanalog part of the system is powered by the voltage of 3 V,while the digital part uses 3.3 V.The three separated surface-mounted printed circuit boardsare combined into a single module by relying on board-to-board mezzanine micro-connectors. The PCB with MCUacts as a base plate to which the analogue blocks with theantennas are attached.Fig. 5. The integrated electromagnetic field receiver with visiblebattery and PCBs.Figure 5 presents the complete receiver unit. It is 500 mmlong with 40 mm diameter.100Autonomous Instrumentation for Measuring Spontaneous Electromagnetic Emissions in MiningFig. 6. Frequency characteristics of electric field sensor: (a) rel-ative amplitude sensitivity E/Emax, (b) phase response.Fig. 7. Frequency characteristics of magnetic field receiver:(a) relative amplitude sensitivity H/Hmax, (b) phase response.3.1. Instrument ValidationA series of tests and verification measurements has beenperformed to determine the operating frequency range ofthe device’s sensors. Figures 6a and 7a show frequency sen-sitivity of both sensing elements and their phase response.For the electric field, the upper frequency of 47 kHz wasobtained, and for the magnetic receiver, it equaled 48 kHz.The sensitivity regularities observed in the operating bandare below 1 dB. Out of band amplitude characteristicsshow the slope of −12 dB/octave, which corresponds tothe double-pole transmittance type.Figures 6b and 7b show phase characteristics of the sensingsystem. Both exceed the 3 dB frequency limit at the phaseof −180◦, which is a typical response for a double poletransmittance system [14].3.2. Tests under Real Mining ConditionsThe designed unit was installed in a real, operable hard coalmine. The receiver was placed in a borehole made in an in-active wall, where it was taking autonomous measurementsover a period of 10 days.To achieve reliable results, first the natural EM backgrounddisturbances present at the location in question were de-Fig. 8. EM background in an inactive mine for: (a) elec-tric component, (b) E component spectrum, (c) H component,(d) H component spectrum.101Krzysztof Maniak and Remigiusz Mydlikowskitermined. The time waveforms obtained are periodical innature and their peak-to-peak values are relatively low(Fig. 8a–c).Spectral analysis shows that the majority of interferencesoriginate from the power mains (with spectrum peaksvisible at 50, 100 and 150 Hz).Fig. 9. Received series of EM pulses exceeding the backgroundlevel registered at the measurement location.Fig. 10. Detailed analysis of recorded signal presented in Fig. 9:(a) E component, (b) H component, (c) E signal frequency spec-trum, (d) H signal spectra.During the 10-day test conducted under real mine condi-tions, at least several events of increased spontaneous elec-tromagnetic emissions were recorded. They had the formof a series of pulses with different amplitudes and shortrepetition times. Since the receiver was installed in an in-active mine, the disturbances from equipment working atthe mine could be excluded. Figure 9 shows an example ofevents related to a low-energy mine collapse. The resultsshown are representative of the entire set of EM eventsrecorded during the test and of other seismic events thatoccurred in the mine.Figure 10 presents an analysis of the series of recorded EMpulses from Fig. 9. The fading waveform is visible there(Fig. 10a–b) lasting up to 200 ms and up to 100 ms forthe E component and for the H component, respectively.After performing the Fourier transformation (Fig. 10c–d),it was determined that a significant portion of the frequencyresponse is within the 1 kHz band.4. ConclusionsThe use of an integrated, autonomous EM receiver in a realmine allows for early detection of upcoming seismic events.During its operation in the mine, the receiver recorded sev-eral such events. A single event of this type is character-ized by a series of pulses with short repetition intervals.The period of the recorded pulses depends on the dynam-ics of the seismic process. Low-energy seismic events wererecorded during mining. Interference from the power mainsadversely affected the quality of the recorded data. How-ever, with proper shielding of the receiver’s sensing device,it is possible to significantly reduce the interference level.In addition, the digitized signal obtained should be pro-cessed to lower the impact of noise.The frequency band of the recorded seismic pulses in-creases up to several kilohertz. This was also confirmedduring laboratory tests performed as part of the researchproject.Measurements of EM emissions generated by force-stressedrocks may be used as an additional warning factor. Cur-rently, the seismic method is relied upon in mines for thisparticular purpose. However, new detection methods arebeing developed constantly and may offer a valuable addi-tion to the classic approach. Fortunately, it is possible tocreate an early warning system for identifying mine col-lapse hazards by using a network of EM field receiversdistributed throughout the mine and installed at the lo-cations that are most sensitive, from the geological pointof view.References[1] M. Akgün, “Coal mine accidents”, Turkish Thoracic J., vol. 16,no. S1–S2, 2015 (DOI: 10.5152/ttd.2015.008).[2] J. L. X. Meng, Y. Wang, and Z. Yang, “Prediction of coal seamdetails and mining safety using multicomponent seismic data: A casehistory from China”, Geophysics, vol. 81, no. 5, pp. B149–B165,2016 (DOI: 10.1190/geo2016-0009.1).102Autonomous Instrumentation for Measuring Spontaneous Electromagnetic Emissions in Mining[3] A. Prałat, K. Maniak, and I. Pompura, “Electromagnetic phenomenain landslides”, Acta Geodynamica and Geomaterialia, vol. 2, no. 3,pp. 131–138, 2005 [Online]. Available: https://www.irsm.cas.cz/materialy/acta content/2005 03/16 Pralat.pdf[4] V. Frid, “Calculation of electromagnetic radiation criterion of rock-burst hazard forecast in coal mines”, Pure and Applied Geophysics,vol. 158, pp. 931–944, 2001 (DOI: 10.1007/PL00001214).[5] V. Frid and K. Vozoff, “Electromagnetic radiation induced by miningrock failure”, Int. J. of Coal Geology, vol. 64, no. 1–2, pp. 57–65,2005 (DOI: 10.1016/j.coal.2005.03.005).[6] D. Lin-ming, L. Cai-ping, M. Zong-long, and G. Ming-shi, “Pre-vention and forecasting of rock burst hazards in coal mines”, MiningSci. and Technol. (China), vol. 19, no. 5, pp. 585–591, 2009(DOI: 10.1016/S1674-5264(09)60109-5).[7] L. Guang-Jian, Lu Cai-Ping, W. Hong-Yu, L. Peng-Fei, and LiuYang, “Warning method of coal bursting failure danger by electro-magnetic radiation”, Shock and Vibration, vol. 2015, 2015(DOI: 10.1155/2015/583862).[8] E. Wang, H. Jia, D. Song, N. Li, and W. Qian, “Use of ultra-low-frequency electromagnetic emission to monitor stress and failure incoal mines”, Int. J. of Rock Mechanics & Mining Sciences, vol. 70,pp. 16–25, 2014 (DOI: 10.1016/j.ijrmms.2014.02.004).[9] Z. Shijiea, S. Xiaoyuanc, L. Chengwub, X. Xiaoxuan, and X Zhuang,“The analysis of coal or rock electromagnetic radiation (EMR) sig-nals based on Hilbert-Huang transform (HHT)”, in Procedia Engin.,vol. 26, pp. 689–698, 2011(DOI: 10.1016/j.proeng.2011.11.2224).[10] R. Mydlikowski and K. Maniak, “Measurement of electromagneticfield component emission as a precursor of emerging hazard in coalmines”, J. of Telecommun. and Inf. Technol., no. 4, pp. 30–35, 2019(DOI: 10.26636/jtit.2019.135419).[11] Z. Loni, H. Espinosa, and D. Thiel, “Insulated wire fed float-ing monopole antenna for coastal monitoring”, Radioengineering,vol. 27, no. 1, pp. 127–133, 2018(DOI: 10.13164/re.2018.0127).[12] V. Dyo, T. Ajmal, B. Allen, D. Jazani, and I. Ivanov, “Design ofa ferrite rod antenna for harvesting energy from medium wave broad-cast signals”, The J. of Engin., vol. 2013, no. 12, pp. 89–96, 2013(DOI: 10.1049/joe.2013.0126).[13] T. Bolton and M. B. Cohen, “Optimal design of electrically-smallloop receiving antenna”, Progress In Electromagnetics Research C,vol. 98, pp. 155–169, 2020 (DOI: 10.2528/PIERC19090911).[14] U. Tietze, Ch. Schenk, and E. Gamm, Electronic Circuits–Handbookfor Design and Application 2nd Edition. Springer, 2011(ISBN-13: 9783540004295).Krzysztof Maniak received hisM.Sc. and Ph.D. degrees inElectronics from Wroclaw Uni-versity of Technology, Poland,in 2001 and 2008, respec-tively. Since 2012, he has beenwith the National Institute ofTelecommunications in Wroc-ław. He is an author and co-author of numerous research pa-pers and publications. His pro-fessional work focuses on: power systems, electromag-netic compatibility and design of measuring equipment.He also participates in research and measurement-re-lated activities. His research interests focus on the test-ing of power grid quality and its broadband interferenceprotection.https://orcid.org/0000-0003-1974-909XE-mail: k.maniak@itl.waw.plNational Institute of TelecommunicationsSwojczycka 3851-501 Wrocław, PolandRemigiusz Mydlikowski re-ceived his M.Sc. and Ph.D.degrees in Electrical Engi-neering from Wroclaw Univer-sity of Science and Technol-ogy, Wrocław, Poland, in 1997and 2002, respectively. He iscurrently with the Departmentof Theory of Field Theory,Electronic Systems and Opto-electronics at Wroclaw Univer-sity of Science and Technology. His research interests arein non-invasive studies of ground and rock properties.https://orcid.org/0000-0002-8670-2913E-mail: remigiusz.mydlikowski@pwr.edu.plWroclaw University of Science and TechnologyWybrzeże Wyspiańskiego 2750-370 Wrocław, Poland103

Autonomous Instrumentation for Measuring Spontaneous Electromagnetic Emissions in Mining, Journal of Telecommunications and Information Technology, 2020, nr 4

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Autonomous Instrumentation for Measuring Spontaneous Electromagnetic Emissions in Mining, Journal of Telecommunications and Information Technology, 2020, nr 4

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