Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Seismic Wave Characteristic of Ground Surface Layer Produced by Pulsed Discharges in Closed Water Domain Chengyan Ren, Rongyao Fu, Ailong Fan Xuzhe Xu, Yaohong Sun, Ping Yan, Tao Shao Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China [email protected] Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory of Power Electronics and Electric Drive, Chinese Academy of Sciences, Beijing 100190, China [email protected] Abstract—The technology of portable spark source, independent of surrounding water environment, needs to be developed for some special situations. The transfer efficiency from electric energy to seismic wave energy is the most important factor in this technology. The electrode filled with water was firstly designed to develop the pulsed discharge experiment. The seismic wave generated by pulsed discharge was measured by seismic wave detector. The relation of seismic wave amplitude and discharge parameter was observed. The electrode component is machined by stainless steel and other materials which have similar acoustic impedance with salt water to enhance the transfer efficiency from electric energy to acoustic energy. The piston-like electrodes were also designed to enhance the transfer efficiency from electric energy to mechanical energy, in which some components could be active up and down due to the impulsive force produced by pulsed discharge. The seismic wave in different electrode construction was measured and analyzed. The experiment result indicates that the amplitude of seismic wave is largest in piston-like electrodes, which also indirectly indicates that the energy of pulsed discharge in closed electrode filled with water is transferred to the ground surface layer mainly by mechanical shock. phenomena and production mechanism of pulsed discharge in water, such as the characteristics of pressure, electric, discharge plasma, bubble, optical and sound radiation [4-5]. Keywords—pulsed discharge; seismic wave; transfer efficiency; electrode construction component I. INTRODUCTION The technology of pulsed discharge in water has been used in many industrial applications based on the electro-hydraulic effects, such as seismic exploration, electro-hydraulic forming, extracorporeal shock wave lithotripsy and rock fragmentation [1-3]. The high-voltage capacitor is charged in direct current voltage firstly, and then discharges instantly in water by switcher and transmission cable. At last the discharge plasma channel is formed in water. The discharge channel will obtain huge energy in very short time due to the short discharge time (μs-ms) and high discharge current (104 A), which produces the high pressure and high density plasma in water body. The plasma channel expands fast in huge pressure and the shock wave is formed in water. Many applications have been exploited based on the mechanical effect of shock wave. The extensive study has been developed about the experiment This work was supported by R&D of Key Instruments and Technologies for Deep Resources Prospecting (National R&D Projects for Key Scientific Instruments) under contract ZDYZ2012-1-06, the National Natural Science Foundation of China under contracts 51477163 and 51277171, the Science and Technology Project of State Grid Corporation of China. The pulsed discharge in water is also widely used in seismic exploration. The sparkers used in the sea or on the land have formed series of products, which is based on the pulsed discharge in expansive water domain, such as in the sea or in the deep well. But the conventional sparker is not applicable in some places short of water, especially in the mountain area. However, most of metallogenic belt of metal ores are in the orogenic belt, and the surface and subsurface geological conditions are very complicated [6-7]. The conventional seismic exploration techniques (such as dynamite source, vibrator, air gun and the sparker, etc) can not meet the requirements of the seismic exploration in these areas. The phased array sparker technology can make the seismic wave focus on one direction. When the target stratum reflects, the reflection waves will just point to the surface geophone array. Thus the signal-to-noise ratio can be increased, and the high resolution seismic profile can be obtained. A phased array sparker was developed to verify the feasibility of this concept. The discharge electrode, namely energy transducer, is key component in phased array sparker. The discharge electrode filled with water was designed to develop pulsed discharge experiment in closed water domain, which means the narrower water space closed in the discharge electrode. The seismic wave generated by pulsed discharge was measured by seismic wave detector. The electrodes in different structures were also designed to enhance the transfer efficiency from electric energy to mechanical energy. II. EXPERIMENT SETUP AND MEASUREMENT A. Pulsed Power Module The phased array sparker system consists of a diesel generator, a charging system, multiple identical pulsed power modules, multiple transducers, a phase shift controller and a seismograph [7]. The seismic waves produced by multiple transducers were focused on one point by controlling the discharge time sequence using phase shift controller. The geophone array and seismograph were used to measure the seismic waves of ground surface layer by pulsed discharges. Geophone array 1m Discharge electrode Fig. 3. Picture of discharge experiment. Fig. 1. Schematic of one pulsed power module. Each pulsed power module mainly consists of an isolating diode D, a capacitor C (MKMJ5-50, 50 μF) with a stored energy of 625 J, a discharging switch SCR (KP8 800-65), a one-way diode D1 and an inductor L, as shown in Fig.1. Discharging coaxial cable (SYV50-9) is about 15 m, and the discharging current up to 15 kA. The voltage applied to capacitor C is measured by a Lecroy high voltage probe (Model: PMK-14k), which has a division ratio of 1000: 1. The discharge current is measured by a coaxial current divider with the resistance of 5 mΩ. A Tektronix oscilloscope (Model: DPO 2024) is used to record the voltage and current signals. It has a bandwidth of 200 MHz and a time resolution of 1 GS/s. B. Measurement of Seismic Wave The wired distributed seismograph, offered by the Institute of Geology and Geophysics, Chinese Academy of Sciences, was used to measure the seismic wave. It mainly consists of the power station, the data acquisition station (namely geophone array), the crosslink unit (used for data conversion) and the seismic data acquisition software, which is shown in Fig.2. The power station was used to provide power supply for the crosslink unit. The data collected were transmitted to the computer by network line. seismic software Data acquisition station Crosslink unit Fig. 2. Measurement system of seismic wave. III. DISCHARGE EXPERIMENT AND MEASUREMENT RESULTS The picture of discharge experiment is shown in Fig.3, where two discharge electrodes were used. The discharge experiment was mainly done using one electrode in this paper. The geophone array was used to measure the seismic wave of ground surface layer, including six geophones. The distance from the discharge electrode to the first geophone is 1 m, and the distance between two geophones is also 1 m. The cable was used to connect the pulsed power module with the discharge electrode. High voltage electrode External cap Ground electrode Internal insulation High voltage electrode Internal insulation Ground electrode Fixed part Upper cylinder Bottom cylinder Insulation sleeve Insulation sleeve (a) Fixed electrode (b) Piston-like electrode Fig. 4. Discharge electrodes of different structures. The construction of discharge electrodes is shown in Fig.4. Two kinds of electrodes in different structures were used in our experiment. The high voltage output of pulsed power module is connected to the high voltage electrode, and the ground terminal is connected with the ground electrode. The discharge electrode is a closed cylinder, and its internal cavity is filled with salt water. The first kind of electrode (Fig.4a) is fixed, and the electrode component could not move when the discharge occurs. The second kind of electrode (Fig.4b) is piston-like, and the upper components could be active up and down due to the impulsive force produced by pulsed discharge. Its maximum travel is determined by the fixed part in piston-like electrode. To ensure the upper portion movable, it matches the bottom one relatively loose, so the static friction between them is not big. The high voltage electrode consists of multiple centre conductors of cables, and the diameter of centre conductor is 1 mm, the thickness of insulation layer outside of centre conductor is about 1 mm. The existence of insulation layer could reduce the erosion of high voltage electrode. The ground electrode is integral to the upper cylinder and the bottom cylinder, and they are all made of stainless steel. The internal insulation and the insulation sleeve are made of epoxy glass fiber. The discharge distance is adjustable from 0-10cm. One end of high voltage electrode is put in the salt water, and the proportion of salt and water is 30 g/L, its conductivity is about 40-60 mS/cm. 21600 1 V/div 4 1 V/div 2 21650 2 3 3 t (ms) 4 1 1 25 μs/div 200 A/div Arc discharge There are two kinds of discharge modes in our experiment, corona discharge and arc discharge, shown in Fig.5. The curve 1-3 are current waveforms and the curve 4 is the triggering signal waveform. The three current waveforms were the measurement results in three different elements in the array. The time delay between triggering signal and corona discharge is quite short, besides, the starting time of multiple corona discharges is in complete accord. But the time delay in arc discharge is long and the starting time varies. To assure the synchronism of multichannel discharge in phased array sparker, the corona discharge was chosen in our experiment. The typical current waveform of corona discharge is shown in Fig.6. CH1 is the discharge voltage waveform, CH2 is the discharge current waveform, and CH3 is the triggering signal waveform. The discharge voltage and current are respectively 4.8 kV and 780A. The high voltage electrode consists of 70 centre conductors of cables, and the discharge distance is about 4 cm. The typical seismic wave waveforms produced by corona discharge are shown in Fig.7. The sample interval of seismograph is 1 ms, and the record length is 30 s. The seismic wave waveforms in Fig.7 are only a window of the 30 s record length and there are no seismic wave in other record time. So the whole record length was not given here. There were six geophones in the data acquisition system, but the data of front three channels were only given to observe clearly the seismic wave. It can be seen that the amplitude of the seismic wave decreases with the increasing distance between the discharge electrode and the geophone, the initial arrival time of seismic wave increases correspondingly. The scale of horizontal ordinate is not marked because the measured seismic wave amplitude is a relative value. The seismic wave produced by pulsed discharge in closed salt water is an oscillation pulse, decaying fast with the measurement distance. CH1: 2 kV/div CH2: 200A/div Fig. 6. Typical current waveforms of corona discharge. C2 C3 Fig. 7. Typical seismic wave waveforms produced by discharge 3.5 1 7 40 70 100 3.0 2.5 Current (kA) Fig. 5. Current waveforms of two discharge modes. C1 Arc discharge 2.0 1.5 1.0 0.5 (a) 0.0 2 3 4 5 4 5 Voltage (kV) 2.00 1 7 40 70 100 1.75 1.50 Seismic wave 250 μs/div 200 A/div Corona discharge 21700 1.25 1.00 0.75 0.50 0.25 (b) 0.00 2 3 Voltage (kV) Fig. 8. Effect of high voltage electrode on results IV. EFFECT OF ELECTRODE STRUCTURE ON SEISMIC WAVE The effect of high voltage electrode, applied voltage and discharge distance was studied to obtain the seismic wave in different conditions. The discharge distance is from 1 cm to 6 cm in our experiment. The experiment result indicates that the corona discharge is relatively stable and the arc discharge is not apt to occur at 4 cm. The results of different high voltage electrodes in different applied voltages are shown in Fig.8. The high voltage electrodes respectively consist of 1, 7, 40, 70 and 100 centre conductors of cables, the applied voltage is from 2 kV to 5 kV. The discharge current increases with the applied voltage and the number of centre conductors from Fig.8a. The arc discharge occurred with 100 centre conductors at 4.8 kV, and the current of arc discharge is about 3 kA. The amplitude of seismic wave produced by discharge increases with the applied voltage in most conditions, but the increasing speed is relatively slow. The effect of centre conductor number is not very clear, but the result of 70 centre conductors is relatively good, where the amplitude of seismic wave increases linearly with the applied voltage. At the same time we can find the seismic wave produced by arc discharge with 100 centre conductors at 4.8 kV is about the same with that by corona discharge with 70 centre conductors at 4.8 kV. Even if the discharge current is large in arc discharge, the amplitude of seismic wave produced by arc discharge is not always large. In general, the experiment result with 70 centre conductors and the discharge distance of 4 cm is relatively good in our experiment conditions. Only the internal insulation and high voltage electrode are movable in the internal movable electrode. The seismic wave produced by discharge using the movable electrode is larger than using the fixed electrode, especially in the higher voltage or using the external movable electrode. 6 5 4 Epoxy Rubber Nylon PMMA PE PTFE 0.7 Seismic wave 0.6 0.5 0.4 0.3 0.2 0.1 2.5 3.0 3.5 4.0 Voltage (kV) 4.5 5.0 Fig. 9. Effect of bottom materials on results The experiment in Fig.9 has been done with the discharge distance of 6 cm, where the amplitude of seismic wave with Epoxy and PTFE materials is relatively large. But a large number of experiments indicate that the data are dispersal in different discharge distances. When the bottom cylinder is made of insulation materials, the bottom portion can not be used as ground electrode, and the upper portion made of stainless steel is ground electrode in this structure. So the discharge path changed and the discharge energy declined compared with the previous structure, which could have effect on the experiment results. The amplitude of seismic wave using the insulation bottoms is relatively lower, so the effect of different insulation materials is not clear. B. Movable Electrode The previous experiment indicates that the amplitude of seismic wave is comparatively large when the discharge electrode has no external cap. In other words, when the discharge occurs, part of electrode will move. In this conditions, the seismic wave is large. To verify the idea, the piston-like electrode in Fig.4b was designed. The experiment result is shown in Fig.10. Three electrodes were used in the experiment, including the external movable electrode (out, Fig.4b), the internal movable one (in) and the fixed one (in-fixed, Fig.4a). Seismic wave A. Materials of Bottom Cylinder The above experiment was done using the fixed electrode in Fig.4a and its bottom cylinder is made of stainless steel. The shock wave produced by pulsed discharge propagates in closed water of discharge electrode. It will be reflected partly and the partial energy will be lost when it reaches the bottom cylinder. In theory, when the bottom cylinder has the similar acoustic impedance with salt water, the propagation efficiency of shock wave should be highest from the closed water to the ground. So the experiments were done with different bottom materials, and the results are shown in Fig.9. 3 out in in-fixed 2 1 0 2 3 Voltage (kV) 4 5 Fig. 10. Seismic wave of fixed and movable electrodes V. CONCLUSIONS A phased control sparker has been developed for the basic research on the seismic exploration of the metal ores. Several kinds of discharge electrodes were designed to obtain larger seismic wave. The experiment results indicate that corona discharge is more suitable in phased control sparker due to short time delay and good synchronism. The movable discharge electrode produces larger seismic wave than the fixed electrode. The measured wave in our experiment is mainly the seismic wave which propagates along the surface layer of ground. The vibration wave produced by mechanical shock is easy to be detected by geophone near the electrode. So the energy transition in present experiment mainly depends on the mechanical shock from the pulsed discharge in closed electrode to the seismic wave on the ground surface layer. REFERENCES [1] [2] [3] [4] [5] [6] [7] Z. Y. Qin, G. N. Zuo, Y. R. Wang, H. Wu, G. S. Sun, and Y. H. Sun, “High voltage impulse discharge and its applications,” Beijing: Beijing university of technology Press, 2000. Z. Y. Li, Y. Liu, Y. B. Han, Q. P. Luo, and F. C. Lin, “Erosion mechanism research of metal electrode under pulse current in water,” High Power Laser and Particle Beams, vol. 28, art. no. 045007, 2016. P. D. Kumar, S. Kumar, and R. Thakur, “Erosion and lifetime evaluation of molybdenum electrode under high energy impulse current,” IEEE Trans. Plasma Sci., vol. 39, pp. 1180-1186, 2011. M. Hamelin, M. Kitzinger, and S. Pronko, “Hard rock fragmentation with pulsed power, ” in Proc. 9th IEEE Pulsed Power Conference, Albuquerque, NM, 1993, p. 11. T. H. Weise, J. Hofmann, and M. J. Loffler, “Fragmentation of composite materials by electrothermally generated pressure pulses,” 10th IEEE Pulsed Power Conference, Albuquerque, NM, 1995, pp. 1194-1199. Y. H. Sun, Y. H. Gao, and P. Yan, “Experimental study of a 20kJ multielectrodes sparker,” in Proc. IEEE International Power Modulator and High Voltage Conference, Atlanta, GA, 2010, pp. 564-567. Y. H. Sun, R. Y. Fu, Y. H. Gao, X. Z. Xu, C. Y. Ren, and P. Yan, “Development of a phased array sparker,” in Proc. IEEE International Power Modulator and High Voltage Conference, Santa Fe, NM, 2014, pp. 348-351.