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  • 8288 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 33, NO. 10, OCTOBER 2018

    Multifunction Electromagnetic Transmitting System for Mineral Exploration

    Meng Wang , Sheng Jin, Ming Deng, Wenbo Wei, and Kai Chen

    Abstract—Electrical and electromagnetic methods have long played an important role in mineral exploration. For example, in active source prospecting methods, when measuring the val- ues of many electrical parameters, the signal-to-noise ratio can be improved by increasing the transmitting power. In this paper, we present a multifunction borehole ground electromagnetic trans- mitting system that transmits rectangular waves at a frequency of 0.01–10 kHz with a frequency error less than 10−8 Hz @ 1 Hz (de- pending on the high-precision temperature compensation crystal) and the initial synchronization error between the transmitter and receiver is less than ±0.1 µs. During operation, either the maxi- mum transmitting voltage output can reach 950 V or the maximum transmitting current can exceed 90 A, but both cannot be achieved simultaneously. We tested the transmitting system at its maximum power output continuously for over 8 h and the power output was more than 48 kW at a current above 60 A. The energy was deliv- ered above ground or underground using existing drill casings to produce the high-power transmissions. The field experiments show that the system meets the demands of the controlled-source audio- frequency magnetotelluric and induced polarization methods used in mineral exploration.

    Index Terms—High power, mineral exploration, multifunction borehole ground electromagnetic transmitter.

    I. INTRODUCTION

    FOR many years, electrical and electromagnetic (EM) meth-ods have played important roles in mineral explorations [1], [2]. There has been a trend in the development of those methods toward reaching deep earth [3], [4]. For explorations that leverage active transmitters, the signal-to-noise ratios can be improved by enhancing the transmitting power, while the inversion accuracy can be improved by taking advantage of the electrical parameters. However, this requires the performance of EM instruments to be enhanced to increase the detection depth, accuracy, and resolutions of measurements [5]–[7].

    Much research has been conducted by instrument manufac- turers, research institutes, and academies toward increasing the

    Manuscript received July 30, 2017; revised October 15, 2017; accepted November 7, 2017. Date of publication November 17, 2017; date of current version July 15, 2018. This work was supported in part by the National High Technology Research and Development Program of China (2014AA06A603) and in part by the National Science Foundation of China (41504138). Rec- ommended for publication by Associate Editor T. M. Lebey. (Corresponding author: Meng Wang.)

    The authors are with the School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, China (e-mail: wangmeng@ cugb.edu.cn; jinsheng@cugb.edu.cn; dengming@cugb.edu.cn; wwb5130@ cugb.edu.cn; ck@cugb.edu.cn).

    Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

    Digital Object Identifier 10.1109/TPEL.2017.2774817

    Fig. 1. Schematic diagram of the multifunction borehole ground EM imaging system.

    transmitting power of EM instruments. Current equipment sup- ports transmit powers of 30 kW (GGT-30 and T-30 are made by Zonge and Phoenix, respectively), 50 kW [8], [9], 70 kW [10], [11], to 160 kW (T-200 is made by Phoenix) [12] and 200 kW [13]. In particular, the T-200 instrument was designed for superpower controlled-source audio-frequency magnetotel- luric (CSAMT) and time-frequency EM methods, while the sys- tem designed by Central South University is used for high-power wide-area EM and pseudorandom signal methods [14]–[16].

    We developed a high-power borehole ground EM imaging system to detect deep mineral resources [17], [18], as shown in the schematic diagram in Fig. 1. An important component in this system is the high-power multifunction borehole ground EM transmitter. The receiver arrays are deployed on land or in existing shafts, such as test drill holes in the mineral field where metal casings have been left in the borehole. In these cases, the existing metal casings are used as the transmitting electrodes, and the transmitters and receivers are offset to form a quasi-three-dimensional prospecting configuration.

    In the following, we first introduce the design and important characteristics of the developed system.

    A. Modular Design

    A schematic diagram of the high-power electromagnetic transmitter is shown in Fig. 2. The system is composed of a modular structure that include a high-power generator (shown in the left of Fig. 3), an adjustable direct current (dc) switching

    0885-8993 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

    https://orcid.org/0000-0002-5393-0418

  • WANG et al.: MULTIFUNCTION ELECTROMAGNETIC TRANSMITTING SYSTEM FOR MINERAL EXPLORATION 8289

    Fig. 2. Schematic diagram of the high-power EM transmitting system.

    Fig. 3. High-power generator (left) and dc switching power supply (right).

    Fig. 4. Multifunction borehole ground electromagnetic transmitter (left) and the transmitting current recorder (right).

    power supply (shown on the right of Fig. 3), a transmitter (shown in the left of Fig. 4), a transmitting current recorder (shown in the right of Fig. 4), an external control box (shown on the left of Fig. 5), a portable tablet, which runs the monitoring and control

    software, and a set of transmitting antennas and electrodes. An advantage of this type of modular structure is that it can be easily tested, repaired, and maintained.

    The adjustable dc switching power supply is used to con- vert the fixed triple-phase 50 Hz/380 VAC (alternating current) from the high-power generator into the variable 0–1 kV dc/0– 120 A power required by the multifunction borehole ground EM transmitter. The dc switching power supply contains four power modules and each module can deliver 500 V/60 A. Multiple- series connections were used to produce the desired output of 1 kV dc/120 A. The topology of the four power modules is as follows. The power modules shown near the top of Fig. 2 are denoted from left to right as first, second, third, and fourth. The first and second 500 V/60 A power modules are connected in series to form a 1 kV/60 A power module group, as are the third and fourth. The final outputs of these pairs are then connected in parallel to realize a 1 kV/120 A power module group.

    The power modules are connected and controlled through the communication bus by the dc switching power supply con- trol unit. The touch keyboard shown in the figure can be used to specify the voltage and current output, and the output can be dis- played on the LCD screen. Each power module includes a low frequency rectifier [19], insulated-gate bipolar transistor (IGBT) H full bridge that operates at 20 kHz [20], [21], pulse width mod- ulation (PWM) control unit [22], high frequency transformer, and rectifier [23], [24]. The control unit monitors the output of each module and makes adjustments as required, and facilitates the current and voltage sharing of the four power modules.

    A schematic diagram of the EM transmitter control system is also shown in Fig. 2. While there are five power inverter modules (IGBTs) in a traditional transmitter topological diagram, [25], [26], our design only used four IGBTs and did not use a dummy

  • 8290 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 33, NO. 10, OCTOBER 2018

    Fig. 5. External transmitter control box (left) and screenshot of the monitoring and control software on the portable tablet (right).

    load. We conducted a time domain induced polarization (TDIP) test, and the power output was about 950 V × 10 A � 10 kW, which was much less than the maximum power output of the generator. The transmitter supports the CSAMT method and is able to operate continuously for up to 5 μs without stopping. Thus, a dummy load is not required to protect the generator. The transmitter also incorporates a full H-bridge structure to support inversion from dc to ac and includes protection and clamping circuits that employ special high-voltage metallized polypropylene film capacitors to absorb and buffer high-voltage spikes and ensure the stable and reliable performance of the transmitter. The auxiliary acquisition circuit is used to capture the temperature, output voltage, and output current information.

    When the power switching devices in the transmitter are op- erational, significant amounts of noise is generated from the voltage spikes, which influences the low-voltage control unit [27]. For this reason, the high- and low-voltage circuits were separated in the design. The remote-control bus is a shielded twelve core cable that includes the signals for the IGBT drivers and the auxiliary signals that are used to connect the transmitter to the external control box [28]. The external control box, which is used to measure the output voltage, internal temperature, root mean square (RMS) value of the current, and alarm signals from the transmitter drive boards, was placed outside the body of the transmitter to minimize the space EM interference. A portable tablet is included in the system to connect wirelessly to the exter- nal control box for control purposes. The host control software was implemented in the C# programming language with a flex- ible control pattern and a convenient user-friendly interactive in