Due to the pressing demand for metallic ore exploration technology in China, several new technologies are being employed in the relevant exploration instruments. In addition to possessing the high resolution of the traditional transient electromagnetic method, high-efficiency measurements, and a short measurement time, the multichannel transient electromagnetic method (MTEM) technology can also sensitively determine the characteristics of a low-resistivity geologic body, without being affected by the terrain. Besides, the MTEM technology also solves the critical, existing interference problem in electrical exploration technology. This study develops a full-waveform voltage and current recording device for MTEM transmitters. After continuous acquisition and storage of the large, pseudo-random current signals emitted by the MTEM transmitter, these signals are then convoluted with the signals collected by the receiver to obtain the earth's impulse response. In this paper, the overall design of the full-waveform recording apparatus, including the hardware and upper-computer software designs, the software interface display, and the results of field test, is discussed in detail.
The full-waveform voltage and current recording device of a multichannel transient electromagnetic method (MTEM) transmitter is an important component of MTEM exploration equipment. This device is responsible for the continuous acquisition and storage of the large pseudo-random current signals emitted by the MTEM transmitter. These signals are then convoluted with the signals collected by the receiver to obtain the impulse response of the ground under test (Zhong and Xue, 2014; Xue et al., 2015). Electrical methods (including conductive and inductive electrical methods) are crucial in metallic ore exploration. The conductivity, dielectric constant, and magnetic permeability of the rock ore are the main physical parameters measured in electrical geological exploration (Xue et al., 2007; Newman, 1989). Currently, the most critical problem with these methods is the interference from the area being explored. Solutions to reduce the effects of interference include increasing the transmission power of the field source, expanding the distance between the transmitter and receiver (conductive electrical methods), and reducing the observation signal frequency (inductive electrical methods). However, these measures often result in insignificant improvements in the signal-to-noise ratio (SNR) and higher costs. Therefore, they cannot satisfy the demands of the metallic ore exploration industry (Wright et al., 2001). The device examined in this study uses pseudo-random code to transmit certain artificial signals; the induced electromagnetic signals are then collected using a receiver at the remote end. Coherent decoding can significantly enhance the SNR of the acquired data and improve the exploration outcome; hence, it is suitable for monitoring high-power signals. Furthermore, the device also uses related technology to ensure the accuracy of the acquired signal amplitude and acquisition time, thereby achieving simultaneous acquisition of the high- and low-speed voltage and current.
Block diagram of the conditioning process for voltage acquisition.
Block diagram of the conditioning process for current acquisition.
In the future, the full-waveform voltage and current recording device can be used for marine exploration, marine electromagnetic exploration as a supplement to seismic exploration technology, can directly detect oil and gas in the structure of oil and gas, its status in the field of marine exploration is increasing. MTEM is accompanied by marine electromagnetic exploration came into being, if this technology is applied to land and sea oil and gas exploration, will greatly reduce the risk of three-dimensional seismic exploration, non-seismic methods of oil and gas resources exploration will have a very positive impact. In the future, the full-waveform voltage and current recording device will be used for marine electromagnetic exploration as a supplement to seismic exploration technology. The device can directly detect oil and gas; its status in the field of marine electromagnetic exploration is increasing. If this technology can be applied to land and sea oil and gas exploration, it will greatly reduce the risk of three-dimensional seismic exploration. Additionally, non-seismic methods of oil and gas resources exploration will have a very positive impact.
This study discusses the development of the full-waveform voltage and current recording device for MTEM transmitters. The article is structured as follows: Sect. 2 presents the hardware circuit design of the full-waveform recording device, Sect. 3 describes the upper-computer software design, and Sect. 4 presents the test results.
The hardware circuit design of the full-waveform recording device for MTEM
transmitters can be divided into the voltage and current channel designs
(Ziolkowski et al., 2007). The design principles are depicted in Figs. 1
and 2. The voltage channel divides the input signal into two levels:
large range and small range. For large-range levels, the maximum input for
the transmitter voltage amplitude is 1000
The calculation formula for high hysteresis comparison voltage is as
follows:
Schematic circuit diagram of high-speed optocoupler isolation.
The signals emitted by the transmitter are strong with large voltages and
currents, whereas those of the master FPGA are weak (Huang et al., 2015).
Hence, an isolation barrier is needed between the two for protecting the
master circuit and the upper computer connected to it. Therefore, we
designed a high-speed optocoupler isolation circuit. Every digital
input–output of the ADC on the acquisition board requires a high-speed
optocoupler for isolation; the input and output directions of the
optocoupler were verified one-by-one to ensure circuit correctness (Jhin et
al., 2014). HCPL0723 is a high-speed, positive logic CMOS optocoupler of
Avago Technologies company, which supports a transmission
speed of up to 50
Block diagram of the FPGA master circuit.
Block diagram of high precision atomic clock circuit.
The acquisition circuit of the full-waveform recording device can be divided
into low speed and high speed. The low-speed acquisition circuit uses the
ADC ADS1271 to achieve a sampling rate of 32
Block diagram of the Bluetooth and 485 module transmission.
Interface display of the main window form on the upper computer.
Test results of the low-speed voltage acquisition
channel.
Test results of the high-speed voltage acquisition
channel.
Test results of the low-speed current acquisition
channel.
Test results of the high-speed current acquisition
channel.
The FPGA master circuit is the control core of the MTEM transmitter
full-waveform voltage and current recording (Sun et al., 2016). It is
responsible for the core functions of the recording device, including
control acquisition and data transfer (Ziolkowski et al., 2011). The master
circuit is mainly composed of the FPGA and a few external storage devices;
its block diagram is depicted in Fig. 4. The storage devices include an
SRAM, SDRAM, and serial and parallel flashes. Among them, the SDRAM is used
as the data buffer for low-speed acquisition and the parallel flash, as the
memory for the FPGA-configuration firmware. In order to ensure data
integrity in the low-speed acquisition, an external SDRAM was used as a
buffer for the data from this circuit. The 256
After the upper computer receives the transmission signal waveform, it calculates the voltage and current peak values, frequency, timestamp, and the other transmission waveform information (Zhong et al., 2016). This information is then transferred to the lower computer via the command control channel. The lower-computer forwards these data, as per the serial communication protocol, to the 485 and Bluetooth modules; the data are then transferred to a remote PC via wired or wireless methods. Figure 6 shows the process block diagram.
The MTEM method requires the simultaneous recording of the transmitter current and receiver voltage signals followed by the convolution of the two to calculate the earth's impulse response. Hence, the acquisition time for these two signals is an important parameter (Wright, 2003). This design uses an atomic clock as the clock source for the master circuit and all the ADC clocks are based on stable clock frequency dividers or multipliers of the atomic clock outputs (Xu et al., 2004; Zhang et al., 2015). By synchronizing the pulse per second (PPS) of the atomic clock and the GPS, and then synchronizing the PPS of the receiver and GPS, we could approximate the simultaneous acquisition of the two acquisition systems (Olalekan and Di, 2015).
The block diagram of the high-precision atomic clock circuit is shown in Fig. 5. The clock board consists of three parts: the MSP430 microcontroller, GPS module, and atomic clock module. In addition, the clock board also includes some necessary download ports, configuration interface, and the interface with the main control board.
The MSP430 microcontroller, the master, downloads the program through the JTAG download port. The microcontroller communicates with the atomic clock SA.45S through serial port 1, receives the serial data to obtain the atomic clock running status, and sends the serial port command to control its taming time. MSP430 and GPS module communicate with each other through serial port 2.
The transmitter of the GPS serial port is also connected to the input–output of the FPGA so that the acquisition data contains the location coordinate information. The GPS second pulse signal is connected to the atomic clock of the PPS_IN pin to tame the atomic clock module. GPS sets the mode of operation through its full-speed USB interface; after setting the work mode of host computer software, you can store the work mode information in the internal of GPS, so that the host computer can automatically read the last saved work mode information from the internal of GPS when the module power was cut off.
The upper-computer program was built on the Visual Studio platform using C# Windows forms. The program mainly includes the main window form (Form1), the sub-window form (Form2), and a self-defined class function. The program framework is described below: in the main window form, the read and write functions of each channel were assigned to eight corresponding threads. Sampling or interception was performed on the data flags and channel types to obtain data points for plotting the waveform; four drawing threads were opened to plot the waveforms. Class functions were formed by packaging each channel of the USB equipment into a class, based on the endpoint address. Each class included the endpoint parameters and two methods to read (using the asynchronous read mode) and write data into the files. The read and write methods were assigned to the threads of their own channels, in the main function. The sub-window form was used to open and review the stored file data. The entire software uses the Metro style, which provides a more humanistic human–computer interface. Figure 7 displays the main window form.
Overall performance testing was conducted after developing the full-waveform
recording device. For the low-speed voltage, 20
Figure 8 shows the test results of the low-speed voltage acquisition
channel. Figure 8a is the time domain waveform of the sinusoid acquired by
the recording device, whereas Fig. 6b is the frequency domain waveform. As
seen from Fig. 8b, the corresponding amplitude of the input 20
Figure 9 displays the test results of the high-speed voltage acquisition
channel. In Fig. 9b, harmonic distortion at an amplitude of approximately
Figure 10 shows the test results of the low-speed current acquisition
channel. We can see from the amplitude-frequency characteristics that there
was a 50
Figure 11 displays the test results of the high-speed current acquisition channel.
In this study, the development of a full-waveform voltage and current
recording device for MTEM transmitters was presented. First, the hardware
circuit was designed to complete the pre-conditioning and acquisition of the
signals, which were then transferred to the FPGA for data processing. Then,
the upper-computer software was designed to further process the data and
present them as graphic plots. The overall performance was tested, with
individual tests for the high- and low-speed channels for the voltage and
current. The maximum transmission current and voltage acquired by the device
were 50
No data sets were used in this article.
The authors declare that they have no conflict of interest.
This work was supported by the Fundamental Research Funds for the Central Universities of China (no. 2652015213), the National Natural Science Foundation of China (no. 41574131), and the National Major Scientific Research Equipment Research Projects of China (no. ZDYZ2012-1-05-01). Edited by: Luis Vazquez Reviewed by: three anonymous referees