IoT-based 4-dimensional high-density electrical instrument for geophysical prospecting

The high-density electrical method is a primary method used in shallow geophysical prospecting. With the rapid industrial development of recent years, the function and performance of high-density electrical instruments have been considerably improved in several aspects. However, most of the electrical instruments currently available on the market still 10 exhibit some shortcomings, such as being bulky, heavy, limited in their data acquisition accuracy, and difficult to connect to the Internet for remote monitoring. To address these problems, this study developed a new multifunctional 4-dimensional (4D) high-density electrical instrument based on remote wireless communication technology. The system is small and lightweight, includes an integrated transceiver, has high data acquisition accuracy, and is capable of remote wireless real-time control. In this study, the hardware circuit was designed. The Arm all-in-one (AIO) LJD-eWinV5-ST7 with a 154.4cm × 87cm, 800 × 15 480 high-brightness wide-temperature-range display is used as the host computer, which has the advantages of small size, low power consumption, and abundant hardware resources. IoT technology is incorporated in the system, and a 4G module is employed to provide a real-time remote control and data acquisition monitoring system based on the cloud platform. Tests showed that this instrument is stable and convenient to use and can meet the requirements for use in field prospecting.

Thus, earth resistivity can be obtained as follows: The theory underlying the high-density resistivity method is the same as in the conventional resistivity method; the difference is that the former has a higher-density arrangement of measurement points for taking observations. During the measurement process ( Fig. 2), once all the electrodes are placed on the measurement points at the specified interval, the host can 70 automatically control the changes in the power supply electrodes and the receiving electrodes, thereby completing the measurement (Dong and Wang, 2003). In terms of design and technical implementation, the high-density electrical measurement system is based on advanced automatic control theory and uses large-scale integrated circuits. A large number of electrodes are used, which can be combined freely. In this way, more geoelectric information can be extracted, and the multi-coverage observation approach used in seismic prospecting can be realized in electrical prospecting as well (Di et al., 75 2003).

Design of 4-dimensional (4D) high-density electrical instrument based on IoT 80
3.1 Overview of architecture Development of the system used in this study included designing hardware circuits, software programs, and mechanical structures. The system's stability and accuracy in actual use are particularly important (Mendecki et al., 2010).
A block diagram of the IoT-based 4D high-density electrical instrument is shown in Fig. 3. The instrument consists mainly of a high-voltage board, an acquisition board (AB), and an Arm all-in-one (AIO) PC, which form a complete system with the 85 electrodes and electrode multiplexers. The AIO is the Arm LJD-eWinV5-ST7 with a 154.4cm × 87cm, 800 × 480 highbrightness wide-temperature-range display. It is small in size, consumes little power, and has abundant hardware resources, including Secure Digital (SD) cards and multiple RS232 and RS485 ports. The system additionally contains a 4G module, which uses the USR-G780 module. This enables the user to not only control the instrument at the measurement site directly via the mouse or touch screen but also control it remotely with a computer via the cloud platform. To carry out the 4D high-90 density electrical prospecting, 480 electrodes are available for use.
The system design is described in detail in the following subsections.

Design of acquisition circuit
The AB serves as the front-end circuit, which is responsible for processing the analog signals acquired by the electrodes and converting them into digital signals to be processed. The ADC chip is a high-precision 24-bit serial A/D converter CS5532.
The chip's gain can be controlled, and it contains a very-low-noise, chopper-stabilized instrumentation amplifier, and a fourthorder delta-sigma modulator followed by a digital filter. 100 The front ends of the ADC chip's two channels (ch1 and ch2) are respectively connected to the precision resistors of the MN and AB electrodes' front ends. The voltage and current values can be read simultaneously using dual channels. Figure 4 shows a block diagram of the acquisition device. The processing of the two analog channels is similar, the difference being that the MN voltage channel has self-potential compensation. Figure 5 shows a schematic of the MN channel of the acquisition device.
The signals first undergo impedance matching and then pass through the low-pass filter to filter out high-frequency clutter. 105 Because the input to the ADC is a differential signal, the single-ended signal must be converted into a differential signal to enter the ADC chip, which then outputs the digital signal. The ADC chip of this circuit performs sampling at 20 ms (50 Hz corresponds to a 20 ms period), thereby effectively suppressing the interference from the 50 Hz grid power and multiple waves. In addition, the channel connecting AD and MN has an INA128 as a subtractor, which subtracts the natural self-potential (VSP) converted by the DAC chip, thereby accomplishing self-potential compensation, for a more accurate calculation result. 110

Design of transmitting circuit
The transmitting circuit can be divided into alternating current (AC) transmission and direct current (DC) transmission, and different devices are applied in different environments and for different measurements (Li et al., 2020;Zeng et al., 2018).
This system uses a DC power supply. Compared with an AC power supply, DC provides a greater prospecting depth and 120 exhibits advantages in reducing electromagnetic coupling interference. The transmitting circuit consists mainly of a protection circuit, an isolation circuit, the intelligent power module (IPM) (10 A / 1200 V) (Xing, 2001), a power conversion circuit, the output electrodes, and a high-voltage source. The external DC high-voltage power supply is connected to the IPM via the protection circuit. The main control unit sends control commands through the multiple optocoupler to control the IPM. The optocoupler used here is the HCPL-4504 from Agilent, which is an optocoupler dedicated to the IPM interface. This product's 125 emitting light source and receiving light source are insulated by a transparent insulator, and there is no actual electrical connection. It has high speed, a high common-mode ratio, and an extremely short parasitic delay. Its instantaneous commonmode ratio is greater than 10 kV µs −1 . It is an electrical isolation device dedicated to the IPM, which effectively reduces mutual interference between signals.
This system is an integrated transceiver having no separate transmitter; it exhibits the advantages of being lightweight, small, 130 and portable.

Software design
The Arm AIO LJD-eWinV5-ST7 has abundant hardware resources and a complete set of software functions. It contains, for example, an I/O port, SD card, dual-channel ADC, multiple RS232 ports, and multiple RS485 ports. The built-in ADC sampling rate is fixed at 2 µs, which is too fast and produces too much noise. Therefore, an external ADC chip is required. 135 Then, because the I/O port's timing accuracy cannot meet the requirement of this external ADC chip, an STC15 single-chip microcomputer is added as a slave computer to control commands. It communicates with the Arm AIO through an RS485 interface. Therefore, the software system includes the STC15 software and the Arm AIO software.
The slave computer program is the STC15 microcontroller program based on the Keil4 platform and is used to perform current and voltage acquisition, self-potential compensation, data transmission, communication, and other functions. It also sends the 140 acquired current and voltage values to the Arm AIO for calculation and processing.
C# is an object-oriented programming language derived from C and C++. It inherits the powerful functions of both C and C++ while excluding some of their complex features (Perkins et al., 2016;Zhao et al., 2013) and combines the simple visual operation of VB with the operating efficiency of C++ (Ji, 2008). This study used the C# language, a control platform based on the Windows CE operating system, and the VS2008 development software. The software functions include creating new 145 projects, setting parameters (device type, measurement mode, electrode distance, power supply time, power-off time, starting electrode, ending electrode, starting measurement layer, ending measurement layer, power supply current, secondary delay, use/nonuse of VSP compensation, etc.), opening a project, assigning addresses, inspecting electrodes, executing measurement commands, monitoring commands, viewing profiles, viewing curves, executing the END command, saving files, viewing data, pausing, and stopping. The various device types include the Wenner device, Wenner-Schlumberger device, Schlumberger 150 device, β device, Wenner roll-along, pole-pole device, pole-dipole device, differential device, and dipole-dipole device. An overview of the instrument's program flow is given in Fig. 6.

Key performance indicators of the instrument and a comparison 170
The 4D high-density electrical method adds a time dimension to the 3-dimensional (3D) method; that is, ρ = ρ(x,y,z,t). The data acquisition is performed by arranging the same set of electrodes at a given location and then repeating the 3D data acquisition at different time points (Loke, 1999). Table 1

Laboratory and campus tests 180
After the instrument was developed, its practical application capabilities needed to be tested. To test the instrument's accuracy in the laboratory, an experiment was conducted using the resistor string method. A resistor string network was soldered as shown in Fig. 8 to form the equivalent of a dummy load. There were forty-eight 510 Ω resistors, with 1 Ω resistors placed between adjacent 510 Ω resistors, forming a voltage divider network. From each 510 Ω resistor, a wire was soldered from the board to the cable, forming the equivalent of 48 measurement electrodes. The voltage and current data measured at a 60 V 185 power supply were recorded, with results as shown in Fig. 9. After several experimental runs, including tests with large and small signals (range from 1mV to 500V ) and a variety of resistance values (range from 0.01 Ω to 510 Ω), the final accuracy was calculated to be within 1/1000.

195
Further testing was conducted on the China University of Geosciences (Beijing) campus, as shown in the Fig. 10 photo. A large lawn in front of the main building was selected as the ground to be tested; there is a tube well on the lawn. The electrodes were arranged in a space of approximately 60 m. A real-time profile view of the test site as displayed on the instrument is shown in Fig. 11; it can be seen that the lawn and the tube well have different colors on the profile view. To validate the remote solution, in addition to adjusting the 4G module in the laboratory, transparent cloud transmission was used to monitor the data 200 remotely on the PC in the laboratory. The profile view displayed on the notebook PC (Fig. 12) was the same as that displayed on the electrical instrument. Thus, the feasibility of using the program remotely was confirmed.

Field tests
To further verify the stability and feasibility of the instrument, it was used to conduct high-density electrical prospecting in Lantian Village, Anxi County, in the province of Fujian, China. In correspondence with the actual terrain and topography of the farmland on the east side of Huazhang Natural Village, Lantian Village, Lantian Township, three high-density electrical 215 survey lines were arranged (Fig. 13 shows their schematic layout), with electrodes spaced at 10 m on each survey line. Survey Line L1 had 96 electrodes, whose starting and ending coordinates were (25.132667° N, 117.879972° E) and (25.124778° N, 117.878306° E), respectively; L2 had 64 electrodes, whose starting and ending coordinates were (25.124000° N, 117.883417° E) and (25.126611° N, 117.878528° E), respectively; and L3 had 80 electrodes, whose starting and ending coordinates were (25.132250° N, 117.880250° E) and (25.125889° N, 117.878750° E), respectively. A Garmin Vista handheld GPS was used 220 to position each electrode. Figure 14 shows a photo of the field testing.

Survey Line L2
Survey Line L2 had 64 electrodes, a trace spacing of 10 m, and a profile length of 640 m. The hill is more than 40 m higher 245 than the flat ground. The first electrode measurement point was located on the hill platform, and its latitude, longitude, and elevation were 25.124000° N, 117.883417° E, and 663 m, respectively. There were 4 electrode measurement points on the hill platform and 13 electrode measurement points on the slope. The last electrode measurement point was close to the village roadside; its latitude, longitude, and elevation were 25.126611° N, 117.878528° E, and 624 m, respectively.
According to the profile view obtained using the high-density electrical method (Fig. 16), the highest part of the hill is relatively 250 flat, where the underground medium is relatively uniform and dense. At the foot of the hill is a low-resistance zone containing a larger amount of underground water; the western tail of the survey line passes through the water-bearing region.

Survey Line L3
Survey Line L3 was east of L1 and closer to the hill. It had 80 electrodes, a trace spacing of 10 m, and a profile length of 800 m. The latitude, longitude, and elevation of the first electrode were 25.132250° N, 117.880250° E, and 614 m, respectively.
Taking the 15th electrode measurement point (with latitude, longitude, and elevation of 25.131250° N, 117.880000° E, and 613 m, respectively) as the center, the zone beneath exhibited low resistance, which corresponds to the low-resistance zone on 260 L1 but is narrower. From the 48th electrode measurement point (with latitude, longitude, and elevation of 25.128250° N, 117.880306° E, and 616 m, respectively) to the 58th electrode measurement point (with latitude, longitude, and elevation of 25.127417° N, 117.880028° E, and 617 m, respectively) was a low-resistance zone with a width of approximately 90 m. The uppermost layer after the 58th electrode measurement point was a low-resistance section, beneath which was a high-resistance zone, similar to the tail of L1. The profile view of L3 obtained using the high-density electrical method is shown in Fig. 17. 265 The intersection of the 72nd electrode of L3 with the 57th electrode of L2 is rich in underground water.