Development of Multi-Channel Seismic–Electrical Combined Rolling Coverage Measurement System

Abstract

Electrical and seismic exploration are two widely used geophysical methods in geological surveys. They reflect different geophysical properties of underground rocks, but each method can only provide information from a single perspective of the rock structure’s physical parameters. As a result, relying on a single geophysical method can lead to ambiguous interpretations. To address this issue, this paper presents the development of a multi-functional, high-power, multi-channel, rolling, fast measurement system for combined electrical and seismic exploration. The system features the following capabilities: it can be used simultaneously for both electrical and seismic exploration; it supports high-power operation, with a transmission power of up to 10 kW during electrical exploration; it includes multiple measurement channels for rolling measurement and data acquisition, with a sampling rate of up to 100 ksps, significantly improving work efficiency and expanding the frequency range. The distance between channels can be selected from 1 to 20 m, greatly enhancing the system’s adaptability to various environments. Additionally, we have designed accompanying upper-level software that not only stores data internally but also displays waveforms in real-time on a computer for monitoring and control. The experimental results demonstrate that the instrument operates stably and meets the requirements for field exploration.

1. Introduction

Today’s society is in an era of rapid technological development, and human demand for resources has reached unprecedented heights [1]. The security of supply for strategic energy sources, such as oil and gas, is crucial [2]. To meet the increasing demand for resources like these, it is of great importance to continuously improve traditional geophysical instruments [3].

Electrical and seismic exploration are two widely used geophysical methods in geological surveys [4]. Electrical exploration technology is a geophysical method used to detect geological structures, locate minerals, and find water based on the differences in resistivity and polarization characteristics of soil and rock [5]. It has many advantages, such as a high resolution and accurate results. Common electrical exploration methods include the natural potential method, resistivity method, induced polarization method, and controlled-source audio-frequency magnetotellurics (CSAMT) method [6]. The resistivity method is used to detect underground anomalies based on the differences in resistivity of subsurface materials [7]. As shown in Figure 1, the instrument supplies power to the earth through electrodes $AB$, creating a stable electric field underground, while measuring the supply current I. The instrument measures the potential difference $\Delta U$ between two points on the surface using electrodes $MN$, and calculates the apparent resistivity value (${\rho }_s$, in units of $\Omega ·m$) at the midpoint between electrodes M and N using the formula ${\rho }_s=K·\Delta U/I$.

Seismic exploration infers the structure and properties of underground rock layers by analyzing how artificially generated seismic waves travel through media with varying elasticity and density [8]. The elastic waves generated by artificial excitation produce reflected and refracted waves when they encounter the boundaries between rock layers. These waves are recorded by highly sensitive instruments as they return to the surface. By analyzing the travel distance and travel time of the waves, the burial depth and shape of the rock layer interfaces where the elastic waves are reflected or refracted can be determined, thus allowing the inference of underground geological structures [9].

Traditional electrical exploration measures current and potential sequentially via a single channel, leading to low efficiency. Currently, high-density electrical instruments are mainly designed for engineering surveys. Although they offer a greater number of measurement channels (typically dozens or more), their output power is relatively low (usually only a few kilowatts), resulting in limited depth penetration. As such, they are not suitable for mineral or other resource exploration at depths of several hundred meters. Additionally, high-density electrical instruments typically do not support rolling coverage measurements. For long survey lines, the instrument must be repositioned multiple times, leading to increased operational workload and reduced efficiency. Once measurements at the current electrode positions are completed, the entire electrode and cable system must be repositioned to continue the survey, making the process time-consuming and labor-intensive. Furthermore, traditional geophysical exploration systems typically employ a single method, which limits their effectiveness in complex geological environments. Under such conditions, strong scattering and shielding effects can significantly weaken subsurface signal illumination [10]. Seismic and electrical exploration data can complement each other, leading to more reliable imaging of underground structures. Therefore, fully utilizing the advantages of multiple exploration methods for combined exploration has become a current research highlight [11,12,13]. Seismic and electrical data contain complementary information, and their joint inversion can effectively improve the imaging resolution of underground geological structures and the accuracy of reservoir descriptions [14].

Due to the fundamental differences in physical sensitivity and spatial resolution, integrating seismic and resistivity data presents several inherent challenges. Seismic methods are primarily sensitive to mechanical properties such as velocity and density, while resistivity methods reflect conductivity, which is influenced by fluid content, porosity, and mineralogy. As a result, the parameters derived from these two methods cannot be directly compared. Furthermore, seismic imaging generally offers higher spatial resolution than resistivity tomography, which may lead to misalignments when performing joint interpretation or inversion. Therefore, their effective integration requires careful spatial registration and the development of inversion strategies that address the differences in sensitivity and scale. However, this integration holds significant potential. The complementary constraints between different geophysical methods can improve the ambiguity in inversion results and enhance model reliability. Dobróka performed a joint inversion using seismic profile data, resistivity data, and underground coal seam resistivity data [15]. Gallardo developed a robust two-dimensional joint inversion scheme that combines the cross-gradient of resistivity and seismic velocity as a constraint [16]. Beatriz used a soft clustering method to integrate the independently derived electrical and seismic models [17]. Liao proposed a new inversion strategy based on the alternating direction method of multipliers [18]. Karpiah applied anisotropic marine controlled-source electromagnetic (CSEM) data and seismic refraction imaging data for a geological hazard assessment of the seabed cover layer [19]. Hu introduced multi-channel seismoelectric spectral ratios by considering an active seismic source acting on the ground surface [20]. Kang studied the seismic waves generated in fluid-saturated porous interlayers and the reflection/transmission characteristics at the interface of the interlayer structure [21]. Zhao regarded an oil–water mixture as an effective fluid and derived a seismic–electric coupling equation for porous media containing oil and water [22].

Therefore, there is a need for instruments capable of simultaneously collecting seismic and electrical data. The Italian company PASI launched the 16SG24-N system, which is suitable for a variety of engineering geological surveys [23,24]. The parameters of the 16SG24-N system are shown in

Table 1. Seismic–electrical combined acquisition system parameters.

Parameter	16SG24-N	KMS-820	The Instrument Developed in This Paper
Number of channels	up to 24	up to 6	up to 120
Sampling rate	500 sps to 31.25 ksps	up to 80 ksps	up to 100 ksps
Geophysical methods	seismic method, high-density electrical method	seismic methods, electromagnetic methods	seismic method, high-density electrical method
Analog-to-digital conversion	24-bit	24-bit and 32-bit	24-bit
Acquisition time	up to 65,536 ms	unlimited	unlimited
Cable type	centralized cable	single-core cable	coverage cable

, with a maximum of 24 channels, supporting both seismic methods and high-density electrical methods. The KMS-820 system developed by the American company KMS supports microseismic and electromagnetic methods, with a maximum of six channels [25]. The KMS-820 includes an analog-to-digital converter with a maximum sampling rate of 80 ksps and a 32-bit fluxgate magnetometer digital interface with a maximum sampling rate of 4 ksps.

In summary, traditional geophysical instruments have limitations such as low power, single functionality, few channels, fixed arrangement measurements, and slow measurement speed. To address the limitation of single functionality, Qiao et al. proposed a hybrid seismic–electrical acquisition station based on cloud technology and the green IoT [26], but this acquisition system still faces issues of low power and limited number of channels. Multi-method combined exploration has become a research highlights, but the development of related combined acquisition instruments has been slow. Therefore, developing a seismic–electrical combined acquisition system for geophysical exploration has significant scientific importance. With the increase in detection depth, improvement in observation accuracy, and diversification of observation methods, seismic and electrical combined acquisition technology has broad development prospects. Multi-channel, multi-functional, multi-parameter, high-power, and wide-range capabilities will become a trend in the development of geophysical instruments [27].

This paper improves upon the limitations of current traditional geophysical instruments by proposing a multi-functional, high-power, multi-channel, rolling, fast measurement electrical–seismic combined measurement system. By integrating both seismic and electrical methods for joint exploration, the system breaks away from the traditional approach of relying on a single geophysical method. It enables high-precision, multi-parameter geophysical data acquisition, effectively reducing ambiguity and enhancing the accuracy of exploration results. When measuring electrical signals, the power can exceed 10 kW. The instrument supports rolling coverage measurements, significantly improving work efficiency.

2. Design of the Combined Measurement System

2.1. Overall Architecture

The overall hardware architecture is shown in Figure 2, and mainly includes the power supply layer, signal acquisition layer, and digital circuit layer. The power supply layer (shown in purple in Figure 2) provides high and low voltage power to the entire system, including high-voltage power supply, a high-voltage bridge, low-voltage power supply, and other circuits. The signal acquisition layer (shown in yellow in Figure 2) controls the switching of channels and collects voltage and current information, including sensors, input/output control, preamplification, filters, and acquisition circuits. The digital circuit layer (shown in green in Figure 2) is mainly responsible for controlling the instrument’s workflow, including the system on a programmable chip (SoPC), data storage, communication, and other circuits. Communication between the SoPC control circuit and the computer is established via Wi-Fi or an Ethernet cable. The computer issues instructions to the SoPC control circuit, which controls all channel inputs or high-voltage outputs, adjusts the preamplifier gain, and manages the AD converter to convert analog signals into digital signals. The SoPC control circuit uses an field programmable gate array (FPGA) + advanced RISC machine (ARM) structure, where the FPGA is primarily responsible for data acquisition, and the ARM is mainly responsible for data storage and communication with the host computer.

2.2. Design of Input and Output Control Circuit

The channel input/output control circuit is shown in Figure 3. Each channel has two relays controlled by the MC1413 chip. The MC1413 operates at a high voltage and current, with the ability to sink a current of up to 500 mA and withstand a voltage of 50 V in the off state. It can also operate in parallel with high-load currents during output [28]. One of the relays, H, is used to control the power supply. Relay H is responsible for controlling the high-voltage power supply. After selecting a particular electrode, the MC1413 drives relay H to connect the electrode to the high-voltage power supply, allowing a high-voltage current to flow through the electrode to the ground. Relay L is used to control the measurement. After selecting a specific electrode, the MC1413 drives relay L to connect the electrode to the analog channel circuit, where the ADC in the analog channel measures the potential at the electrode contact point. Relays H and L will not be activated simultaneously, as this would cause a short circuit. The high-voltage power supply is adjustable, with a maximum voltage of 2000 V. The high-voltage power supply is used to input a high-voltage current into the ground to measure the apparent resistivity, polarization rate, phase difference, complex resistivity, and other data. As shown in Figure 4, during the time-domain electrical exploration process, the bridge converts the DC voltage into alternating waveforms of positive, zero, and negative voltages. As shown in Figure 5, during frequency-domain electrical exploration, the high-voltage bridge converts the DC voltage into square waveforms with frequencies ranging from $2^{-6}$ Hz to $2^4$ Hz. Additionally, the high-voltage bridge integrates protections against over-voltage, over-current, and other faults to ensure operator safety.

When the instrument is used for seismic data acquisition (as shown in Figure 6), each channel’s signal wire is connected to one output terminal of the geophone, while the shielding layer is connected to the other terminal. In addition, the shielding layer is grounded at the instrument end. The seismic geophone used with the instrument has a coil resistance of 1800 $\Omega$, a natural frequency of 10 Hz, and a sensitivity of 85.8 V/m/s. After selecting a particular channel, the MC1413 drives the relay L to connect the corresponding seismic geophone output to the analog channel circuit. The ADC in the analog channel then measures the output voltage waveform of the seismic geophone.

2.3. Design of Analog Channel Circuit

The analog channel filters and amplifies signals from geophones or electrodes, then digitizes them via an ADC chip before passing them to the main control unit. As shown in Figure 7, each channel is equipped with an independent amplification and filtering circuit, along with a dedicated 24-bit ADC. The system uses the AD7175 [29], a 16-channel, 24-bit, 250 ksps $\Sigma -\Delta$ ADC featuring a 20 ms settling time and a true rail-to-rail buffer, which communicates with the main controller via SPI.

As shown in Figure 8, the SoPC control circuit adjusts the preamplifier gain via an analog switch to accommodate varying input signal amplitudes during seismic and electrical data acquisition. Compared to seismic signals, electrical signals are more susceptible to power-frequency interference. Considering the versatility of the analog channels, we have added a twin-T notch filter circuit to each analog channel. As shown in Figure 9, the SoPC uses an analog switch to connect or bypass the twin-T notch filter circuit, allowing the power-frequency interference to be filtered from the signal when needed. The twin-T notch filter uses 0.1%-tolerance resistors and low-temperature-drift C0G capacitors to ensure a stable frequency response, achieving a 50 dB notch at 50 Hz.

The front-end filter of the ADC converter is used to eliminate high-frequency components from the input signal, preventing frequency aliasing during sampling. Each channel is equipped with an independent preamplifier, filter, and ADC, enabling simultaneous data acquisition from all channels and thereby enhancing efficiency. The SoPC processes and stores the voltage and current data acquired by the ADC. The computer retrieves the data from the instrument’s storage via Wi-Fi or Ethernet.

2.4. Design of Coverage Cable

This instrument supports the rolling coverage measurement method during electrical or seismic exploration, which is realized by a new type of coverage cable. Taking high-density electrical exploration as an example, as shown in Figure 10, the commonly used centralized high-density resistivity instrument cables (referred to as centralized cables) have enough taps set along a single cable to serve as connection points for the electrodes [30]. If the profile length to be measured is greater than the cable length, the cable and electrodes must be moved to cover the profile (as shown in Figure 11). All electrodes must be disconnected from the cable before moving them. After relocating the electrodes, the cable must be repositioned and aligned with the new electrode positions before reconnecting. This process is inconvenient, time-consuming, and labor-intensive.

This paper uses an improved structure of cable, called the coverage cable, which is composed of several identical unit cables. Each unit cable has a male connector at one end, a female connector at the other, and several evenly spaced taps along its length to connect electrodes or geophones. Two cable units can be connected to each other through the male and female connectors. The core wires of the cable are offset and welded to the connectors in a regular pattern. If a group of cables consists of k unit cables, and each unit cable has m taps, the number of core wires in the cable is $n=k\times m$. The i-th pin of the unit cable’s female connector is connected to the $(i-m)\mathrm{mod}m$-th pin of the male connector at the other end via the core wire. Taking an eight-core cable as an example, as shown in Figure 12, this cable group is composed of four unit cables, each with two taps. As shown in Figure 12, core wires of the same color are internally connected, linking the eight electrodes to pins 1 through 8 of the instrument’s female connector.

Due to the use of a segmented unit cable structure that connects with each other, the movement of the cable electrodes and the cable itself becomes very convenient. The first unit cable can be detached and reconnected to the end of the last unit cable, as shown in Figure 13. Taking the two types of cables in Figure 13 and Figure 14 as examples, the processes required for the centralized cable and coverage cable are shown in

Table 2. Comparison of the number of processes required for centralized cable and coverage cable measurements.

Process	Centralized Cable	Coverage Cable	Difference
Bury electrode	8 + 4 + 4 = 16 times	8 + 4 + 4 = 16 times	0 times
Connect electrode	8 + 8 + 8 = 24 times	4 + 4 + 8 = 16 times	8 times
Disconnect electrode	8 + 8 + 8 = 24 times	8 + 4 + 4 = 16 times	8 times
Move electrode	4 + 4 = 8 times	4 + 4 = 8 times	0 times
Move cable	2 + 2 = 4 times	1 + 1 = 2 times	2 times
Move instrument	1 + 1 = 2 times	0 times	2 times

. Because adjacent coverage cables remain connected during relocation, their movement is counted as a single cable operation. Table 2 shows that, compared to the centralized cable system, the use of coverage cables reduces to eight electrode connections and disconnections, along with two cable movements and two instrument relocations. Therefore, using the coverage cable for resistivity profiling can effectively reduce both operational complexity and acquisition time.

When the coverage cable is used for seismic measurements, the two output terminals of each geophone are connected, respectively, to the core wire taps and the shield ground taps of the cable, as shown in Figure 15. The grounding shield layers of each unit cable are interconnected and connected to the instrument’s ground. The shield layer of the cable provides both a current return path for the geophone’s output and noise shielding for the low-voltage analog seismic signal, ensuring the integrity of the signal. Additionally, the sensor signals are amplified in two stages by the instrument’s analog channels, enabling the system to effectively capture weak seismic signals.

2.5. Program Design

The overall program framework of the instrument is shown in Figure 16. The instrument program is mainly divided into the upper computer program, embedded ARM program, and FPGA program. The embedded ARM program includes function modules such as file saving, GNSS synchronization, and instrument status control. The FPGA program is mainly responsible for high-voltage output control, data acquisition control, channel switching control, and other functions directly related to the analog circuit hardware. This system uses the Xilinx xc3s2000-4fg456 FPGA chip, so the program is designed and implemented in the Vivado development environment [31,32,33].

Developed in $C\#$, the upper computer handles data reception, waveform display, and instrument control. $C\#$ is a strongly typed, object-oriented language widely used for professional development on Windows platforms [34]. The functional architecture of the upper computer software control system designed for this system is shown in Figure 17. The upper computer display interface is shown in Figure 18.

Figure 19 shows the software workflow, with green, yellow, and blue indicating the FPGA, ARM, and PC layers, respectively. After power-on, the FPGA performs time synchronization, and the PC connects to the ARM via TCP. Once connected, the user configures the measurement type and parameters in the PC software, which are then transmitted to the ARM layer. The ARM layer sends hardware-related commands to the FPGA, which executes data collection and returns the results to the ARM layer. The ARM layer packages and stores the data locally, then sends it to the PC, where the upper computer software visualizes it for the user.

The system collects seismic and electrical signals simultaneously through separate channels. To ensure synchronization, the SoPC module sends trigger signals to each acquisition circuit at the same time, with equal-length signal lines designed to minimize delay mismatches. This architecture achieves temporally coherent multi-modal data acquisition, laying the foundation for joint inversion and integrated interpretation.

Although the current field validation focuses on DC resistivity acquisition, preliminary work has also been performed on the seismic data processing workflow. The system supports high-resolution, timestamped seismic waveform acquisition, with data saved to the PC via the upper computer software. In the initial processing stage, standard preprocessing steps such as band-pass filtering, baseline correction, and time windowing are applied using MATLAB (R2024b). Future work will incorporate automatic first-arrival picking and waveform inversion modules as part of the ongoing system integration and enhancement plan, enabling more comprehensive seismic analysis.

3. Instrument Testing and Result Analysis

3.1. Field Test Environment and Setup Description

To verify the feasibility of the integrated measurement system, a field survey was carried out in Changping District, Beijing, China, with field photographs shown in Figure 20. The survey used a high-density electrical method with a Wenner array, which involves setting up all electrodes in advance and sequentially energizing different pairs. This automated technique is widely used in mineral exploration, geological hazard assessment, and related fields [35,36,37]. The Wenner array arranges the current electrodes A and B and the potential electrodes M and N at equal spacing in the sequence $A˘M˘N˘B$ during power injection and measurement. By using coverage cables instead of centralized ones, the time required for instrument relocation is significantly reduced. As shown in Figure 21, the $L1$ survey line was investigated, starting at point $P1$ (40.242054° N, 116.159484° E) and ending at point $P2$ (40.237318° N, 116.16251° E), with a total length of 580 m.

3.2. Measurement Result and Analysis

The survey results of the $L1$ survey line are shown in Figure 22. The three subplots from top to bottom display the measured apparent resistivity pseudo-section, the calculated apparent resistivity pseudo-section, and the inverted resistivity model. A least-squares inversion method was applied to derive the subsurface resistivity model from the measured data. The inverted resistivity model reveals a region of lower resistivity at the deeper part of the $L1$ line, around the 400 m coordinate.

Figure 23 shows the core sampling analysis at the 400 m point of the $L1$ survey line. The ’w’ column represents soil moisture content, while the ’$\rho$’ column indicates soil density. The surface layer, up to 50 cm in depth, consists of artificial fill soil with gravel particles typically ranging from 4 to 6 cm in diameter and approximately 20% clay content. The layer extending from the surface to a depth of 4.3 m consists of moderately dense variegated gravel mixed with clay. The layer between 4.3 and 5.5 m in depth consists of yellow-brown, moist subclay containing mica and coarse particles. The section from 5.5 to 12.3 m in depth is composed of moist variegated gravel mixed with clay, with gravel particle diameters typically ranging from 2 to 3 cm, approximately 30% clay, and locally distributed fine sand and subclay layers. The layer between 12.3 and 17.4 m in depth consists of variegated cobbles, with diameters typically ranging from 6 to 8 cm, and containing small amounts of medium sand, clay, and gravel. The section from 17.4 to 32.3 m in depth is composed of yellow–brown, moist subclay containing mica, iron oxide, and a small amount of gingerstone, with a pebble interlayer present between 20.6 and 20.9 m. The section from 32.3 to 37 m in depth consists of red–brown, moist clay soil mixed with gravel, containing approximately 30% weathered gravel, with gravel particle diameters typically ranging from 2 to 3 cm. The section from 37 to 49.5 m in depth is composed of gray, strongly weathered limestone, with drill core samples predominantly broken and blocky, and occasionally cylindrical. The section from 49.5 to 53 m in depth consists of gray, moderately weathered limestone, with drill core samples in columnar form, each approximately 20 cm in length. The core sampling results align with the low resistivity observed in the electrical inversion of the underground structure. Specifically, the section from 32.3 to 37 m, with a density of 1.99 g/cm³ and 24.9% moisture content, matches the low resistivity feature in the inverted model (red arrow in Figure 22). This demonstrates the system’s effectiveness and efficiency in surveying underground structures.

4. Conclusions

This paper presents a multi-functional, high-power, multi-channel, rolling fast measurement system that integrates electrical and seismic methods, with a comprehensive description of its hardware and software design. The system supports both electrical and seismic measurements, offering equipment for integrated seismic–electrical exploration. Its high-power capability enables deeper electrical exploration. Additionally, the system introduces a new coverage cable that reduces exploration wiring time and saves manpower. The test results demonstrate that the system provides complete data, maintains normal communication with the host computer, and meets the practical requirements of field exploration.

Although the developed system is designed for multi-method geophysical surveys, including AC resistivity, induced polarization (IP), and integrated seismic–electrical acquisition, the current field validation has been limited to DC resistivity surveys. This constitutes a limitation in the present work. Due to time and logistical constraints during the field campaign, no seismic data acquisition was performed. We recognize that the absence of demonstrative seismic validation, particularly seismic refraction, limits the reader’s ability to evaluate the effectiveness of the integrated cable design. In future work, we plan to conduct comprehensive field experiments covering AC resistivity, IP, and seismic data acquisition to fully verify the system’s multi-functional capabilities. Preliminary seismic validation is scheduled for the upcoming field season (Q3 2025). These tests will include seismic waveform quality evaluation, signal-to-noise ratio analysis, and joint inversion feasibility.