## 1 Introduction  
Boulders, defined as spheroidal weathering bodies of granite, are commonly encountered in coastal areas [1]. These boulders, which can reach diameters of up to 30 cm, are often embedded within soft sediments. Their presence can significantly impact tunnel boring machine (TBM) cutter tools and cutterheads during subway excavation, frequently causing construction delays and increased costs. As a result, boulders represent a substantial risk factor for shield tunneling projects [2; 3; 4; 5]. Effective detection of boulders before the commencement of shield construction is essential for thorough planning and mitigation. Accurate reconstruction detection allows for the development of strategies to address potential challenges, ensuring both the safety and efficiency of the tunneling process. However, despite advancements in detection technologies, boulder detection remains a persistent global challenge [6; 7]. The complexity of accurately identifying boulders, particularly in varying geological conditions and depths, continues to hinder efforts to mitigate associated risks and optimize construction operations.  
The primary exploration methods currently employed for addressing the challenge of boulder detection in subway construction include geotechnical investigations and geophysical surveys. Geotechnical investigations, such as borehole drilling, provide detailed subsurface data but are time-consuming and expensive when conducted on a large scale [8]. Geophysical surveys, including electrical resistivity tomography (ERT), seismic refraction, and ground-penetrating radar (GPR), are effective for identifying anomalies in subsurface resistivity or seismic wave velocities that may indicate the presence of boulders [9; 10; 11].  
Various studies have been conducted to improve the accuracy and efficiency of boulder detection methods [12; 13]. During the construction of the Guangzhou metro in southern China in 2012, Dang conducted comprehensive geophysical tests and specialized research on boulder detection [14]. Over ten geophysical methods were carefully chosen to suit the subway construction environment, demonstrating the variety and adaptability of geophysical techniques in such complex settings. Geophysical methods offer noninvasive and rapid surveys but may have limited penetration depth or resolution in challenging geological conditions. The article suggests that geophysical methods conducted in boreholes may yield better results. Common geophysical methods in wells include borehole seismic tomography and borehole electrical resistivity tomography [15; 16]. Compared to borehole seismic tomography, ERT can delineate subsurface anomalies based on variations in electrical resistivity without requiring an electric spark as the source, making it effective for identifying boulder locations [17]. However, the current cross-hole ERT method acquires the electrical resistivity profile between two boreholes, which is essentially a 2D detection method with significant limitations [18; 19]. This 2D approach may not fully capture the three-dimensional nature of the subsurface anomalies, thus potentially overlooking critical details about boulder size and distribution [20].  
In this paper, we propose a method that enhances the detection of boulders by arranging electrodes in boreholes and utilizing parallel electrical method technology for hole-to-hole electrical penetration. This unique data collection approach allows us to obtain a massive amount of data, significantly improving collection efficiency. By arranging a certain number of electrodes in multiple boreholes, we create a three-dimensional vertical observation system, enabling the acquisition of three-dimensional detection results between the holes. This method provides a more comprehensive and accurate characterization of the subsurface, improving the detection and assessment of boulders in subway construction environments.