## 1 Introduction  
In recent years, subways have been extensively constructed because of the limitations and increasing pressure from the demand on land resources and the growing urban population. Meanwhile, shield tunneling has been more and more widely used in metro tunnel construction, and the excavation depth of shield tunnels is becoming deeper than before. Deep boulders have become one of the problems threatening subway tunnel construction. However, the spacing between drilling holes in metro engineering is usually 40-50 m [1]. Such a distance leads to a low probability of discovering boulders. As a result, the detection of boulders is tremendously challenging.  
In the last few decades, geophysical prospecting techniques [2; 3; 4] and inversion methods [5] have made great progress, and some have been used in engineering, proving their validity [6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. At the same time, some geophysical exploration methods have been applied to identify boulders, including ground-penetrating radar [12] and microtremor surveying [1]. However, the resolution of surface prospecting methods is limited, and the low signal-to-noise ratio creates the possibility of increased artifacts [1; 16]. As it stands, the underground surroundings of subway tunnel construction projects are usually complicated,and the occurrence and condition of boulders within the strata are unpredictable. Therefore, methods for improving the accuracy of boulder detection need to be developed.  
[PERSON] et al. [17] applied cross-hole seismic tomography analysis to detect boulders, and [PERSON] et al. [2] analyzed the effect of cross-hole ERT in the detection of boulders using model testing. By comparing with surface prospecting methods, cross-hole methods were found to be effective for improving the resolution at depth. The capability of cross-hole ERT exceeds the performance limits of surface electrical surveying, because it extends the detection range [18]. [PERSON] et al. [19] used an optimized refraction tomography method to accurately determine a tunnel's geology. [PERSON] et al. [20; 21] used the ERT method to detect underground abnormal bodies in physical models and practical engineering. In previous studies, cross-hole ERT was used for many aspects. For instance, cross-hole ERT was applied to hydrogeological and fluid/gas plume migration studies, allowing better monitoring of the variations of those processes and the characterization of subsurface structures [22]. These studies have shown the prospects of success with this technique in our study area. Hence, it is natural to choose cross-hole ERT as one of the geophysical methods for detecting boulders. Previous studies on cross-hole ERT found that every configuration has its own advantages and disadvantages [18; 23]. Therefore, we proposed multi-configuration combinations of cross-hole ERT.  
The study herein focused on the application of the cross-hole ERT method in detecting boulders. First, we put forward an weighted inversion model, based on a multi-configuration combination of cross-hole ERT. We set up several numerical examples, to verify the correctness of the method. Then, the method was applied to practical projects. A multi-configuration combination of cross-hole ERT was used to collect data, followed by data processing that used the weighted inversion to obtain four profiles. Finally, the study evaluated the effectiveness of this method, by analyzing the results of field tests.