## 3 Field Case
### Results and Discussion  
In this section, four profiles and four coring results of test boreholes are described, to indicate the position of boulders in the studied area. Each profile was obtained by weighted inversion of the multi-configuration combination, as shown in Figure 9. In this process, the smoothness-constrained least-squares inversion method was applied to invert resistivity data and the number of iterations of inversion was six. The coring result of the test boreholes is shown in Figure 9, in comparison with the weighted inversion results of the field data.  
The four profiles showed high resistivity values in areas corresponding to the boulders. Figure 9a shows three high-resistivity bodies between ZK4 and ZK5 at different positions.  
In YZK1 (Figure 9a), cross-hole ERT revealed three boulders, located at a depth of 5-10 m, depth of 18-22 m, and depth of 27-30 m. Correspondingly, the borehole revealed an abnormal body located at 23-27 m.  
The profile for the site between ZK4 and ZK1 is shown in Figure 9b. Cross-hole ERT also revealed three boulders, which were located at a depth of 5-10 m, depth of 18-22 m, and depth of 27-30 m. Correspondingly, the borehole revealed an abnormal body located at 25-28 m.  
The profiles in Figure 9c were acquired between ZK3 and ZK1, showing three high resistivity bodies. The test borehole revealed two boulders at 21-23 m and 27-31 m. It is noted that the results obtained by geophysical prospecting found one more high resistivity body than the coring results of YZK3.  
Figure 9d shows very similar features to Figure 9c, for the site between ZK2 and ZK1. Three high resistivity bodies occurred at 12-15, 17-21, and 23-27 m. The results of the test borehole presented in Figure 9d included two boulders. One was at 20-23 m and another was at 23-27 m. Compared with three the bodies in the profile between ZK2 and ZK1 (Figure 9d), there existed one different body, at 12-15 m.  
From the perspective of numerical simulation, the new cross-hole ERT method has good vertical resolution and weak horizontal resolution, but it may be difficult to distinguish boulders of the same horizontal position and size, independently. Moreover, from the numerical simulation and engineering measurement, when setting a reasonable borehole-depth/borehole-spacing ratio (>1.5) and borehole spacing and electrode spacing, boulders with a size of 0.5 m-2 m can be distinguished.  
Based on the analysis herein, the results of the geophysical prospecting showed a high degree of similarity with the results of the test boreholes. Moreover, the ERT results show more boulder anomalies. The cross-hole ERT results showed more high resistivity bodies, and they were some distance from the test boreholes. These could have been boulders that were outside the test boreholes. Results using the weighted inverted geoelectrical model were reasonable and agreed with the results validated by the boreholes. This study demonstrates that the use of weighted inversion of a multi-configuration combination in the investigation of boulder detection is effective. Therefore, we recommend the use of weighted inversion of a multi-configuration combination as a higher resolution approach to detection. However, in the application of this method, a few disadvantages were found. For instance, the boundaries of bodies were unclear. Therefore, further basic research is required, to solve problems such as the multi-scale inversion [25] and L1 (blocky) inversion.