## 5 Application
The inversion results of separate inversion and joint inversion are shown in Figure 8. The first and second columns of Figure 8 show the resistivity results of separate and joint inversion, respectively, and the third and fourth columns show the polarizability results of separate and joint inversion, respectively. The separate inversion results of resistivity had good stratification, and showed that the farther away from the river, the thicker the high-resistivity layer and the larger the resistivity value. The separate inversion achieved good results, while there was little difference between separate inversion and joint inversion. The separate inversion results of polarizability show a low layer on the surface, which increases with increasing depth, but it does not present a layered distribution with depth. In particular, as the groundwater level becomes increasingly deeper in the area far from the river, the IP effect becomes weaker, the value of polarizability obtained by inversion is too small, the polarizability model appears distorted, and it is difficult to reflect the information of the groundwater level. Compared with the separate inversion results of the polarizability, the polarizability model obtained by the joint inversion results has clearer morphology, more obvious interface, and better stratification, and can more directly reflect the layered distribution of groundwater. Figure 7: The distribution map of the survey line. The red circle denotes transmitters, and black lines denote receiving electrodes; the purple point is the origin O; the black point is the location of the groundwater monitoring well MY2; the cyan area is the Chaobai River, the blue line is the right bank of the river, and the positive X-axis represents the flow direction of the Chaobai River, with an azimuth angle of 233\({}^{\circ}\). The \(RMS\) values of the resistivity method and TDIP obtained in separate inversion were 2.351 and 2.72, respectively, and that in joint inversion were 2.278 and 2.68, respectively. The variation in \(RMS\) values of the resistivity method and TDIP is not obvious, and the \(\zeta\) cannot be calculated since the true model is unknown. Therefore, we compared the separate inversion and the joint inversion through the value of \(\overline{t}\) and the groundwater level of the borehole. The value of \(\overline{t}\) obtained in separate inversion was \(6.14\times 10^{-6}\), while it was \(5.77\times 10^{-6}\) in joint inversion, which indicates that the resistivity and polarizability models obtained by the joint inversion are more similar in structure. After joint inversion, the average cross-gradient value decreased, and the cross-gradient values of the five sections in Figure 8 were calculated, as shown in Figure 9. As shown in Figure 9, the cross-gradient value is larger at the surface and does not decrease in joint inversion, which is caused by the influence of various factors, such as silty clay, corrosive material, pebbles and water stagnation in the upper layer. However, below 10 m underground, the cross-gradient values decreased significantly, and the farther from the river, the more obvious the cross-gradient value decreased. The polarizability results of joint inversion have fewer false anomalies, and can better reflect the groundwater level. Figure 8: Imaging results from separate inversion and joint inversion of the measured data. The resistivity results of separate inversion and joint inversion are shown in the first column and second column, respectively, and the polarizability results of separate inversion and joint inversion are shown in the third column and fourth column, respectively. Panels (**a–e**) are the vertical slice diagrams of the inversion models with Y = 30 m, 70 m, 100 m, 130 m and 170 m, respectively. Combined with local geological data, geological interpretation of the inversion results was made (Figure 10). As shown in Figure 10, the surface layer of 0\(\sim\)3 m is mainly a silty clay layer.