## 4 Data Acquisition and Analysis
### 2D Inversion Results  
For the NW-SE profile, the TE mode inversion (Figure 8a) reveals a thin low-resistivity layer (<100 \(\Omega\)m) near the surface, with a thickness of less than 250-300 m. This layer overlies a high-resistivity layer of more than 1000 \(\Omega\)m, which extends beyond the depth limits of the study. At the lower edges of the profile, both the NW and SE portions hinted at the existence of lower-resistivity features at around 2.5 km deep. The TM mode inversion (Figure 8b) reveals nearly vertical conductive features between N1-N2 (marked as S1) and F2-F1 (marked as S2), dipping southward and extending to the bottom of the profile. Similar to the other two modes, the joint-mode inversion (Figure 8c) portrays the thin conductive layer at depths of nearly 500 m from the surface, followed by a high-resistivity layer of more than 1000 \(\Omega\)m. A low-resistivity layer below 100 \(\Omega\)m is observed at approximately  
Figure 7: Geoelectrical stitched sections from 1D inversion for the NE-SW profile, shown in TE mode (**top**) and TM mode (**bottom**). Higher resistivity is represented in red. The white dashed lines indicate the estimated sediment interfaces. The red triangle marks the station at the intersection of the NE-SW and NW-SE profiles.  
below 3 km. The nearly vertical structure S1 is less distinguishable in the joint mode, but S2 remains visible.  
For the NE-SW profile (Figure 9), the inversion results from the TE mode show similar characteristics to the NW-SE profile, where the shallow layer is of low resistivity of less than 100 \(\Omega\)m up to 500 m deep. The underlying high-resistivity layer of approximately 1000 \(\Omega\)m, extends to the bottom of the profile. Low-resistivity-layer features of less than 100 \(\Omega\)m can be observed at nearly 2 km. The TM mode inversion shows a series of conductive vertical discontinuity marked as S3-S5. In the joint-mode inversion, the shallow conductive layer extends to approximately 500 m while the underlying resistive layer thinned out to the  
Figure 8: NW-SE profile images obtained from 2D inversion of **(a)** TE mode, **(b)** TM mode, and **(c)** TE and TM mode. Resistivity values are shown on a logarithmic scale, with higher resistivity indicated by red. The dashed line highlights potential structures identified in the inversion profiles.  
southwest end of the profile. Low-resistivity features below this layer are also observed at approximately 3 km deep.  
The general consistency between the 1D and 2D inversion results suggests that the subsurface beneath the study area, down to a depth of 4 km, can be characterized by three main layers: a low-resistivity shallow layer, a high-resistivity middle layer, and a low-resistivity bottom layer. The top low-resistivity layer, extending to a variable depth of less than 500 m, represents the Quaternary sediments, which include recent alluvium and the Linkou Conglomerate, overlying marine sediment formations. The low resistivity may be due to the presence of the groundwater system beneath the Plain. The presence of fluid  
Figure 9: NE-SW profile images obtained from 2D inversion of (**a**) TE mode, (**b**) TM mode, and (**c**) TE and TM mode. Resistivity values are shown on a logarithmic scale, with higher resistivity indicated by red. The dashed line highlights potential structures identified in the inversion profiles.  
within the highly porous layer can decrease resistivity, resulting in lower measurements. The high-resistivity middle layer up to 2.5-3 km may represent the Pliocene bedrock made up of sandstone and shale, which has undergone compaction that causes it to have lower porosity as the sediments expel more fluid and become more tightly packed. The low-resistivity layer at the bottom may imply the Miocene muddy sandstone and sandy mudstone of the Wushan formation, which may extend deeper to more than 4 km.