###### Abstract  
m m  
the late Pleistocene and Holocene epochs. These alluvial sediments are largely derived from the major rivers in the Plain (Figure 1), while the older, deeper sediments are likely from shallow marine environments that existed during the subduction of the oceanic plate. GPS measurements indicate that the Pingtung Plain is rapidly subsiding, with the southern portion experiencing the greatest rates of subsidence. The subsidence is attributed to both tectonic extrusion and groundwater extraction [15; 16; 17]. Gravity and magnetic studies reveal that the Plain is characterized by low gravity and magnetic anomalies, suggesting the presence of sediment-filled basins [12; 18; 19; 20]. Seismic analyses further indicate that the Pingtung Plain is a low-velocity zone relative to its surrounding regions, reinforcing the idea of sediment accumulation [21; 22; 23; 24]. Numerous boreholes have been drilled in the northern portion of the Plain, with depths ranging from 100 to 250 m. However, only a few studies have addressed the deeper subsurface structure. Consequently, there is a significant gap in data regarding the deeper extent of these subsurface formations. The MT method has been used since the 1950s to image the Earth's subsurface electrical structure by recording naturally occurring electromagnetic (EM) waves. The EM waves span a broad frequency range of 10\({}^{-4}\) to 10\({}^{4}\) Hz [25]. This wide frequency band allows MT to achieve subsurface penetration of tens to hundreds of kilometers. The method assumes that the Earth only absorbs or dissipates the EM field, and it measures the time-varying electric field (\(E\)) and magnetic field (\(H\)) generated in the atmosphere and magnetosphere [26]. Low-frequency signals (<1 Hz) are generated by magnetic storms resulting from the interaction of solar wind with the Earth's \(H\) in the magnetosphere. In contrast, higher-frequency signals (>1 Hz) originate from a small fraction of the energy produced by worldwide lightning activity. Figure 1: Geological map of the Pingtung Plain. Coordinates are in TWD97. The study area is indicated by the black square. Ten observation stations are deployed along the north–south and east–west profile lines (black dots). A remote reference station is located in the Meinong Hills, approximately 15–20 km from the local stations (red diamond). The stratigraphical well southern of the study area is indicated by the purple star. The transfer function in MT data is represented by the impedance (\(Z\)), which reflects the Earth's response to EM induction and carries information about subsurface resistivity. The impedance (\(Z\)) is linearly related to the orthogonal horizontal \(E\) and \(H\), as expressed in the following equation:  
\[ \begin{pmatrix}E_{x}\\ E_{y}\end{pmatrix} = \begin{pmatrix}Z_{xx}Z_{xy}\\ Z_{yx}Z_{yy}\end{pmatrix}\begin{pmatrix}H_{x}\\ H_{y}\end{pmatrix}  \tag{1}\]  
Each tensor element can be presented as:  
\[Z_{ij}=\frac{E_{i}}{H_{j}} \tag{2}\]  
where \(i\) and \(j\) are the field components \(x\) and \(y\), respectively. In the 2D Earth model, \(x\) is assumed to be parallel to the strike and \(y\) is perpendicular to the strike. This gives rise to two polarization modes of MT: Transverse Electric (TE) and Transverse Magnetic (TM). TE is the polarization where \(E\) is parallel to the strike, while TM is where \(H\) is parallel to the strike. In TM mode, the measured \(E\) is perpendicular to the strike. The current is flowing across the boundary between the different media. The heterogeneity of the medium will cause the electric charge to gather on the interface, thus causing \(E\) discontinuity in the lateral direction. Therefore, the TM mode normally provides a better resolution for interfaces between different resistivity zones. In contrast, TE mode is measured where the current flows along the strike and may omit the interface; thus, it will carry less information about lateral inhomogeneity.