## 3 Data Acquisition and Analysis
### Dimensionality and Strike Analysis  
In complex geological conditions, the impedance tensor is often affected by galvanic distortions, which are caused by localized, near-surface heterogeneities near the measurement location [38]. Galvanic distortions may result in subsurface artifacts during inversion. The distortion analysis can eliminate the influence of the near-surface heterogeneities from the measured MT impedance tensor. The distortion analysis methods used for distortion removal are deriving a solution for the regional (undistorted) impedance tensor. These methods also determine the impedance tensor that represents the regional structure alone and analyzes the dimensionality of the regional structure [39; 40].  
The dimensionality and directionality analysis provides an important basis to determine if 2D or 3D modeling methods could be proper for the MT data. In 2D modeling methods, MT data can be decomposed into the transverse electric (TE) mode data and transverse magnetic (TM) mode data, along two orthogonal directions that are perpendicular and parallel to the strike direction. Then these two groups of data can be inverted respectively or jointly. Thus, a regional strike direction along the profile must be determined in advance. We applied a tensor decomposition to estimate geoelectric strike directions [41], which are shown in Figure 3 by the format of the statistic rose diagram. The rose diagrams show that the electrical strike directions are relatively complex, the regional geological strike direction is NW-SE in general. Figure 3 shows the strike directions in the intervals of 0.001-0.01, 0.01-0.1, 0.1-1, 1-10, 10-100, 100-1000, 1000-10,000 s, which shows that the electrical strike directions are \(\sim\)N37.5\({}^{\circ}\) E. The principal axis describes the regional strike direction, which has an uncertainty of 90\({}^{\circ}\). Therefore, the direction of electrical strike was determined to be S127.5\({}^{\circ}\) E, considering the geological structure direction of the faults and structures.  
Phase tensors in Figure 4 were calculated to reveal the dimensionality of the subsurface structure [41], which has the advantage of avoiding near-surface conductivity heterogeneity effects. We found most of the stations are of relatively small skew angles (<5\({}^{\circ}\)) over different period bands. The shallow part of the study area indicates the smallest skew, while the ellipses of some of the stations approach circles, which represent a 1D character that may be related to surface cover. Larger skew angles for some stations are distributed beneath the Wulungunan fault zone and the shapes of phase tensor ellipses are strong ellipticity, which suggests that the crust is unstable and complex. For the stations beneath the Wulungu Depression, the shapes of phase tensor ellipses are close to round circles, which suggest that the uniform crust in this region is generally of 1D character. The stations beneath the East Junggar Orogenic Belt in longer periods (in 100-1000 s, skew angles >5\({}^{\circ}\)) indicate complex subsurface structures in this region. In general, we conclude the dimensionality of the profile is 2D, and 2D inversion procedures are applicable to our dataset.  
Figure 3: Geoelectric strike direction for different period bands: 10\({}^{-5}\)-10\({}^{-4}\) s; 10\({}^{-4}\)-10\({}^{-3}\) s; 10\({}^{-3}\)-10\({}^{-2}\) s; 10\({}^{-2}\)-10\({}^{1}\) s; 10\({}^{0}\)-10\({}^{1}\) s; 10\({}^{1}\)-10\({}^{2}\) s; 10\({}^{2}\)-10\({}^{3}\) s; 10\