# Key Strata Identification of Overburden Based on Magnetotelluric Detection: A Case Study
## 2 MT Detection Method  
### Technical Principle  
With the development of MT detection technique, new technical means have been provided for the detection of overburden structure and fractures. It is an important geophysical means to study the electrical structure of the earth's interior with the natural electromagnetic field as the field source [24]. Although rock resistivity is affected by many factors, under certain conditions, its resistivity is a constant value and has certain regularity. For example, the resistivity of rocks in the same area, age, and lithology is generally similar. Therefore, as long as the resistivity characteristics of various rocks are mastered, geological sections can be divided. The results of data inversion technology reveal that the geological characteristics of rock strata are visual, accurate, and efficient. Different lithology has obvious differences in material composition, cements, and consolidation. According to the skin theory, different strata contain their corresponding frequencies and amplitudes. By detecting the electromagnetic wave signal, lithology and fluid information of different depth strata, the underground lithology interface, and the structure can be analyzed. Therefore, its basic principle is to analyze the surface material depth and electrical structures of electromagnetic waves with different frequencies when they pass through the strata, and to obtain the electromagnetic response array of the earth with these frequencies from high to low, as shown in Figure 2. This is because different strata have different compositional, cement, and consolidation properties [25], which can be used to analyze the overburden nature, activity, interface, coal seam, aquifer, geothermal activity, and geological structure. \begin{table}
\begin{tabular}{c c c c} \hline
**Year** & **Site** & **Purpose** & **Depth (m)** \\ \hline
2007 & Jiaozuo mining section of South-to-North Water & & \\  & Diversion Project, Henan & & \\ \hline
2008 & Concealed mine area, Fujian & “Three-under” coal mining & 1500 \\
2009 & Bautie mining area of Manchi county, Henan & Identifying thin interbed & 600 \\
2010 & Hongliu Coalmine, Ningytia & Height of water flowing fracture zone & 350 \\
2011 & Panyi Coalmine, Anhui & Overburden failure characteristics & 360 \\
2012 & Coalmine of Xinjiang \& Sharxi & Goal location and aquifer distribution & 400 \\
2014 & Jingdong Coalmine, Shamxi & Goal distribution & 200 \\
2015 & Tectonic fault basin, Northeast Inner Mongolia & Coal mining & 500 \\
2016 & Liudong Coaimmine, Anhui & Structure exploration & 900 \\ \hline \end{tabular}
\end{table} 
Table 1: Examples with magnetotelluric (MT) detection technique in mines. Figure 2: A sketch of the MT detection principle. Based on its technical principle, it has the advantages of higher resolution, great depth detection, low cost, and easy construction. In theory, wave impedance is a ratio that electric field \(E\) and horizontal component of magnetic field \(H\) in the uniform and horizontal layered strata (Equations (1)-(3)). \[Z=\left|\frac{E}{H}\right|e^{i(\varphi_{E}-\varphi_{H})} \tag{1}\]  
\[\rho_{xy}=\frac{1}{5f}\big{|}Z_{xy}\big{|}^{2}=\frac{1}{5f}\bigg{|}\frac{E_{x}} {H_{y}}\bigg{|}^{2} \tag{2}\]  
\[\rho_{yx}=\frac{1}{5f}\big{|}Z_{yx}\big{|}^{2}=\frac{1}{5f}\bigg{|}\frac{E_{y}} {H_{x}}\bigg{|}^{2}, \tag{3}\]  
where: \(f\)--frequency, Hz; \(\rho\)--resistivity, \(\Omega\) m; \(E\)--electric field intensity, mv/km; \(H\)--magnetic field intensity, nT; \(\varphi_{E}\)--electric filed phase; \(\varphi_{H}\)--magnetic field phase, mrad. The skin depth (\(\delta\)) is propagation depth when the amplitude attenuation of electromagnetic wave (\(E,H\)) decays to 1/e of the initial value in the strata, as shown in Formula (4). \[\delta{=}\ 503\sqrt{\frac{\rho}{f}} \tag{4}\]  
According to the above formula, skin depth varies with resistivity and frequency.