# Key Strata Identification of Overburden Based on Magnetotelluric Detection: A Case Study
## 1 Introduction  
China is a large coal producer and energy consumer, which is mainly because coal is the primary energy source and provides the basis of the energy infrastructure necessary for long-term, rapid economic growth [1]. With the continuous development of mining methods, technical equipment, and the Internet, the high-intensity mining of thick coal seams has become an important development direction of China's coal mining technology, especially in the fragile eco-environment area of Northwest China [2]. However, the exploitation of coal resources can lead to a series of serious environmental problems, the essence of which is that the overburden failure is relatively severe [3, 4]. Moreover, the degree of overburden failure is closely related to the location and failure of key strata (KS) [5, 6, 7, 8, 9, 10]. As is well-known, the overburden is composed of different strata with variable characteristics; thus, the mining effects on each stratum is different. For the hard and thick strata, they may have a stronger bearing capacity, supporting those weak and thin strata that only act as loads. Based on these differences, the KS theory was proposed [11]. According to the KS theory, the hard and thick stratum is named KS; as a result, overburden has at least one KS. If there are more than two KS, as shown in Figure 1[12], the top KS is named the primary KS (PKS), as it bears the entire load from its roof toground surface. Correspondingly, the other KS is named the sub-key strata (SKS), that controls partial overburden strata [13]. At present, the identification method of KS mainly relies on the lithology calculation, ground drilling, and laboratory simulation. [PERSON] et al. [14] found that the key stratum has a significant impact on the behavior of the strata in a stope by theoretical calculation, and divided it into a \"single key stratum\" structure and a \"double key strata\" structure. [PERSON] et al. [15] adopted in-situ investigation of surface boreholes and found a strong correlation between dislocation inside the boreholes and the location of the key strata, from which the fracture and movement of the key strata with the advancing process of the working face was inverted. [PERSON] et al. [16] carried out an in-situ investigation to determine the law of fracture and movement of a key stratum. [PERSON] et al. [17] defined the movement type of SKS by theoretical analysis and numerical simulation, and verified the correctness of the support working resistance formula under different types by field measurement. [PERSON] and [PERSON] [18] used numerical simulation methods to study the key strata with different thicknesses and heights, and got the relationship between the key strata and the abutment pressure. Meanwhile, [PERSON] and [PERSON] [19] developed the distinguishing software of key strata with the C# language. Based on the existing references, the identification of KS needs to be calculated according to the rock's mechanical parameters. However, for the mining areas with unknown mechanical parameters, it is important to find a proper way to accurately identify the KS in order to avoid the time-consuming, complex, and labor-intensive of the laboratory simulation method, so as to ensure the safety production and mine pressure control. Combined with the development of geophysical exploration, it has become a hot topic in detecting overburden structure and failure [20; 21; 22; 23]. Among the methods, magnetotelluric (MT) detection is a simple and convenient one, which has been applied to the identification and detection of water-rich coal seams, thin interbeds, overburden failure, geothermal resources, karst, collapse areas, fault structures, and overburden fractures in the goaf area.