# Petrophysics Parameter Inversion and Its Application Based on the Transient Electromagnetic Method
## 4 Field Application  
The proposed methodology was applied to the Tongxin Coal Mine to analyze the water saturation and porosity of the subsurface rock formations and infer the distribution of water-bearing zones and goat (mined-out) areas. The Tongxin Coal Mine is located at the northeastern margin of the Datong Coalfield and is part of a dual-system mining area. The No. 3-5 coal seam is one of the primary mining seams. Due to its significant thickness, it is prone to hydraulic connectivity with the overlying water-filled goat zones of the Jurassic coal seams through water-conducting fracture zones formed after mining, posing a serious risk of roof water inrush. To address this hazard, a surface transient electromagnetic (TEM) survey was conducted at the 8311 working faces to investigate the water content within the goat zone of the overlying No. 14 Jurassic coal seam. Geologically, the Tongxin minefield is underlain by the Upper Archean Jining Group, with overlying strata deposited sequentially. The dominant lithology of the sedimentary layers is sandstone. Accordingly, [PERSON]'s equation was adopted as the constitutive relationship between formation resistivity, water saturation, and porosity for the inversion of the petrophysical parameters. The location of the exploration area and the layout of the survey lines are shown in Figure 14. A large-loop TEM system was employed in the field test, with a transmitting loop measuring 500 m \(\times\) 500 m. Three survey lines were deployed with a line spacing of 50 m and a station spacing of 20 m. The transmitting current was set to 20 A, and a three-component, high-precision electromagnetic sensor was used as the receiving system. Figure 15 presents the multi-channel response curves of the preprocessed original data. Significant fluctuations in the induced electromotive force are observed between measurement points 7 and 15 (lateral positions 140-300 m), indicating potential anomalies in this region. Subsequently, inversion was performed for all measurement points to obtain resistivity cross-sections for each survey line, as shown in Figure 16. The horizontal axis represents the lateral distance along the survey line, and the vertical axis corresponds to the burial depth of the formation, corrected for surface elevation. The inversion results reveal a characteristic high-low-high resistivity distribution. According to geological data, the elevation of the goat zone of the overlying Jurassic No. 14 coal seam ranges from 1030 to 1050 m. The inversion resistivity sections show a prominent low-resistivity anomaly at this depth, suggesting the presence of water accumulation within the goat. Figure 14: Exploration area location and survey line layout. Mining activities lead to the redistribution of in-situ stress, causing varying degrees of damage from the coal seam roof to the overlying strata. The deformation, movement, and failure of these overlying rock layers give rise to characteristic subsidence, fractured, and caved zones, collectively referred to as the \"three upper zones\". Similarly, the underlying strata are also affected by mining. When the induced stress exceeds the elastic limit of the rock, plastic deformation occurs, accompanied by the formation of numerous new fractures. As a result, the porosity of the rock near the roof and floor of the goat associated with the No. 14 coal seam increases. The water saturation in these zones also increases due to the accumulation of water within the goat. Using the method described in Section 3.3, the formation of petrophysical parameters was further inverted based on [PERSON]'s equation, utilizing the previously obtained resistivity results. The resulting porosity and water saturation cross-sections are shown in Figures 17 and 18. Both porosity and water saturation are relatively high in the 800-1200 m range. The elevated porosity suggests that the formation has been affected by coal mining, resulting in extensive fracturing.