To reduce all kinds of noise interference in the transient electromagnetic signal, and improve the resolution of abnormal resistance in detection area. In this paper, a method based on Slime Mould Algorithm (SMA) optimized Variational Mode Decomposition (VMD) combined with Wavelet Threshold Denoising (SMA-VMD-WTD) was proposed to identify and eliminate the noise. Firstly, the slime mould algorithm (SMA) was used to select the key parameters in VMD. Then, the signal was adaptively decomposed by VMD according to the optimized parameters to obtain certain modal components, and divided them into effective signal components and noise components. Finally, the effective signal components were further processed by Wavelet Threshold Denoising (WTD) and reconstructed to obtain the transient electromagnetic signal with noise removal. The simulation signal tests with different noise intensity were built, and this method was compared with Ensemble Empirical Mode Decomposition (EEMD), WTD, unoptimized VMD and SMA to optimize VMD for reducing noise analysis of simulation tests. The results showed that SMA-VMD-WTD was more effective in identifying and eliminating transient electromagnetic signal noise. The method was applied to a geological exploration area, which verified the field applicability of this method. The results of the simulation tests and field test showed that SMA-VMD-WTD was a better denoising method for transient electromagnetic signal.

A two-dimensional unsteady seepage model for coal using a finite element program is developed, and the temporal variations of key factors such as water pressure and hydraulic gradient are analyzed in this paper. Additionally, the triaxial rock mechanical experiment and utilized pneumatic fracturing equipment on raw coal samples to investigate both hydraulic and pneumatic fracturing processes are conducted. Through these experiments, the relationship between pressure and crack formation and expansion are examined. The analysis reveals that the pore pressure gradient at the coal inlet reaches its peak during rapid surges in water pressure but diminishes over time. Conversely, the pore pressure gradient at the outlet side exhibits a gradual increase. Hydraulic fracturing is most likely to occur at the water inlet during sudden increases in water pressure. Besides, as the permeability of coal decreases, the duration for seepage stabilization prolongs due to the intensified pore pressure gradient resulting from sudden increases in water pressure. Moreover, an extended period of high hydraulic gradient further increases the risk of hydraulic fracturing. The experimental findings indicate that coal samples initially experience tensile failure influenced by water and air pressure. Subsequently, mode I cracks form under pressure, propagating along the fracture surface and becoming visible. The main types of failure observed in hydraulic and pneumatic fracturing are diametrical tensile failure, and the development of fractures can be categorized into three distinct stages, which contains the initial stage characterized by slight volume changes while water pressure increases, the expansion stage when pressure reaches the failure strength, and the crack closure stage marked by little or even decreasing volume changes during pressure unloading. The acoustic emission signal accurately corresponds to these three stages.