Background Accurately identifying subsurface anomalies is crucial to building a geological transparency system in the transformation to intelligent coal mines. The precise exploration of fold anomalies is a challenge of underground geophysical exploration technology, holding great significance for reducing excavation costs, mining difficulties, and safety risks.
Objective and Methods Previous in-seam seismic explorations reveal that folds pose significant impacts on the propagation characteristics of in-seam waves. However, conventional inversion techniques of in-seam waves are difficult to distinguish folds from faults. This study constructed numerical models for faults and folds with similar geometric shapes and equal exploration distances. Specifically, the differences in wave field responses between faults and folds were determined by mining information on wave fields using the 3D wave field simulation technology. Subsequently, images were obtained through inversion using diffraction migration imaging.
Results and Conclusions The results of this study are as follows: (1) Compared to fault anomalies, fold anomalies exhibit more dispersed and fuzzier waveforms of reflected in-seam waves, with energy unevenly distributed across different reflection points. (2) The direct in-seam waves corresponding to fold anomalies display higher signal-to-noise ratios at far offsets than those associated with faults. Moreover, fold anomalies can reduce the attenuation speed of direct in-seam waves. As a result, the distances that direct in-seam waves propagate in folds are about 1.5 times those in faults. (3) The energy difference between the reflected and the direct in-seam waves in faults is slight but that in folds is significant. (4) Fold anomalies manifest a lower dominant frequency of reflected in-seam waves than faults. This results in broadened frequency bands of direct and reflected in-seam waves. The reflected in-seam waves of fold anomalies exhibit slightly higher frequencies and velocity than their direct in-seam waves, while this feature is subtle for faults. (5) The reflecting surfaces of faults are typically located in the middle-upper part of fault planes, whereas the reflecting surfaces of fold anomalies tend to be in the middle part of the fold planes. Besides, fold anomalies display less continuous reflecting surfaces for diffraction migration imaging than faults. Based on the thorough analysis of wave fields and dispersion characteristics mentioned above, this study investigated the No. 15 coal seam in a coal mine in Yangquan, Shanxi, as an example. Using the seismic exploration technique through diffraction migration imaging of reflected in-seam waves, this study qualitatively identified faults and folds in the coal seam on the side walls of the roadway in the coal mine and clearly described the morphologies of folds. The findings of this study provide both a more accurate geological basis for the rational layout of the mining face in mining areas and strong technical support for the safe production and efficient mining of coal mines.
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