Objective The central and western regions of China hold abundant shallowly buried coal resources. However, the accurate exploration and safe, efficient development of coal seams in these regions are confronted with serious challenges due to weathering and oxidation. To achieve the cost-effective delineation of weathered and oxidized zones in shallowly buried coal seams, this study proposed a novel exploration method that integrated logging and dense microtremor array technology based on differences in physical properties and geophysical responses between weathered and oxidized zones and normal strata.
Methods Initially, based on the analysis of the laws of variations in the physical properties and logging responses of strata with burial depth, log curve-based criteria for identifying the lower boundaries of weathered and oxidized zones were established. These boundaries were then compared with dispersion spectrum characteristics of near-borehole sub-arrays to verify their correlation. Finally, the surface-wave dispersion curves were determined through inversion using a genetic algorithm, and a 3D shear wave velocity model was constructed. Accordingly, the spatial three-dimensional distribution of the lower boundary of the weathered and oxidized zone in the study area was delineated based on the characteristic shear wave velocities corresponding to the boundary. The delineation results were validated through comparative analysis.
Results and Conclusions The findings indicate that the fitted trend line based on the statistical maximum values of density logs enabled the accurate identification of the depth of the lower boundary of a weathered and oxidized zone (corresponding to the inflection point of the trend line), with such burial depths in boreholes M1103 and M1105 determined at 100 m and 105 m, respectively. The presence of sandstone lenses in the coal seam roof and variations in the burial depth of the lower boundary of the weathered and oxidized zone led to significantly frequency-varying energy clusters in the dispersion spectra of sub-arrays. The lower boundary of the weathered and oxidized zone in the study area corresponded to a characteristic shear wave velocity of approximately 1 380 m/s. Accordingly, the lower boundary of the weathered and oxidized zone was delineated, aligning well with the low-resistivity anomaly area detected using the transient electromagnetic method. The lower boundary of the weathered and oxidized zone in the study area is controlled by sandstone lenses in the coal seam roof and anticlinal structures. The proposed method that integrates logging and microtremor enables the economical and effective delineation of the lower boundaries of weathered and oxidized zones, thereby supporting safe, efficient development and accurate resource assessment of shallowly buried coal seams.
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