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Fine detection on water abundance of sandstone by joint inversion of seismic and CSAMT: a case study in the western Ordos Basin, China

Dong Yi, Cheng Jiulong, Wen Laifu, Cheng Peng, Li Fei, Jiao Junjun, Huang Qisong

Acta Geophysica

2022

Vol. 70, no 5

2045--2056

The Ningdong mining area in the western Ordos Basin, China, mainly mines coal seam in Yan'an group, where its overlying rock is a thick sandstone layer of the Middle Jurassic Zhiluo Formation. This rock layer poses a direct water hazard threat to the coal mining if it is water-rich. The water abundant rock layer in the upper strata of Zhiluo Formation forms a low-resistivity overburden layer, decreasing the resolution of controlled source audio magnetotelluric (CSAMT) method in detecting water-bearing in the lower part of Zhiluo Formation sandstone layer of coal seam direct roof. Therefore, under the influence of the low resistivity overburden of the upper sandstone, how to accurately detect the aquosity of the lower sandstone layer is of great importance to the safe mining of coal mines in the region. On the basis of CSAMT detection, combined with high-resolution seismic exploration method, the joint inversion of seismic and CSAMT is realized by using cross-gradient operation between the seismic wave impedance attributes clustered by particle swarm algorithm and CSAMT inversion model. The seismic data fitting term in the joint inversion objective function is discarded, and the pseudo-2D inversion method is used for CSAMT to reduce the calculation cost of the inversion. A 3D geological model conforming to the hydrogeological characteristics of the Ningdong mining area is established, and the joint inversion test between seismic and CSAMT is conducted, proving the feasibility and applicability of joint inversion to detect the water enrichment of sandstone in this area. The accuracy of the seismic and CSAMT joint inversion results is verified by combining the engineering example of water abundance detection in the sandstone layer in Maiduoshan coal mine that accorded with the typical hydrogeological characteristics of Ningdong mining area and the results of later downhole drilling exposures, which is remarkably better than the single method. The research shows that the joint inversion of seismic and CSAMT can accurately identify the water abundance of the lower sandstone layer and its range under the influence of the upper low resistance sandstone overburden and achieve the purpose of fine detection of the water abundances of the lower sandstone layer of the Zhiluo Formation. The joint inversion can provide important safety geological guarantee for the mining of coal seams in the Ningdong mining area in the western part of Ordos Basin.