3D Inversion of Controlled-Source Electromagnetic Data for Coalbed Methane Exploration in Southern Yanchuan

Abstract

Coal-measure natural gas exploration is of great significance for developing the global natural gas industry. To explore the feasibility of 3D controlled-source audio-frequency magnetotelluric (CSAMT) inversion for coalbed methane (CBM) exploration in southern Yanchuan, we collected orthogonal dual-source CSAMT data. The inversion algorithm uses the Data Space Conjugate Gradient (DCG) method. The research mainly includes three aspects: (1) The influence of orthogonal dual-source tensor and scalar observations on the inversion results of 3D CSAMT are analyzed; and (2) based on the geological formation of the known boreholes, the deep formation is calibrated according to the large differences in physical properties between CBM and surrounding rocks, left and right monoclinic structures, and relatively simple geological structure in the southern Yanchuan experimental work area. A 3D CSAMT inversion initial model constrained by the known formation information is established. The final results show that, compared with scalar observations, the 3D CSAMT inversion results of tensor observations can efficiently delineate the spatial location of anomalies; the initial model of the 3D CSAMT inversion is constrained by the boreholes in the survey area, which improves the compatibility between the 3D CSAMT inversion results and the actual geological stratigraphic information in the southern Yanchuan work area. The results show that the method has high reliability and can be popularized and applied in southern Yanchuan and similar regions.