Forward modeling of amplitude variation with offset and azimuth (AVOA) response for fracture parameters of horizontal transversely isotropic coal seams

Fractures serve as a key factor controlling the permeability of coal seams and play a crucial role in the exploration and exploitation of coalbed methane. This study focuses on the Amplitude Variation with offset and Azimuth (AVOA) response mechanisms of the fracture parameters of horizontal transversely isotropic (HTI) coal seams. To this end, this study calculated the equivalent elastic modulus of coal seams with vertical dry and fluid-saturated fractures using the Mori-Tanaka model and the Brown-Korringa anisotropic fluid substitution equation, respectively. Moreover, based on the petrophysical characterization of fractures in coal seams and the forward modeling using the HTI coal seam model, this study analyzed the seismic responses of fracture parameters. The results of this study are as follows: (1) The variation in the fracture density significantly changed the horizontal P- and S- wave velocities, anisotropic coefficients, and the anisotropic gradient of the coal seams’ top interface; (2) The horizontal P-wave velocity and anisotropic coefficient ε^(v) were primarily controlled and affected by the fracture porosity, while the S-wave velocity, anisotropic coefficients γ^(v) and δ^(v), and the anisotropic gradient of the top interface of coal seams were sensitive to the variations in the porosity and aspect ratio of fractures; (3) When incidence angle θ was greater than 0, the azimuthal anisotropy of both the P-P reflection coefficient of the coal seams’ top interface and the maximum positive amplitude of the coal seams’ reflected composite waves was enhanced with an increase in θ or the fracture density. Moreover, they changed significantly with a variation in the fracture density when azimuth was equal to 0. Therefore, analyzing and determining the AVOA response mechanisms of the fracture parameters of HTI coal seams can provide theoretical support for the seismic detection of fractures in coal seams.