A preliminary study on the multiscale effect of seismic anisotropy of coal seams at long wavelengths

Terrestrial coal-bearing strata in China are all typical thin interbeds given the seismic frequency bands of coal exploration. The isotropic thin interbeds are superimposed in the coal-bearing strata, exhibiting seismic anisotropy at long wavelengths among different seismic wavelengths, for which simplified equivalent processing, such as vertical transverse isotropy (VTI) or isotropy (ISO) approximation, is generally adopted in seismic exploration. However, the long-wave approximation has different applicable conditions for strata with different elastic properties. Taking the coal-bearing strata in the Huainan Coalfield as an example, this study established two-dimensional models of horizontal thin interbeds using acoustic and density logging curves of boreholes and explored the multiscale effect of the seismic anisotropy of coal-bearing strata at long wavelengths. Based on logging curves homogenized with different window lengths, this study emphatically investigated the differences in the elastic parameters and their anisotropy of the equivalent models that were established at different window lengths using the two equivalent modeling methods. Finally, this study analyzed the applicability of different equivalent models through numerical simulations of the elastic wave fields of the models. The results are as follows: (1) the single-point anomalous data of velocity, density, and anisotropy parameters of the equivalent models decreased with an increase in the window length; (2) the ISO approximation yielded high P-wave velocities than the VTI approximation; (3) there existed a critical window length (λ_min/L≈3 for the coal-bearing strata in the Huainan Coalfield) at which the laminar anisotropy of the equivalent models began to weaken; (4) the propagation characteristics of elastic waves of the equivalent models of the coal-bearing thin interbeds approximated those of the original model only when both the ratio of the average length to the stratum thickness (L/d) and the ratio of the minimum principal wavelength to the average length (λ_min/L) satisfied the assumptions of the long-wavelength approximation. The results of this study help determine the seismic anisotropy characteristics of coal-bearing strata at long wavelengths and provide theoretical guidance for the forward simulation and inverse modeling of coal-bearing strata, thus further improving the accuracy of the seismic exploration of coal fields.