Joint Lamé constants and P-wave anisotropic parameter estimation of vertical transverse isotropy coal-measure gas reservoir

The Lamé constants (λ and μ) and anisotropic parameters (ε, γ, and δ) are the key quantities to calculate the brittleness indices and fluid factors in unconventional reservoirs, supporting high-precision seismic data processing as well as anisotropy characterization. However, accurately estimating these quantities using the traditional amplitude variation with offset (AVO) inversion based on the assumption of isotropic media is difficult. Therefore, we conducted a three-parameter AVO inversion and estimated the Lamé constants and P-wave anisotropic parameter (ε) for media with vertically transverse isotropy (VTI), using a coal-measure gas reservoir as an example. A new PP-wave reflection coefficient approximation equation for VTI media was derived based on Rüger's approximation. Subsequently, we analyzed the accuracy of the new equation and designed a strategy for estimating the Lamé constants and P-wave anisotropic parameter. The method was tested and applied using a synthetic seismic record and a cross-well seismic profile. The results demonstrate that the Lamé constants and P-wave anisotropic parameter estimated by both the synthetic seismic record (with or without noise) and cross-well seismic profile (at the well location), agree closely with the well-logging calculated curves. Therefore, the method can reasonably be extended to other similar unconventional reservoirs with certain constraints.