Estimation of fracture indicators using azimuthal PP-wave amplitudes without NMO correction

Huaizhen Chen, Kristopher A. Innanen

Seismic waveforms at large offsets are stretched during NMO correction, which are usually muted in seismic processing. However, in horizontal vertically isotropic (HTI) media or hydrocarbon reservoirs with vertical fractures, variation of seismic wave reflection with azimuthal angle is more obvious at large offsets, which means seismic waveforms of large offsets are indispensable to implement a better characterization of HTI media and fractured reservoirs. To circumvent the NMO stretching and reserve seismic waveforms at large offsets, we present an approach and workflow of employing seismic waveforms without NMO correction to estimate elastic impedance EI and fracture parameters (i.e. fracture weaknesses). Starting with re-expression of P- and S-wave velocities of HTI media, we derive PP-wave reflection coefficient and azimuthal EI as a function of the normal and tangential fracture weaknesses based on the solution of Zoeppritz equations. Using the derived azimuthal EI, we introduce a NMO operator to the convolution model to generate PP-wave seismic data without NMO correction. A two-step inversion workflow is established to estimate fracture weaknesses using seismic data, which is implemented as: 1) using seismic data without NMO correction at different incidence and azimuthal angles to invert for EI, and 2) using difference in EI at different azimuthal angles as input data to estimate fracture weaknesses. Bayesian inversion algorithm is employed in the two-step inversion. Synthetic seismic data of signal-to-noise ratio (SNR) of 2 is generated to verify the robustness of the proposed inversion approach. In the case of applying the proposed inversion approach and workflow to real datasets acquired over fractured reservoirs, reliable results of fracture weaknesses are obtained, which may guide the identification of potential fractured reservoirs.