The determination of the P- and S-wave propagation directions is crucial for wavefield decomposition, polarity reversal correction, and noise suppression in elastic reverse time migration (RTM). Compared with the conventional decoupled elastic wave equation, the first-order velocity-dilatation-rotation equations enable more accurate computation of propagation directions for P and S waves. Moreover, compared with the Poynting vector, the optical flow vector signifies the wavefield propagation directions more accurately. To effectively enhance the accuracy of elastic wave imaging, we develop an elastic RTM based on first-order velocity-dilatation-rotation equations using the optical flow vector. Numerical tests illustrate that our method, with or without noise, can better eliminate migration artifacts and improve the imaging accuracy of the elastic RTM than conventional methods, achieving more accurate wavefield decomposition and superior S-wave polarity reversal correction.

Despite additional availability of laboratory data from water-saturated sandstone at seismic frequencies, measurements of rock samples saturated with high viscous fluids, particularly at partial saturation, are still rare. To quantify the effects of fluid viscosity and saturation levels on seismic dispersion and attenuation characteristics, we conducted two comparative forced-oscillation measurements in partially saturated sandstone with varying fluid viscosity (e.g., water, glycerin) at seismic frequencies (2–400 Hz). The results demonstrate that fluid viscosity and saturation levels substantially influence the dispersion and attenuation characteristics at the measured frequencies. Significant dispersion and attenuation are observed in the presence of a relatively small amount of gas (approximately 6%–8%) for glycerin and water saturation cases but vary in their magnitudes and characteristic frequencies. Specifically, the maximum extensional attenuation (approximately 0.024) occurs at approximately 200 Hz for water-saturated rock at 94% saturation, whereas at approximately 30 Hz with a peak of 0.032 for glycerin-saturated rock at 92% saturation. Based on theoretical modeling analysis, we suggest that mesoscopic fluid flow might be a dominant mechanism accounting for the observed attenuation in partial water or glycerin saturation, while the microscopic (squirt) flow mechanism possibly dominates the fully saturated cases.

In multicomponent surveys, the travel path of seismic waves from the subface interfaces to geophones may not be vertical to the ground; consequently, vertical and horizontal geophones receive mixed wavefields comprising P- and S-waves. Vector separation of wavefields is crucial for subsequent imaging and shear wave splitting analyses. However, conventional methods cannot address the nonlinear polarization of pure wave and can also produce incorrect results due to the presence of dipping interface. To address the above issues, we propose a vector wavefield separation method based on the slope of events and polarizability. The proposed method calculates the slope of the events for determining the propagation direction of the wavefield. The polarizability is used to evaluate if the resulting wavefield is purely composed of one type of body wave while the wavefield type is determined based on the angle between the polarization and propagation directions. When the wavefield is mixed, P/S separation is realized based on vector decomposition in an affine coordinate system optimized by propagation direction. We use both numerical and field data to demonstrate the effectiveness and applicability of the proposed method. The numerical example confirms that the proposed method yields more accurate results than the conventional vector decomposition method. The field data application also indicates that the new method can effectively resolve the P- and S-waves from the noisy seismic wavefield.

Young’s modulus and Poisson’s ratio are critical parameters for reservoir characterization using marine seismic data. Conventional amplitude versus offset (AVO) inversion methods often assume a constant S-to-P wave velocity ratio to simplify the inversion, leading to significant errors, particularly in heterogeneous reservoirs. To address this, we derive a novel four-term PP-wave reflection coefficient by reparameterizing Poisson’s ratio, effectively reducing the nonlinearity associated with the velocity ratio and enhancing the stability of Poisson’s ratio estimation. Building on this, we propose a Bayesian AVO inversion framework incorporating Cauchy prior and low-frequency model regularizations. The elastic parameters are estimated using a maximum a posteriori (MAP) approach by minimizing the negative log-posterior function. Numerical simulations and seismic gather data from East China demonstrate that the proposed inversion method yields more accurate estimates of Young’s modulus and Poisson’s ratio compared to conventional approaches. This improved AVO approximation offers a more reliable tool for delineating reservoir heterogeneity in complex geological settings using marine seismic data.