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Cracks are extensively existing in rocks and play a significant role in the acoustic anisotropy of cracked rocks. Rocks in nature are affected by pore pressure, whereas the crack deformation with pore pressure and the impacts of the crack deformation on the anisotropic acoustic properties remain little known. Combining the theoretical model with the laboratory measurements of the anisotropic velocities of artificial sandstone samples with and without aligned penny‐shaped cracks, we invert for the crack parameters that characterize the crack deformation as a function of pore pressure and theoretically simulate the impacts of pore pressure–induced variation in the crack parameters on the anisotropic velocities. The results show that with increasing pore pressure, the inverted crack porosity increases exponentially, whereas the inverted crack aspect ratio decreases exponentially and the two crack parameters are linearly correlated. Moreover, model calculation demonstrates that the anisotropic velocities exhibit distinct reductions with the variation in the crack parameters caused by increasing pore pressure. In particular, the reduction in the velocity of the shear wave travelling parallel to the crack plane with polarization perpendicular to the crack plane is the most pronounced. We also demonstrate that the effects of the pore pressure–induced increasing crack porosity on the anisotropic velocities are more pronounced than the impacts of the decreasing crack aspect ratio. The findings not only reveal the variation of the crack geometry with pore pressure and the effects of the crack deformation on the anisotropic velocities of the cracked rocks but also can provide theoretical support for improving the characterization of the cracks through seismic survey. [ABSTRACT FROM AUTHOR]

The utilization of the inversion-based algorithm for spectral decomposition using constrained least-squares spectral analysis (CLSSA) facilitates a time–frequency spectrum with higher temporal and frequency resolution. The conventional CLSSA algorithm is solved by optimizing an L2-norm regularized least-squares misfit function using Gaussian elimination, which suffers from intensive computational cost. Instead of solving an L2-norm regularized misfit function, we propose to use an L1-norm regularized objective function to enhance the sparsity of the resulting time–frequency spectra. Then, we utilize a faster, smarter, and greedier algorithm named greedy-FISTA to enhance the computational efficiency. Compared to the short-time Fourier transform, continuous wavelet transform, and the conventional CLSSA method, the sparsity-enhanced CLSSA with the greedy-FISTA is capable of achieving time–frequency spectra with higher resolution but with much less computational cost. The applicability of this sparsity-enhanced CLSSA method is demonstrated through synthetic and real data examples. [ABSTRACT FROM AUTHOR]

Joint inversion of horizontal-to-vertical spectral ratios (HVSRs) and dispersion curves (DCs) from seismic noise recordings has been extensively used to overcome the lack of inversion uniqueness in the noise-based HVSR (NHV) or DC inversions alone. Earthquake recordings contain information about the structural properties of sedimentary layers and provide body-wave data complementary to seismic noise recordings to estimate site velocity structures, particularly in the high-frequency band. We propose a joint inversion of the Rayleigh wave DC obtained from array measurements and earthquake-based HVSR (EHV). The EHV is derived from earthquake motions rather than from microtremors based on the diffuse-field theory of plane waves. We investigated the complementarity of EHV and surface-wave DC in the joint inversion through sensitivity analyses. The DC is sensitive to bedrock shear-wave velocities in the low-frequency range and is supplemented to some degree by the EHV in the high-frequency range. The EHV is more sensitive to sediment thicknesses almost over the entire frequency range. The joint inversion is implemented by a hybrid global optimization scheme that combines genetic algorithm (GA) and simulated annealing (SA) to avoid premature convergence in the GA. The sensitivity of inversion parameters was tested to demonstrate that the P- and S-wave velocities and thicknesses of soil layers are the dominant parameters influencing EHV and DC responses. The proposed method was validated by using synthetic models to compare the joint inversion with EHV or DC inversions alone. The joint inversion was applied to the Garner Valley Downhole Array (GVDA) data for identifying the velocity structures of the site based on earthquake and noise observations. The inversion results for the P- and S-wave velocities and thicknesses of soil layers strongly suggest that the joint inversion is an efficient method to estimate site velocity structures.

In a large body of rock-physics research, seismic wave velocity dispersion and attenuation in fluid-saturated porous rock are studied by constructing analytical or numerical models for time- or frequency-dependent dynamic (effective, or viscoelastic) moduli. A key and broadly used model of such kind is the Zener's, or the standard linear solid (SLS). This model is qualitatively successful in explaining many field and laboratory observations and serves as the key element of many generalizations such as the Burgers model for plastic deformations or the generalized SLS explaining band-limited or near-constant seismic attenuation. However, as a physical model of fluid-saturated porous rock, the SLS has several major limitations: disregard of inertial effects, absence of secondary wave modes and lack of key physical parameters such as porosity and Skempton coefficients. Grainy and porous rock is an unconsolidated material in which the effective density is frequency-dependent, and its effects on wave velocities may exceed those of the dynamic modulus. To overcome these limitations of the empirical SLS, we propose a rigorous rheologic model based on classical continuum mechanics and called the extended SLS, or eSLS. This rheology explains the available attenuation and dispersion observations equally well, but it is also close to Biot's model, honours all poroelastic relations, includes inertial effects, and reveals several new physical properties of the material. Detailed comparison of the eSLS and Biot's models gives a physical-mechanism-based classification of wave-induced fluid flow (WIFF) phenomena. In this classification, the so-called 'global-scale' flows occur in Biot's type structures within the material, whereas the 'local-scale' WIFF occurs in eSLS-type structures. Combining Biot's and eSLS models gives a broad class of rheologies for linear anelastic phenomena within rock with a single type of porosity. The model can be readily generalized to multiple porosities and different types of internal variables, such as describing squirt flows, wetting or thermoelastic effects. Modelling is conducted with relatively little effort, using a single matrix equation similar to a mechanical form of the standard SLS. By combining the eSLS and Biot's models, observations of dynamic-modulus dispersion and attenuation can be inverted for macroscopic mechanical properties of porous materials. [ABSTRACT FROM AUTHOR]