Abstract To simulate seismic images of a highly heterogeneous and anelastic shallow subsurface requires full waveform techniques that properly account for scattering and attenuation. A viscoelastic finite-difference technique is employed to quantify the effects that typical near-surface features (i. e., approximately the uppermost 20 m) have on high-resolution seismic refraction and reflection data. Guided waves, ground roll and energy scattered from shallow heterogeneities and surface topography are forms of source-generated noise that may affect data quality. Dispersion of guided waves narrows the ‘optimum reflection window’ between the first arrivals and the ground roll, and the intrinsically shingled nature of guided waves may lead to mis-picking of first breaks for refraction analyses and difficulties in identifying and processing shallow reflections. Numerical modeling demonstrates that even minor topographic features may cause significant scattering of guided waves and ground roll from the earth’s surface. The scattered energy appears as ubiquitous source-generated noise that interferes with reflections and diffractions throughout the seismic record. The amount of noise generated by this scattering mechanism, and thus its impact on the imaging of subsurface structures, depends critically on the degree of attenuation in the uppermost layers.

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Abstract Joint inversion of crosshole ground-penetrating radar and seismic data can improve model resolution and fidelity of the resultant individual models. Model coupling obtained by minimizing or penalizing some measure of structural dissimilarity between models appears to be the most versatile approach because only weak assumptions about petrophysical relationships are required. Nevertheless, experimental results and petrophysical arguments suggest that when porosity variations are weak in saturated unconsolidated environments, then radar wave speed is approximately linearly related to seismic wave speed. Under such circumstances, model coupling also can be achieved by incorporating cross-covariances in the model regularization. In two case studies, structural similarity is imposed by penalizing models for which the model cross-gradients are nonzero. A first case study demonstrates improvements in model resolution by comparing the resulting models with borehole information, whereas a second case study uses point-spread functions. Although radar seismic wave-speed crossplots are very similar for the two case studies, the models plot in different portions of the graph, suggesting variances in porosity. Both examples display a close, quasilinear relationship between radar seismic wave speed in unconsolidated environments that is described rather well by the corresponding lower Hashin-Shtrikman (HS) bounds. Combining crossplots of the joint inversion models with HS bounds can constrain porosity and pore structure better than individual inversion results can.

Abstract Near-surface shear-wave (S-wave) velocities and quality factors are key parameters for a wide range of geotechnical, environmental, and hydrocarbon-exploration research and applications. High-frequency Rayleigh-wave data acquired with a multichannel recording system have been used to determine near-surface S-wave velocities since the early 1980s. Multichannel analysis of surface waves —MASW — is a noninvasive, nondestructive, and cost-effective acoustic approach to estimating near-surface S-wave velocity. Inversion of high-frequency surface waves has been achieved by the geophysics research group at Kansas Geological Survey during the past 15 years, using surface-wave inversion algorithms of both a layered-earth model (commonly used in the MASW method) and a continuously layered-earth model (Gibson half-space). Comparison of the MASW results with direct borehole measurements reveals that the differences between the two are approximately 15% or less and have a random distribution. Studies show that simultaneous inversion of higher modes and the fundamental mode increases model resolution and investigation depth. Another important seismic property—quality factor ( Q )—can be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. A practical algorithm uses the trade-off between model resolution and covariance to assess an inverted model. Real-world examples demonstrate the applicability of inverting high-frequency Rayleigh waves as part of routine MASW applications.

Abstract High-frequency surface-wave methods can provide reliable near-surface shear-wave (S-wave) velocity, which is a key parameter in many shallow-engineering applications, groundwater and environmental studies, and petroleum exploration. Recent research and key accomplishments at the China University of Geosciences at Wuhan into nearfield effects on surface-wave analysis provide not only insight into minimum-source geophone offsets required for generating high-quality surface-wave images but also provide a better understanding of the propagation characteristics of seismic wavefields through near-surface materials. New numerical modeling and dispersion-analysis algorithms are key tools used routinely in those studies. The modeling results illustrate very different energy-partitioning characteristics for Rayleigh and Love waves. Using a high-resolution linear Radon transform produces dispersion images with much better resolution and therefore represents a tool for more accurate separation and determination of phase velocities for different modes. Mode separation results in wavefield components that individually possess great potential for increasing horizontal resolution of S-wave velocity-field determinations. Amplitude corrections can significantly improve the accuracy of phase-velocity estimates from mixed-modal wavefields. Results from two simple models demonstrate how dramatic topographic changes can distort wavefields. This finding was the catalyst for suggesting that a topographic correction should be considered for surface-wave data acquired on a rugged ground surface. Phase-velocity inversion is an ill-posed problem. Rayleigh-wave sensitivity analysis reveals the difficulty in estimating S-wave velocities for a model with a low-velocity layer. Constraints in the model space are therefore necessary. Approximating cutoffs could help build a better initial model and provide critical information about the subsurface when higher modes are present.

Abstract Seismic methods are the primary characterization tools for several engineering and near-surface problems. Noninvasive and invasive methods based on the propagation of either body or surface waves are used widely. Often, more than one method is applied at the same site. In spite of possible synergies that exist between different methods, the data are often processed and interpreted independently. The integration of different data sets could provide more reliable final models and comprehensive site characterization. Acquisition can be optimized to obtain a multipurpose data set. Additional improvements might be obtained by a constrained or joint inversion of different seismic data. These can be demonstrated with real-world examples.