The seismic imaging methods currently in the development stage need to be tested for experimental validation under controlled conditions. Yet natural media are very complex, and moreover, the parameters along the measurement profile prove difficult to evaluate independently of the seismic method itself. To satisfy this need, the ultrasonic measurement laboratory (MUSC) presented in this research has been devised to experimentally model seismic field measurements by using reduced-scale models. This facility is composed of small-scale models of the underground, an optical table with two moving arms, a laser interferometer, and adapted piezoelectric transducers used as the seismic sources. The source system has been adapted to simulate the behavior of a point-surface seismic source. This is essential to reproduce the spatial energy distribution of a surface seismic source and supersedes the sources used in the past for other reduced-scale seismic experimental models. The comparisons of experimental data collected with MUSC and numerical data simulated by means of finite-element viscoelastic modeling indicate very good agreement of time arrivals and amplitudes for a range of propagation distances until the amplitude has decreased to the system noise level. These results demonstrate that the MUSC laboratory is a system with plenty of promise for validating seismic imaging methods through testing on a perfectly known propagation model prior to field application
Surface-wave dispersion inversion is growing in popularity for geotechnical applications, due to its noninvasive character, relative straightforward field procedures and interpretation, especially when the subsurface structure is locally assumed to be one-dimensional (1D). Here, laser-Doppler physical modeling of surface-wave propagation is used to address issues of surface-wave depth penetration, the presence of dipping layers, and the associated limitations and systematic errors propagated in conventional 1D surface-wave inversion. Flat-layered models show that, with an active source and linear spread, the maximum resolvable wavelength of the Rayleigh-wave fundamental mode is on the order of 40% of the spread length. Linearised inversions confirm the rule of thumb that the depth penetration is 20–25% of the spread length, and that correct a priori layer interface depths from refraction analysis allow more accurate results. However, even under optimal conditions, failing to account for a dominant higher mode at low frequency when a stiff shallow layer is present, causes an overestimate of deeper layer shear-wave velocity. Moreover, a layer dip of only a few degrees can significantly bias the surface-wave inversion. If the incorrect a priori information from a single-shot refraction analysis is incorporated in the inverse problem, estimated interface depth depends on the shot position and deeper layer shear-wave velocity is underestimated. Even if correct a priori constraints are used, an underestimate of half-space shear-wave velocity of up to 25% remains.
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