975 research outputs found
Very Fast Simulated Annealing Surface Wave Inversion with Model Constraints
No full textGeophysical Inversion is an ill-posed problem that is inherently affected by the non-uniqueness of the solution. Moreover, several peculiar aspects characterize the surface wave inverse problem as the associated forward problem is implicit, modal identification is often difficult and higher mode solutions may not even exist for certain frequency ranges. To this end, the use of a priori information is of great help in reducing the solution ambiguities. In the heuristic inversion algorithm presented in this note, mathematical measures of the desired nature of the inverted models (e.g. smooth or minimum norm solutions) are introduced into the objective function to bias constructively the solution towards realistic estimates of the ID shear wave profile. In the inversion algorithm, two different forward kernels can be alternatively selected for the direct problem computation: the conventional modal inversion based on modal identification, or the direct minimization of the secular function, to avoid possible pitfalls associated with an ambiguous mode identification of the observed dispersion pattern. The versatility of the inversion algorithm is illustrated using both synthetic and real data. In the latter case, the inverted shear velocity profiles are blind compared with crosshole results. Copyright 2009, European Association of Geoscientists and Engineers
Computation of partial derivatives of Rayleigh-wave phase velocity using second-order subdeterminants
No full textRayleigh-wave propagation in a layered, elastic earth model is frequency-dependent (dispersive) and also function of the S-wave velocity, the P-wave velocity, the density and the thickness of the layers. Inversion of observed surface wave dispersion curves is used in many fields, from seismology to earthquake and environmental engineering. When normal-mode dispersion curves are clearly identified from recorded seismograms, they can be used as input for a so-called surface wave 'modal' inversion, mainly to assess the 1-D profile of S-wave velocity. When using 'local' inversion schemes for surface wave modal inversion, calculation of partial derivatives of dispersion curves with respect to layer parameters is an essential and time-consuming step to update and improve the earth model estimate. Accurate and high-speed computation of partial derivatives is recommended to achieve practical inversion algorithms. Analytical methods exist to calculate the partial derivatives of phase-velocity dispersion curves. In the case of Rayleigh waves, they have been rarely compared in terms of accuracy and computational speed. In order to perform such comparison, we hereby derive a new implementation to calculate analytically the partial derivatives of Rayleigh-mode dispersion curves with respect to the layer parameters of a 1-D layered elastic half-space. This method is based on the Implicit Function Theorem and on the Dunkin restatement of the Haskell recursion for the calculation of the Rayleigh-wave dispersion function. The Implicit Function Theorem permits calculation of the partial derivatives of modal phase velocities by partial differentiation of the dispersion function. Using a recursive scheme, the partial derivatives of the dispersion function are derived by a layer stacking procedure, which involves the determination of the analytical partial derivatives of layer matrix subdeterminants of order two. The resulting algorithm is compared with methods based on the more widely used variational theory in terms of accuracy and computational speed
Effectiveness of group velocity analysis of surface waves for near surface characterization
No full textIn earthquake seismology, group velocity of surface waves is widely used to infer the interior structure of the Earth. Robust techniques exist to compute the group velocity spectrum, also for single trace data. This feature, when applied to shallow shear-wave characterization, seems to be very appealing to practitioners for reducing field operations. The main problem concerning the application of group velocity measurements to active seismograms for shallow targets is the interference between the modes, especially at short offsets. The main objective of this work is to analyse the potential of group velocity dispersion for near surface characterization, with particular focus on signal discrimination, sensitivity with respect to the soil structure and possible sources of interpretation pitfalls. Our analysis demonstrate that the sensitivities and the depth of penetration of group velocity data are almost similar to phase velocity ones. Consequently, when the dispersion analysis is based on active multichannel arrays, group velocity does not add much information to the inversion process. On the other hand, the possibility of using single station measurements has a great value in all those situations when data are fundamental-mode dominated or the deployment of large arrays is not feasible, such as in urban areas
Sensitivity analysis of rayleigh-wave ellipticity with application to near surface characterization
No full textThe joint inversion of surface-wave measurements and Rayleigh-wave ellipticity has gained popularity in recent years for near-surface soil characterization. The common approach is to use low-frequency, singlestation ellipticity data in conjunction with high-frequency dispersion measurements obtained employing small aperture arrays. A complete understanding of the diagnostic potential of ellipticity in such conditions can be assessed only with a complete sensitivity analysis. To this end, a new analytical method is presented for computing the sensitivity of Rayleigh-wave ellipticity with respect to the structural parameters of a layered elastic halfspace. This method employs a layer stacking procedure based on the subdeterminant formulation of the surface-wave forward problem and is numerically stable at high frequencies. The sensitivity of the ellipticity curve is then evaluated quantitatively with specific focus on near-surface examples and compared to the dispersion patterns and sensitivity of modal phase velocity. © (2015) by the European Association of Geoscientists & Engineers (EAGE)
Linearized inversion of MASW data using inequality constraints
No full textInequality constraint formulation of least squares inversion is a flexible method to insert physical constraints (as well as a priori information) into the inversion process. In MASW (Multichannel Analysis of Surface Waves) inversion this has proved to be useful to stabilize convergence when applied to fundamental-mode dominated data as well as to data containing higher modes of propagation. A reliable S-wave velocity profile can be obtained from inversion of surface wave data if all available information is inserted into the inversion algorithm
Global inversion of electrical resistivity tomography data for cavity detection
No full textThis work aims to apply a well-known global inversion method such as the VSFA (Very Fast Simulated Annealing) for the inversion of ERT data for cavity detection. We focus our study on cavity detection, which is a problem where ERT is known to be diagnostic, due to the high resistive contrast with the background medium. Despite of that, local inversion methods are often inaccurate in the assessment of the actual position, shape and resistivity values of the target, due to the smoothness constraints applied and the loss of resolution with depth. Global inversion provides further insight into the problem with relevant information especially on the uncertainty assessment of the inverted profiles
Coupling ERT and SRT data through cross-gradient joint inversion and clustering on structured meshes incorporating topography
No full textThe joint inversion of electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) is a common practice to achieve a more accurate characterization of subsurface features, since these methods are sensitive to different properties of the subsurface. We propose an approach for integrating ERT and SRT, which involves a cross-gradient joint inversion on structured meshes even in cases with complex topography and a post-inversion procedure where a new cross-gradient index and fuzzy c-means analysis are used to assess the reliability of the joint procedure and facilitate the interpretation of the results. Our strategy does not introduce non-strictly necessary elements within the joint inversion procedure for dealing with a non-flat tomography, which can accentuate the illposedness of the inverse problem. The proposed method was applied to both synthetic and a field cases located in Central Italy, where an accurate geophysical reconstruction is needed for rehabilitation of an existing dam and boreholes are available to validate the geophysical survey. For all cases, joint inversion consistently yielded superior results compared to individual inversion, and the post-inversion tools facilitated the assessment of the impact of the joint scheme and allowed for a quantitative reconstruction (position and shape) of the main units at the site
Cross-gradient joint inversion and clustering of ERT and SRT data on structured meshes incorporating topography
No full textThe combination of electrical resistivity and seismic refraction tomography is a common practice for the characterization of subsurface features. Presently, the cross-gradient inversion scheme stands out as one of the most robust joint approaches, and some authors modified it to manage complex topographies on unstructured meshes even if at the expense of introducing additional parameters in the inversion process. We propose in this work a cross-gradient algorithm for jointly inverting electrical and seismic tomographic data on structured meshes in cases with non-flat topography. The proposed approach preserves the benefit of the classical cross-gradient approach without the need to impose physical length scales, as for irregular meshes. The quality of the results is evaluated in comparison with independent inversion through a new standardized cross-gradient index and a fuzzy c-means analysis that provides an assessment of the reconstruction accuracy through the membership function. The proposed method was applied to both synthetic models and field-scale examples located in Central Italy, where an accurate geophysical reconstruction is needed for the rehabilitation of existing dams. For all cases, joint inversion yielded superior results compared to independent inversion, demonstrating better agreement with available borehole data. The effectiveness of the joint approach was also demonstrated by the post-inversion tools, where the new cross-gradient index highlighted changes in structural similarity whilst fuzzy c-means clustering allowed for a quantitative reconstruction (position and shape) of the main units at the sites, facilitating the detection of site layering modifications
Focusing on Soil Foundation Heterogeneity through High-resolution Tomography
No full textAn historical building affected by differential settlements, which were triggered by an earthquake, is
investigated by means of high-resolution tomography, both electrical and seismic. The objective is to
image the geometric structure of the shallow soil below the building and to characterize its stiffness at low
strain.
A preliminary reconstruction of the geological units has been recovered through the combined use of
electrical and seismic data, where the depth of the travertine bedrock varies significantly within the study
site. The range of variation of the main geophysical parameters (resistivity, P- and S-wave velocities)
inferred from these models has been set as reference point for tuning the results obtained from the
geophysical survey performed near the building. The inverted tomographic models obtained from data
acquired alongside the building exhibit heterogeneity of the shallow subsoil, which is partly founded on a
weathered layer and partly on a more rigid lithotype, probably a fractured travertine or a gravel layer.
Therefore the fill anthropic soils can play a relevant role for the structural stability in case of shallow
foundations built on a heterogeneous subsoil
Geophysical Investigation for the rehabilitation of a flood Control Embankment
No full textTo comply with recently published seismic regulations and environmental standards, existing dams and embankments are now being examined for maintenance, repair or rehabilitation. Engineering geophysics is almost the only viable option for investigating these structures and the underlying soil as a whole system. In this contribution, electrical and seismic investigations are performed on an outdated flood control embankment, that has to be put again into service. Geophysical investigation has proven successful to determine the relevant properties of the embankment and the main geometrical features of the underlying subsoil, serving as an important guidance for the rehabilitation intervention
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