A B S T R A C TNear-surface cavities can pose serious hazards to human safety, especially in highly urbanized town centres. The location of subsurface voids, the estimation of their size and the evaluation of the overburden thickness are necessary to assess the risk of collapse.In this study, electrical resistivity tomography (ERT) and seismic refraction tomography data are integrated in a joint interpretation process for cavity location in the city of Rome.ERT is a well established and widely employed method for cavity detection. However, additional information provided by seismic refraction tomography is capable of eliminating some potential pitfalls in resistivity data interpretation. We propose that the structure of the cavities defined by ERT can be used as a base to optimize seismic refraction tomography investigations within the framework of a joint interpretation process.Data integration and the insertion of a priori information are key issues for reducing the uncertainties associated with the inversion process and for optimizing both acquisition procedures and computation time.Herein, the two geophysical methods are tested on both synthetic and real data and the integration of the results is found to be successful in detecting isolated cavities and in assessing their geometrical characteristics. The cavity location inferred by geophysical non-invasive methods has been subsequently confirmed by direct inspection.

The multichannel analysis of the surface waves method is based on the inversion of observed Rayleigh-wave phase-velocity dispersion curves to estimate the shear-wave velocity profile of the site under investigation. This inverse problem is nonlinear and it is often solved using 'local' or linearized inversion strategies. Among linearized inversion algorithms, least-squares methods are widely used in research and prevailing in commercial software; the main drawback of this class of methods is their limited capability to explore the model parameter space. The possibility for the estimated solution to be trapped in local minima of the objective function strongly depends on the degree of nonuniqueness of the problem, which can be reduced by an adequate model parameterization and/or imposing constraints on the solution. In this article, a linearized algorithm based on inequality constraints is introduced for the inversion of observed dispersion curves; this provides a flexible way to insert a priori information as well as physical constraints into the inversion process. As linearized inversion methods are strongly dependent on the choice of the initial model and on the accuracy of partial derivative calculations, these factors are carefully reviewed. Attention is also focused on the appraisal of the inverted solution, using resolution analysis and uncertainty estimation together with a posteriori effective-velocity modelling. Efficiency and stability of the proposed approach are demonstrated using both synthetic and real data; in the latter case, cross-hole S-wave velocity measurements are blind-compared with the results of the inversion process

Rayleigh-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

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Piping sinkholes may naturally develop in the case of a thick overburden overlying calcareous bedrock.\ud Their detection and imaging is a challenging task for geophysical methods, not only because of the required resolution and depth of penetration, but also because major pitfalls may arise, in such\ud geologically complex areas, from the speculative interpretation of geophysical anomalies as geological features. Data integration from different geophysical methods is essential to remove these\ud interpretation ambiguities, caused by large near-surface gradients and heterogeneities in the soil properties, as well as by oscillations of the water table and anomalous water circulation.\ud We present an investigation procedure consisting of the sequential application and integrated interpretation of Electrical Resistivity Tomography (ERT), Seismic Refraction Tomography (SRT) and Self Potential (SP) measurements for locating and monitoring piping sinkholes with application to a site in Central Italy. This approach is a compromise between resolution and cost-effectiveness, and it is designed to be economically affordable by the private end user.\ud In complex geological scenarios, it is usually not possible to rate a single geophysical technique as superior to all the others in terms of resolution, cost-effectiveness and diagnostic capability. The independent information coming from the different geophysical methods is the key to removing interpretation ambiguity when evaluating the position and the development over time of the piping sinkholes. The application of the proposed investigation procedure allowed us to individuate a small area subject to the formation of a piping sinkhole. The geophysical results were confirmed about one year after the execution of the geophysical measurements, as the site exhibited surface evidence\ud of a piping sinkhole, with the formation of a small pond filled with sulphurous water and gases coming from below