Laterally constrained inversion of ground roll from seismic reflection records

Seismic reflection data contain surface waves that can be processed and interpreted to supply shear-wave velocity models along seismic reflection lines. The coverage of seismic reflection data allows the use of automated multifold processing to extract high-quality dispersion curves and experimental uncertainties in amoving spatial window. The dispersion curves are then inverted using a deterministic, laterally constrained inversion to obtain a pseudo-2D model of the shear-wave velocity. A Monte Carlo global search inversion algorithm optimizes the parameterization. When the strategy is used with synthetic and field data, consistent final models ith smooth lateral variations are successfully retrieved. This method constitutes an improvement over the individual inversion of single dispersion curve

Urban underwater ERT for site investigation in lake Mälaren, Sweden

An underwater ERT survey was made along a planned road tunnel for The Stockholm Bypass Project (Förbifart Stockholm). The aim was to assess the ability to identify variations in the depth of the bottom sediments, as well as variations in rock quality and the possible presence of weak zones in the rock. Reference data from a seismic survey and geotechnical drilling was used for evaluation the result. An underwater layout of 121 underwater and 37 land based electrodes at a separation of 5 m was used. Repeated measurements were made to assess the temporal variation in urban noise, which lead to doing the actual survey during the quiet night hours. The resistivity data are of good quality and the inversion results in low residuals. The invested section shows a low resistivity layer corresponding to sediments on top of bedrock with varying resistivity, where the interpreted depth to bedrock is in good accordance with the reference data. A major low resistive zone corresponds well with poor quality rock according to drilling and a low velocity zone

Scale property Monte Carlo driven inversion of surface wave data

Analysis of surface wave data can be made using probabilistic approaches, e.g. Monte Carlo methods that employ a random or pseudorandom generator. A method like this is required to efficiently avoid local minima, evaluate nonuniqueness in the solution and estimating the values and uncertainties of the model parameters. The pure Monte Carlo method applied to surface wave inversion becomes efficient with the introduction of a “smart sampling” rule which exploits the scale property (scaling of the modal solution with the wavelength) of the solution. Introducing this property in the Monte Carlo inversion focuses the scan of model space on high probability density zones. Each model is scaled before evaluating the misfit in order to bring the theoretical dispersion curve obtained by forward algorithm closer to the experimental. An applicative example is presented to support our hypothesis. The main advantage of the proposed approach, based on scale property of dispersion curves, is the possibility of using a pure Monte Carlo method with a limited number of simulations. This leads to a solution which accounts for data uncertainties evidencing equivalent final models. The possible bias of the result from a wrong choice of the initial model is significantly reduced

Laterally constrained inversion of ground roll of seismic reflection records

A new strategy for processing and inverting surface waves retrieved by seismic reflection ground roll is presented. The use of a Laterally Constrained Inversion algorithm allows for estimating a reliable subsoil model also in the case of smooth lateral variations in spite of the 1D approach used for surface wave interpretation. A Montecarlo analysis is also performed for model assessment. An experimental case is used to discuss advantages and limits of the proposed approac

Seismic Characterisation of a Site in the Alps

This study concerns the dynamic characterisation devoted to seismic response analysis of an Alpine test site. We show how integration and combination of different seismic techniques allow a comprehensive characterisation of the site leading to a reliable 2D model for site amplification studies. For the evaluation of the deep structure and the location of the seismic bedrock, combined seismic reflection-refraction tests are used while the shear wave velocity profile is obtained by down-hole measurements and surface wave tests, performed integrating passive and active data to increase the depth of penetration without loosing in resolution of shallow layers. The ground roll affecting reflection-refraction seismic data is also used to reconstruct lateral variation of the shear wave velocity in the soil deposit by means of an innovative inversion approach based on the integration of Monte Carlo and Laterally Constrained Inversion, in which the Vs profiles obtained from the others seismic techniques are used as constrains to improve the reliability of the final solution. Moreover an estimation of the quality factor (Q) is performed from records acquired for surface wave analysis. The use of the same seismic records for different and independent analysis optimize the cost effectiveness of the whole survey

Shear Wave Velocity Model from Surface Wave Analysis - A Field Case Example

Shear wave velocity models can be built through surface wave analysis. For the presented field case, preliminary data analysis has evidenced the possibility of retrieving several modes of vibration of Rayleigh waves and this suggest the use of a multimodal inversion algorithm to estimate a pseudo-3D shear wave velocity model. The full analysis has been performed starting from a preliminary Monte Carlo inversion applied on the representative dispersion curves. After the Monte Carlo inversion, which provides a consistent 1D reference model for each line, the dispersion curves have been inverted simultaneously adopting a linearised laterally constrained inversion (LCI) to obtain a pseudo-3D shear wave velocity model

Seismic characterization of an Alpine site

Site amplification of earthquake ground motion can arise from local stratigraphic and morphological conditions. In this respect, a detailed geo-mechanical 2D model is needed, in particular for peculiar situations such as the ones in Alpine valleys. This paper reports a site characterization case history for seismic response analysis, where a combination of both body and surface wave seismic techniques has been used. In particular, it is shown that synergies and mutual cross-checking of information lead to a reliable 2D model for site amplification studies. The evaluation of the deep structure and the location of the seismic bedrock have been obtained through seismic reflection and combined active-passive surface wave analysis, while the shear-wave velocity profile of the sediments has been obtained by downhole measurements and surface wave tests. Furthermore, a detailed compressional wave velocity model has been retrieved by tomographic inversion of the first arrivals picked on seismic reflection records. This model is useful for geological interpretation as well as for quality control of the other methods. Surface wave analysis has also been performed on the ground roll of the seismic reflection records. The large amount of profile oriented data has made it possible to reconstruct lateral variations of the shear-wave velocity in the soil deposit. The final pseudo-2D shear-wave velocity model is the product of an innovative inversion approach, based on the integration of the Monte Carlo and laterally constrained least-squares techniques. In the laterally constrained inversion the shear-wave velocity profiles obtained from downhole tests and data from passive surface wave measurements have been used to constrain the inversion. This has greatly improved the final solution. This case history underlines the necessity of combining different seismic techniques to derive an accurate and reliable 2D model for site amplification studies. Through several different and independent analyses of the same seismic records, the cost effectiveness of the whole survey is optimized and a full exploitation of the information contained in the reflection seismic data set is achieved

Shear Wave Velocity Model from Surface Wave Analysis – A Field Case Example

Shear wave velocity models can be built through surface wave analysis. For the presented field case, preliminary data analysis has evidenced the possibility of retrieving several modes of vibration of Rayleigh waves and this suggest the use of a multimodal inversion algorithm to estimate a pseudo-3D shear wave velocity model. The full analysis has been performed starting from a preliminary Monte Carlo inversion applied on the representative dispersion curves. After the Monte Carlo inversion, which provides a consistent 1D reference model for each line, the dispersion curves have been inverted simultaneously adopting a linearised laterally constrained inversion (LCI) to obtain a pseudo-3D shear wave velocity model