Inversion of Combined Surface and Borehole First-Arrival Time

Abstract

Site characterization for the design of geotechnical structures such as deep foundations is crucial, as unanticipated site conditions still represent the most common and most significant cause of problems and disputes that occur during construction. Surface-based refraction methods have been widely used recently to assess spatial variation, but one of the biggest limitations of these methods is that they cannot well characterize reverse profiles (decreasing in velocity with depth), buried low-velocity zones, or deep bedrock. An addition of borehole data to surface data is expected to improve inversion results. In this study, the coupling of so-called downhole and refraction tomography techniques using only one borehole is presented. To both qualitatively and quantitatively appraise the capability of the data, a global inversion scheme based on simulated annealing was investigated. Many synthetic and real test data sets with or without boreholes were inverted using the developed technique to obtain both inverted profiles and associated quantitative uncertainties. A comparison of tomograms utilizing the combined borehole and surface data against tomograms developed using just the surface data suggests that significant additional resolution of inverted profiles at depth are obtained with the addition of a borehole. The uncertainty estimates provide a quantitative assessment of the reliability of the interpreted profiles. It is also found that the quantitative uncertainties associated with the inverted profiles are significantly reduced when adding a borehole. In addition, the inversion results of the combined data provide credible information for the design of deep foundations, particularly useful in implementing the new load and resistance factor design methodology that can explicitly account for spatial variability in design parameters.