Shear Wave Velocity Imaging over Quick Clays Using Multiple Seismic Methods

Abstract:

Quick-clays are characterized by an highly unstable particle structure. This structure is usually caused by freshwater leaching of the original high salinity pore water generated by the former marine deposition environment. Given this instability, the clay structure can easily collapse leading to landslides of varying destructiveness. It is, therefore, of major importance to detect the presence of quick-clays. While resistivity based methodologies are commonly used to detect them (lower conductivity of the leached interstitial fluid in respect to the original one) there are also some evidences that leaching can result in a reduction of the undisturbed shear strength of these clays. Multiple integrated shear wave velocity based seismic methods (mainly SH seismic reflection and Love wave dispersion data) have been therefore applied in a case study to evaluate the potential of shear wave velocity imaging for detecting quick clays.

An area near the Göta River in southwest Sweden, which was the scene of a quick clay landslide about 40 years ago, was chosen as experimental site. High-resolution SH reflection data were acquired in the area, as part of a joint project studying clay-related landslides. Seismic reflection processing has evidenced several geologically interesting interfaces related to the presence of quick clays (locally confirmed by boreholes), and sand-gravelly layers strongly contributing to water circulation within them. Dispersion data have been extracted along one of the reflection arrays with a Gaussian windowing approach, and data have been inverted with a Laterally Constrained Inversion using a priori information coming from the seismic reflection imaging. The inversion of dispersion curves has evidenced, in some portion of the seismic line, the presence of a low velocity layer most probably correlatable with quick clays.

Even given the limited dispersion information extracted from the dataset, and the not yet completely understood shear wave velocity properties of quick clays, our work has evidenced the potential of the proposed approach for a more comprehensive imaging of the shear wave velocity distribution. This could be a valuable approach in quick clay identification in general.