High resolution geophysical techniques for small-scale soil model testing

This research focuses on the development and application of high resolution geophysical techniques, based on elastic and electromagnetic waves, for small-scale soil model characterization and process-monitoring. Emphasis is placed on systems that can operate during flight in centrifuge modeling. The second generation of an electrical resistivity needle probe takes into consideration tip shape and corrosion effects. It shows millimetric resolution. Unique results include the identification of layering and porosity changes after liquefaction. P-wave reflection imaging is scaled to small-scale soil models by optimizing the operating frequency and selecting the proper transducers. Emphasis is placed on detecting layers and inclusions in submerged soils. Bender elements are optimal transducers due to their soil-transducer coupling and their high frequency response. It is shown that the resonant frequency of bender element installations depends on the geometry of the bender element, its mounting and the soil stiffness. Multiple reflection techniques and signal matching techniques are developed to solve first arrival and near field issues. Bender element transducers are combined with simple, yet robust inversion algorithms to obtain tomographic images. Travel time S-wave tomography is implemented using pixel- and parametric-based representations. A plane-wave trans-illumination methodology for monitoring short-duration events is implemented whereby an array of transducers is excited at once and a similar array across the model acts as a projection screen. This system permits monitoring the evolution of liquefaction.