The Use of Seismic Trace Characterization to Guide the Analysis of DST Results to Obtain More Accurate Soil Parameters

Downhole Seismic Testing (DST) is a very popular applied seismology site characterizing tool within geotechnical engineering. A challenging aspect of DST is to characterize the acquired seismic data sets to determine the analysis method that will result in the most accurate interval velocity values. BCE has invested considerable resources into developing Seismic Trace Characterization (STC), which uses various independent parameters of the acquired data at a particular depth. Initial work in this area resulted in the selection of the linearity estimate from the polarization analysis, the cross correlation coefficient of the full waveforms at the particular depth and the preceding depth and a uniquely developed parameter referred to as the signal shape parameter for this characterization. Subsequent analysis in STC identified two other parameters: the Signal-Noise-Ratio (SNR) and the Peak Symmetry Differential (PSD). The paper briefly describes these parameters and then outlines how they can guide the data analysis to derive more accurate results, especially near surface, which is especially important to assess the liquefaction potential in areas prone to earthquakes, such as California. The process will be illustrated with actual data from another area prone to earthquakes, namely New Zealand. 1. INTRODUCTION The near surface characterization of low strain in-situ shear wave velocities (VS) has proven critical for liquefaction assessment. Liquefaction is a phenomenon in which dynamic loading of saturated soil results in the material properties to change suddenly from a solid state to a liquefied state. VS is an important parameter for evaluating liquefaction potential due to fact that it is influenced by many of the variables that influence liquefaction (e.g., void ratio, soil density, confining stress, stress history, and geologic age (Andrus et al., 1997)). Aki and Richards (2002) also outline that the amplitude of ground motion should depend on the density and shear wave velocity of near surface soils and rocks according to the theory of wave propagation. Since the change in density with the increase in depth is relative minor compared to that of the shear wave velocity, the latter is a very useful parameter to represent site conditions (Stewart et al., 1997). Bray (2014) and his colleagues carried out an extensive geotechnical analysis of the catastrophic liquefaction that occurred in Christchurch, New Zealand in 2010 and 2011 and found that near surface rather than deep liquefaction resulted in extensive foundation damage.