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Levee evaluation using MASW: Preliminary findings from the Citrus Lakefront Levee, New Orleans, Louisiana

The utility of the multi-channel analysis of surface waves (MASW) seismic method for noninvasive assessment of earthen levees was evaluated for a section of the Citrus Lakefront Levee, New Orleans, Louisiana. This test was conducted after the New Orleans’ area levee system had been stressed by Hurricane Katrina in 2005. The MASW data were acquired in a seismically noisy, urban environment using an accelerated weight-drop seismic source and a towed seismic land streamer. Much of the seismic data were contaminated with higher-order mode guided-waves, requiring application of muting filtering techniques to improve interpretability of the dispersion curves. Comparison of shear-wave velocity sections with boring logs suggests the existence of four distinct horizontal layers within and beneath the levee: (1) the levee core, (2) the levee basal layer of fat clay, (3) a sublevel layer of silty sand, and (4) underlying Pleistocene deposits of sandy lean clay. Along the surveyed section of levee, lateral variations in shearwave velocity are interpreted as changes in material rigidity, suggestive of construction or geologic heterogeneity, or possibly, that dynamic processes (such as differential settlement) are affecting discrete levee areas. The results of this study suggest that the MASW method is a geophysical tool with significant potential for non-invasive characterization of vertical and horizontal variations in levee material shear strength. Additional work, however, is needed to fully understand and address the complex seismic wave propagation in levee structures.