Application of 2D ambient noise tomography to levee safety assessment in New Orleans

Abstract

To develop noninvasive methods for levee inspection, we carry out shallow, active, and passive seismic investigations at three sites along levees in the New Orleans, Louisiana, USA, area: Industrial Canal, London Avenue Canal, and 17th Street Canal. These sites sustained damage from Hurricane Katrina in 2005 and have since been rebuilt. Recorded ambient noise data are processed using the common-midpoint spatial autocorrelation method. Dispersion curves obtained with active surface-wave methods and passive wave methods, which use both L-shaped and linear sensor arrays, show internally consistent similarities. Minimum frequencies range from 0.6 to 2 Hz and maximum frequencies range from 10 to 30 Hz. Nonlinear inversion of 2D S-wave velocity models generates velocity-depth cross sections that extend approximately 400–1000 m along levees and provide information to depths of 40–60 m. Resultant S-wave velocity (VS) profiles are generally consistent with existing drilling logs and the results of laboratory tests. Beneath the London Avenue Canal wall, VS values (130–170 m/s) likely correspond to saturated, unconsolidated sands, and a low-velocity (50–100 m/s) zone at depth to 15 m beneath the 17th Street Canal matches low-rigidity clays observed in geotechnical logs. Comparison to active surface-wave methods at the Industrial Canal site display similar results but highlight that while active methods have better resolution in the upper few meters, passive methods may be acquired more quickly.