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

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Conference Proceeding

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© 2018 J and N Group, Ltd. All rights reserved. Geophysical investigations were carried out at three sites along levees in the New Orleans area. The sites sustained damage from Hurricane Katrina in 2005 and have since been rebuilt. The geophysical methods include active and passive surface wave methods, and capacitively coupled resistivity. This paper summarizes data acquisition and analysis of 2D passive surface wave data. Twelve cableless seismic data acquisition units with vertical component 2 Hz geophones were used to record ambient noise data. Each unit includes a GPS clock so that all units can be synchronized over any distance without cables. Data acquisition used a linear array with geophone spacing of 5 m and four geophones were moved up forward every 10 min. along 450 ~ 1,100 m length lines. Total data acquisition took several hours for each line. Recorded ambient noise data were processed using the common midpoint spatial autocorrelation method (CMP-SPAC) and clear dispersion curves were obtained at all sites. Minimum frequency ranges from 1 to 2 Hz and maximum frequency ranges from 10 to 30 Hz depending on site. Obtained dispersion curves are generally consistent with those obtained from active surface wave methods and L-shaped arrays. Nonlinear inversion was performed and 2D S-wave velocity (VS) models were obtained. The method penetrated to a depth of 40 ~ 60 m and provided 450 ~ 1100 m cross sections along levees. The interpretation focused on identifying zones beneath the levees or canal walls having low VS, corresponding to saturated, unconsolidated sands or low-rigidity clays. Resultant VS profiles are generally consistent with existing drilling logs and the results of laboratory tests.

Publication Source (Journal or Book title)

Proceedings of the Symposium on the Application of Geophyics to Engineering and Environmental Problems, SAGEEP

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