An integrated geophysical approach to scan river embankments

The repeated !oods occurred in the last ten years in Northern and Central Italy as well as in many areas of Europe caused important damages to agriculture, industry and infrastruc-tures, to residential buildings and in some cases there were also casualties. The damages were mostly caused by water overtopping the river embankments or by levee failures. Ex-treme events are becoming more and more frequent and in-tense rainfalls often result in multiple !ood waves in the rivers, causing severe stress conditions on the embankments. The structure and the stratigraphy of earthen levees and of the underlying geology represent vital information and the eventu-ality of a collapse, also on minor rivers, frequently involves un-affordable social cost. The standard approach of monitoring the levees using sparse geotechnical tests and visual analysis is no longer satisfactory as the recent changes in the !ood regime increased the stress on these structures and the asso-ciated risk of failures. Prolonged high-water levels in the river and repeated !ood waves could stress portion of the levees that have not been stressed before causing unpredictable fail-ures. A fast imaging system based on dual geophysical technol-ogy has been set up and experimented in a variety of river embankment scenarios during a long-lasting testing phase. The imaging procedure, named EMAR, is based on Frequency Domain Electro-Magnetic induction (FDEM) and multichan-nel Ground Penetrating Radar (GPR). The system targets the quick detection of anomalies and not their full resolution that could be later achieved with standard two- and three-dimen-sional ERT imaging. The integration of FDEM and GPR re-sponses delivers the highest resolution nowadays achievable in dynamic scanning mode. In the "rst experiments was utilized a multi-frequency FDEM later substituted with a "xed-frequency multi-array FDEM. Multi-frequency FDEM data are mostly qualitative and a spe-ci"c analysis and processing procedure was required to ro-bustly interpret the results and map levee anomalies. The uti-lization of the multi-array FDEM was a major advance as the elevate number of data points allows for data inversion from apparent to a true conductivity model. The system proved to be fast and ef"cient for detecting po-tentially critical condition in the embankments and in the un-derlying deposits. Major system drawbacks have been resolved splitting the EMAR survey in two campaigns. A winter campaign with wet levees is the best choice for EM imaging while a summer cam-paign with dry levees is more appropriate for GPR imaging. This approach permits to get the most out of the EMAR sys-tem. Very tall levees could be outside the range of the test-ed array and adding an extra sensor incrementing the largest spacing of two additional meters could be a perfect solution for the quasi-majority of Italian rivers.