Frequency domain electromagnetic calibration for improved detection of sand intrusions in river embankments

Sand intrusions pose significant risks to river embankments due to potential flow pathways that can lead to instability during flood events. Visual inspection is a first step to recognize critical segments, but it does not deliver information about the subsurface. In this context, the electromagnetic induction (EMI) technique is a useful method for preliminary zoning at regional scale while the electrical resistivity tomography (ERT) method, widely used for hydrological purposes, is considered among the most reliable techniques for local subsurface imaging. A major sand intrusion within the levees of the Brenta River, located near Venice (northern Italy), resulted in water seeping during seasonal floods and posed severe threats to embankment stability. ERT and EMI techniques, along with geotechnical investigations, were the best survey choices to address the problem. Resistivity profiling successfully imaged the sand body geometry within and underneath the levee, and results correlated nicely with borehole stratigraphy. A first multiarray EMI device, which represented a faster and less expensive survey, was deployed to map further anomalies along nearby levees, but results were not satisfactory because the inverted profile failed to image the known intrusion. A second multiarray EMI device, with larger coil spacing, was also tested. Although it performed better in detecting the intrusion, results were still below expectations. A calibration procedure based on Pearson’s coefficients and using ERT as a reference was then devised and implemented to correct the EMI data prior to carrying out inversion. The procedure was successful for both EMI data sets, leading to realistic subsurface resistivity in the inverted sections. EMI measurements could then be recovered and interpreted correctly to estimate subsurface textures. The possibility of calibrating EMI data and obtaining subsurface resistivity images comparable to standard ERT profiling is an important improvement for cost-effective EMI surveying of river embankments to mitigate flood hazards.