Electrical resistivity tomography for the characterization of river embankments: 2D vs 3D inversion

Summary River embankments are increasingly afflicted by intense and frequent flood events owing to climate change. The characterization and monitoring of river embankments are hence becoming crucial for climate change adaptation. Electrical resistivity tomography (ERT) is one of the most common geophysical techniques for the characterization of river embankments to identify local anomalies and potentially prevent damages during floods. ERT surveys are usually executed over the embankment crest and interpreted along a longitudinal section. However, 2D interpretation of ERT data can be strongly affected by the embankment morphology along the investigated portion (i.e., the shape of the longitudinal section and the presence of curves). To overcome this issue, we propose 3D inversion of ERT data acquired over embankments, and compare 2D and 3D inversions of both synthetic and field ERT data in order to understand the differences in results between the two approaches. We created a 3D synthetic model with several types of embedded anomalies to realistically represent the embankment geometry. We considered both a straight and a curved longitudinal profile of the embankment. Synthetic data were then generated with two tailored electrode-configuration arrays (i.e., dipole‒dipole and Wenner‒Schlumberger). 2D field data were acquired close to Turin (NW Italy) over an embankment that has often been damaged by floods. The results from 2D and 3D inversions were analyzed and compared. The main outcomes show that, with synthetic and field ERT data from the crest of the embankment, the subsurface was better characterized by means of 3D inversion, especially in presence of 3D eccentric buried features or complex/curved model domains. From the examples of synthetic data, the presence of the embankment slopes and curvatures negatively affected 2D inversion results, which presented more artefacts and higher misfit values than the 3D inversion results. From the field case study, the 3D resistivity volume presented good sensitivity in the transversal direction, thus allowing useful inspection of feature discontinuities or resistivity contrasts. Moreover, for the synthetic data the 3D results were more coherent than the 2D results with the true resistivity and position of the anomalies. For the field data, the 3D results were more coherent than the 2D results with known information from geological and geotechnical data. The more physically consistent 3D solutions may be further exploited for quantitative analysis on the petrophysical properties of the embankment material. In conclusion, 3D ERT inversion is recommended in presence of irregular domains (slopes, curves, strong discontinuities) even with 2D data.