Resistivity imaging of river embankments: 3D effects due to varying water levels in tidal rivers

Abstract

Electrical resistivity tomography (ERT) has seen increased use in the monitoring the condition of river embankments, due to its spatial subsurface coverage, sensitivity to changes in internal states, such as moisture content, and ability to identify seepage and other erosional process with time-lapse ERT. Twodimensional ERT surveys are commonly used due to time and site constraints, but they are often sensitive to features of anomalous resistivity proximal to the survey line, which can distort the resultant inversion as a three-dimensional (3D) effect. In a tidal embankment, these 3D effects may result from changing water levels and river water salinities. ERT monitoring data at Hadleigh Marsh, UK, showed potential evidence of 3D effects from local water bodies. Synthetic modelling was used to quantify potential 3D effects on tidal embankments. The modelling shows that a 3D effect in a tidal environment occurs (for the geometries studied) when surveys are undertaken at high water levels and at distances less than 4.5 m from the electrode array with 1 m spacing. The 3D effect in the modelling is enhanced in brackish waters, which are common in tidal environments, and with larger electrode spacing. Different geologies, river water compositions, and proximities to the model parameters are expected to induce a varied 3D effect on the ERT data in terms of magnitude, and these should be considered when surveying to minimize artefacts in the data. This research highlights the importance of appropriate geoelectrical measurement design for tidal embankment characterization, particularly with proximal and saline water bodies.

An Iterative 3D Correction plus 2D Inversion Procedure to Remove 3D Effects from 2D ERT Data along Embankments

“…Their results confirmed again that 3D effects are a function of the embankment geometry, geology and water content as well as boundary conditions [ 46 ]. Thus, the problem is further emphasized knowing that 3D effects not only depend on the geometry of the embankment, but they are also affected by seasonal variations in the resistivity values in the embankment body as well as by the fluctuations of the air/water levels in either side of the embankment [ 46 , 47 ]. Zanzi and Hojat (2023) proposed an iterative 3D correction plus 2D inversion procedure to correct 2D ERT data for 3D effects and then invert the corrected data using a 2D inversion algorithm [ 43 ].…”

“…Three-dimensional inversion would be the solution to take into account the 3D effects of the embankment structures during data processing. However, since 2D acquisition systems are the most commonly used systems along such structures because of convenient field work, adequate coverage of subsurface anomalous zones, and relatively rapid data processing and real-time interpretation [ 46 , 47 ], a 3D inversion would be very poorly constrained. Therefore, an iterative 3D correction + 2D inversion procedure was proposed [ 43 ].…”

“…This is due to the fact that in our tests (inspired by our monitoring sites in fresh environments), the conductivity of the river water is not too high to generate an exaggerated contrast with the levee material. However, it should be mentioned that 3D effects are enhanced in the brackish waters common in tidal environments [ 46 ]. The other reason for selecting the simulations when only air is present on both sides of the embankment is that, as described in Section 2 (see also Figure 2 b), Parma River passes at a distance of about 40 m from the ERT line.…”

“…The data acquired along an ERT line is a 2D pseudosection of apparent resistivity values which then undergoes an inversion procedure to produce a tomographic image that represents the real resistivity distribution of the embankment body along the ERT line and with depth. A 2D ERT inversion algorithm assumes that the resistivity does not change in the direction perpendicular to the ERT profile, but the special 3D geometry of earthen embankments ( Figure 1 ) does not satisfy this requirement and thus, distortions are introduced into the 2D data measured along these structures [ 42 , 43 , 44 , 45 , 46 , 47 ]. Such distortions are known as 3D effects and depend on the specific 3D geometry of each site, the resistivity distribution within the embankment, and the boundary conditions of the embankment (i.e., air, water or a combination of air and water on either side).…”

Spreading of Localized Information across an Entire 3D Electrical Resistivity Volume via Constrained EMI Inversion Based on a Realistic Prior Distribution

“…As such, they are useful to infer the electrical resistivity (and chargeability) of the subsurface [1][2][3][4]. ERT data are collected using arrays of dozens of electrodes coupled with the soil to ensure galvanic contact with the ground and are, nowadays, regularly inverted using 2D or 3D forward modeling [5][6][7]. On the contrary, EMI measurements can be performed without any physical contact with the surface and can be generally inverted using simple 1D forward modeling [8][9][10].…”