Evaluation of geoelectrical models efficiency for coastal seawater intrusion by applying uncertainty analysis

Seawater intrusion (SWI) is one of the most important phenomena which occurs in shorelines and islands and affects groundwater storage in the region. This study aims to investigate how far seawater has intruded in a coastal area and what is its extent variation. The results can lead to different management plans. To achieve these goals, a geoelectrical method is applied in the coastal aquifer of Chaouia, Morocco. In order to choose the best inversion process, different settings are applied in two different software packages namely RES2DINV and BERT and the results are compared using the Taylor diagram. Also, to determine the minimum and maximum SWI extent in the region, a new approach of uncertainty analysis is applied in the inversion process by the Monte Carlo method. The general results indicate that the seawater has intruded about 2 km into the shore. The obtained results show that by considering uncertainty, there is a sensible difference between the maximum and minimum extent of SWI (maximum 9% variation in the area of SWI) which should be noted when doing water research management studies. The methodology procedure in this study can be applied to different coastal areas around the world

A geoelectrical pre-study of Älvkarleby test embankment dam : 3D forward modelling and effects of structural constraints on the 3D inversion model of zoned embankment dams

Electrical resistivity tomography has potential as a complementary long-term monitoring method in embankment dams; however, the 3D character of the geometry including the shape of the embankment, its internal zoned construction and the reservoir water make interpretation challenging. To tackle this problem, a qualified inversion model considering the 3D environment is necessary. In this paper, prior information about the resistivity of different parts of a test embankment dam was used as constraints in order to increase the capability of defect detection in a complex 3D context. Five small defects were incorporated into the core of the model. Laboratory measurements were made on samples of the materials intended to be used for the construction of a test embankment dam, and resistivity values provided from the laboratory measurements were used in the forward modelling. A measurement sequence of around 8000 synthetic data points using extended gradient, crossline bipole-bipole and corner arrays between horizontal-horizontal, vertical-vertical, and vertical-horizontal lines were modelled and inverted all at once. The structural constraints were applied to increase the accuracy of inversion, using the L1 and L2 methods. Different mesh qualities with different boundaries for each region and 3D geometric factor calculation were applied for the inversion to evaluate the effects of region control incorporated in the inversion process. The results showed that L1 and L2 norm inversions combined with region control can determine the location of very small defects and finding the defects located near each other, respectively. Removing the region control from the inversion caused unrealistic resistivity prediction for some regions and the inability to discover the dam defects. Therefore, the proposed methodology can decrease non-uniqueness in the inversion and make time-lapse ERT a valuable monitoring tool that complements other instrumentation techniques and based on these results it was concluded promising to continue with the construction of the test dam using the same type of defects and electrode set-up, for verification under field-conditions

Pre-study for geoelectrical monitoring for detection of internal defects and anomalous seepage in the Älvkarleby test embankment dam

Electrical resistivity tomography (ERT) can be used to monitor the interior of hydropower embankments dams, and thereby detect zones with anomalous material properties and flow induced variation in the resistivity caused by changes in total dissolved solids (TDS) and temperature. Furthermore, monitoring of embankment dams in connection with a substantial change in the reservoir water level can detect anomalous leakage paths via differential wetting of zones with different hydraulic properties. In Sweden, where the available hydropower energy capacity is utilised, installation of electrodes must be done post-construction of embankment dams, which for practical reasons generally means installed along its crest, in the top of the core, using a 2D ERT approach. This has the advantage of focusing the sensitivity to the core itself, which is the part of the dam that shall stay impervious over time. However, the orientation of the electrode layout in combination with the 2D approximation leads to severe 3D effects, which distorts the inverted model resistivities and geometry. Furthermore, the resolution decreases with depth, which is a major limitation for high dams. A way ahead would be if electrodes could be installed on deeper levels inside the dam close to the core, which might be possible using modern drilling technology. This electrode lay-out concept was investigated with numerical modelling using extended gradient, cross-line bipole-bipole and corner arrays between horizontal-horizontal, vertical-vertical and vertical-horizontal lines respectively. To interpret the data 3D inversion is required to handle the structure’s geometry due to the zoned construction with materials that have large contrasts in resistivity. A test embankment dam installation was built during the autumn of 2019, with electrodes and various sensors installed inside the dam to evaluate the applicability of the suggested approach. We present results of numerical modelling simulating potential defect scenarios where several measurement sequences of close to 8,000 data points using the abovementioned arrays are inverted all at once. In order to resolve subtle variations inside the core, the finite element grid design is based on prior knowledge about the internal material distribution, with broken smoothness constrains at known material boundaries. In combination with region-based control of the resistivities of the different material zones, the inversion in combination with a time-lapse approach it shows promising results

Monitoring internal erosion in embankment dams using 3D Electrical Resistivity Tomography: Älvkarleby test embankment dam

One major risk threatening embankment dam integrity is internal erosion of the core. Internal erosion progresses inside the dam but it is difficult to detect with conventional methods. Electrical Resistivity Tomography (ERT) is a potential-based method that can sense the interior of the dam, and this study aims to evaluate the capability of ERT as a complementary monitoring technique for discovering internal erosion. A test embankment dam with some defects incorporated inside the core and fine filter in Älvkareby, Sweden, has been constructed with the purpose of assessing different monitoring techniques including ERT. Buried electrodes and sensors for other monitoring instruments were placed inside the dam. The collected ERT data were inverted using a 3D time-lapse inversion model implemented in pyGIMLI/pyBERT package. The results revealed a layered resistivity structure in the core and several unintentional anomalous zones. Furthermore 2 out of 5 defects in the core were located, namely horizontal and vertical crushed rock zones, with a slight location shift for the horizontal zone. A concrete block defect was indicated, although not as distinctly and with a lateral shift. The two remaining defects, a crushed rock zone at the abutment and a wooden block were not discovered