Sérgio Palma Lopes

Nonlinear inversion of geoelectric data acquired across 3D objects using a finite-element approach

We introduce a new finite-element-based scheme for the fast nonlinear inversion of large 3D geoelectric data sets acquired around isolated objects or across the earth’s surface. The principal novelty of this scheme is the combination of a versatile finite-element approach with (1) a method involving minimization of an objective function using a conjugate-gradient algorithm that includes an adjoint-field technique for efficiently establishing the objective-function gradient and (2) parabolic interpolation for estimating suitable inversion step lengths. This scheme is capable of handling large volumes of data acquired using diverse electrode configurations located around or across 3D structures. Only three solutions to the forward problem are required for each iteration. Computation of the Jacobian matrix, which might require computers with a large amount of memory, is not necessary. To minimize artificial irregularities in the inverted models, particularly near the electrodes, we smooth the model parameter...

Quantification and uncertainty analysis of a structural monitoring device: detection of chloride in concrete using DC electrical resistivity measurement

In this work, we seek to assess and optimise the performance of an integrated chloride detection sensor based on the DC electrical resistivity measurement. We specifically seek to evaluate the detection threshold of chlorides. The main problem of the resistivity measurements in concrete is the dispersion of results mainly linked to the material heterogeneity and electrical contact limitations. To take into account the uncertainty of the measurement, we used a measurement device and sensors based on Geoelectrical Imaging methods. It gives richer information than the one delivered by conventional resistivity measurement sensors. Thus, it allows the use of statistical analysis and quality assessment methods. Before performing tests in concrete, we worked in a medium of known and homogenous resistivity in order to assess and optimise the performances of the acquisition system. Then we performed tests on concrete specimens containing different contents of chloride ions. Performance assessment of the resistivity probe is based on an analysis of receiver operating characteristic curves and the detection threshold of chlorides is calculated using the αδ method.

Microstructural Interpretation of Water Content and Dry Density Influence on the DC-Electrical Resistivity of a Fine-Grained Soil

This paper examines the use of direct current-electrical resistivity to quickly and accurately measure the density and water content of soils in situ. The authors develop a new resistivity-cell and experimental procedure to assess the resistivity of compacted samples of a silty soil. The laboratory study was conducted with moisture content and compaction level ranges that are consistent with those used in earthwork construction. The effects of dry density and gravimetric water content on the soil bulk resistivity are considered separately. The effect of air index on electrical resistivity also is demonstrated. The results show that the relationship between DC-resistivity and dry density is fairly linear for given gravimetric water content levels. The corresponding sensitivity decreases as water content increased until reaching a constant sensitivity above a critical water content that lies close to the standard Proctor optimum moisture content. The experimental results are further analyzed within the context of a previously developed soil microfabric model which distinguishes water phase, air phase, particles assumed to be “inert” and clayey aggregates. Two types of electrical conduction help explain the results: first, conduction occurring in the intra-aggregate voids, controlled by the charge density on the surface of clayey particles; and second, inter-aggregate conduction controlled by interstitial water resistivity. The relative effect of each type of conduction is shown to strongly depend on water content. An extrapolation at zero air void allows for presenting an interpretation of the intra-aggregate volume contribution to electrical conduction in dry states. This interpretation suggests a dilution effect on the adsorbed cation concentration within bound water, which is consistent with earlier findings. Limitations of this study and directions for future research are discussed.

Application of Belief Functions to Levee Assessment.

We propose the use of Smets and PCR5 rules to merge artificial geophysical and geotechnical data, as part of fluvial levee assessment. It highlights the ability to characterize the presence of interfaces and a geological anomaly.