P. Côte

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.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams

The recent and dramatic floods of the last years in Europe (Windstorm Xynthia, February 2010) and United-States (Hurricane Katrina, August 2005) showed the vulnerability of flood defence systems. The first key point for avoiding these dramatic damages and the high cost of a failure and its consequences lies in the conception and construction of the dams and dikes, taking into account the past flooding events. A well-designed dike with the correct height avoids failure and overtopping.

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.

A cost-effective 3D electrical resistivity imaging approach applied to dike investigation

Although DC electrical resistivity imaging is widely applied to dike investigation, either rapid 2D or high-resolution 3D approaches fail to address actual needs. An intermediate electrical resistivity imaging approach referred to as “3D–” is introduced in this paper. The methodology is based on existing tools, and it offers useful and sufficiently reliable 3D images of the investigated structure within a cost-effective and flexible procedure. The survey design, the model discretisation, and the thorough integration of a priori information are the main phases of this procedure. To demonstrate the benefits and limitations of this approach, it is applied to an existing stretch of embankment levee along the Loire River. A numerical study was carried out both on synthetic and real data to assess the 3D imaging capability of the approach and the influence of prior information on the inversion outputs. The important role of a priori information is shown to be even more essential here. The results demonstrate the efficiency and versatility of the 3D– approach for reliable and cost-effective investigations of long dikes.

Assessment of Physical Properties of a Sea Dike Using Multichannel Analysis of Surface Waves and 3D Forward Modeling

Seismic surface waves analysis methods (SWM) have been widely developed and tested in the context of subsurface characterization and have demonstrated their effectivness for sounding and monitoring purposes. Given their efficiency, SWM have been transfered to several scale of which civil engineering structures. However, at this scale, many structures may often exhibit 3D surface variations which drastically limit the efficiency of SWM application which are are mostly developed under the assumption of semi-infinite 1D layered medium without topography. Taking advantages of high-performance computing center accessibility and wave propagation modeling algorithm development, it is now possible to consider the use of a 3D elastic forward modeling algorithm in the SWM inversion process. We use a parallelized 3D elastic modeling code based on the spectral element method which allows to obtain accurate synthetic data with very low numerical dispersion and a reasonable numerical cost. In this study, we choose a sea dike as a case example. We first show that their longitudinal geometry and structure may have a significant effect on dispersion diagrams of Rayleigh waves. Then, we demonstrate the necessity of 3D elastic modeling as a forward problem for the inversion of dispersion diagrams.

Monitoring of the Tidal Response of a Sea Dike Using Sea Waves as Seismic Noise

Ambient seismic noise has long been considered as a nuisance for the analysis of seismograms. For some years now, it has shown a great potential for monitoring various geological objects with high accuracy. Here, using ambient seismic noise generated by sea wave impacts, we reconstruct surface waves propagating within a sea dike body, and monitor the tidal response of the structure as the water level changes. Velocity and attenuation variations are obtained respectively from delay and spectral amplitude measurements. We localize lateral areas of variation which temporal evolution is correlated with water level. These variations are interpreted as the consequence of water infiltration inside the structure. The proposed method may provide a solution for detecting internal erosion, generally only observed at late stage by visual inspection.

Resistivity/Induced Polarization/Self-Potential Methods and Applications

Modern multielectrode and multichannel resistivity systems have made it relatively easy and rapid to collect time domain induced polarization (IP) data in near surface surveys. This paper will examine a wide variety of applications through case studies in a variety of geological settings in Western Canada. Case studies will show various applications and complementary features of IP surveys including distinguishing salt water from conductive clays, identifying faults, locating deeply buried structures underneath active facilities, and distinguishing landfilled debris from leachate. IP data sets will be correlated with other data sets including resistivity, seismic reflection, and borehole geophysical parameters.

Refined Experimental Studies for Improving the Reduced-scale Physical Modeling of Seismic Subsurface Measurement

The potential of experimental seismic modeling at reduced scale is explored since several years because it provides an intermediate step between numerical tests and geophysical campaigns on field sites. The MUSC system is designed as a reliable tool, able to produce multi-sources and multi-receivers experimental seismic data suitable for high-resolution imaging techniques like Full Waveform Inversion. However, experimental seismic modeling has limitations such as finite dimensions models used instead of realistic half-space models and three-dimensional seismic data generated by the point-source whereas most of wave propagation and imaging algorithms make use of two-dimensional forward modeling for numerical cost reasons. The main objective of the presented study is thus to improve the experimental seismic modeling to generate reproducible, realistic and suitable data which will be distributed in the scientific community. In this scope, we have: 1) refined the comparison between numerical and experimental data by generating experimental two-dimensional line-sources, 2) assessed the reproducibility of the source emitted in a model by the piezzo-electric transducer and 3) reduced efficiently the boundary reflections with the help of an adapted new geometry of the edges which absorbs a large band of the incident waves spectrum.

A Cost-effective 3D Electrical Resistivity Imaging Approach for Embankment Dike Assessment

Although DC-electrical resistivity imaging techniques are efficient tools for embankment dike and levee investigation and monitoring, the complexity of such structures often leads to misinterpretation when conventional 2D procedures are implemented. Nevertheless, fully 3D procedures can be very slow and costly, and bring results and resolution that may exceed the needs. This paper introduces the ‘3D– inversion’ approach that allows a useful and reliable 3D vision of the investigated structure while maintaining an acceptable number of electrodes and data. To compensate the lack of dense 3D data, the approach is mainly based on i) designing data acquisition based on a reasonable number of electrodes and measurements, ii) designing a well-adapted model discretization and iii) inserting as much a priori information as possible. The building process of the numerical model requires attention, particularly on the model discretization. To exemplify this new approach, synthetic results are presented on a model that is a simplification of a real scale experimentation. Preliminary outcomes show the cost-effective potential of the 3D– approach which is to be tested on various case studies in the near future.