Papers by Frederic Nguyen
Near Surface 2011 - 17th EAGE European Meeting of Environmental and Engineering Geophysics, 2011
Abstract:[en] Many geophysical inverse problems are ill-posed leading to non-uniqueness of the so... more Abstract:[en] Many geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the inverse problem and incorporating all available prior information in the inversion process. We compare three different ways to go beyond standard Occam's inversion for electrical resistivity tomography (ERT) using electromagnetic logging data in the context of salt water infiltration: a simple reference ...
Inversion of time-lapse resistivity data allows obtaining 'snapshots' of changes occurrin... more Inversion of time-lapse resistivity data allows obtaining 'snapshots' of changes occurring in monitored systems for applications such as aquifer storage, geothermal heat exchange, site remediation or tracer tests. Based on these snapshots, one can infer qualitative information on the location and morphology of changes occurring in the subsurface but also quantitative estimates on the degree of changes in certain property such as temperature or total dissolved solid content. Analysis of these changes can provide direct insight into flow and transport and associated processes and controlling parameters. However, the reliability of the analysis is dependent on survey geometry, measurement schemes, data error, and regularization. Survey design parameters may be optimized prior to the monitoring survey. Regularization, on the other hand, may be chosen depending on available information collected during the monitoring. Common approaches consider smoothing model changes both in spa...
Electrical resistivity tomography (ERT) has become a popular imaging methodology in a broad range... more Electrical resistivity tomography (ERT) has become a popular imaging methodology in a broad range of applications given its large sensitivity to subsurface parameters and its relative simplicity to implement. More particularly, time-lapse ERT is now increasingly used for monitoring purposes in many contexts such as water content, per-mafrost, landslide, seawater intrusion, solute transport or heat transport experiments. Specific inversion schemes have been developed for time-lapse data sets. However, in contrast with static inversions for which many techniques including geostatistical, minimum support or structural inversion are commonly applied, most of the methodologies for time-lapse inversion still rely on non-physically based spatial and/or temporal smoothing of the parameters or parameter changes. In this work, we propose a time-lapse ERT inversion scheme based on the difference inversion scheme. We replace the standard smoothness-constraint regularization operator by the para...
Water Resources Research, 2015
ABSTRACT In inverse problems, investigating the relationship between data and prior models and th... more ABSTRACT In inverse problems, investigating the relationship between data and prior models and the uncertainty related to the posterior distribution of model parameters are as important as matching the data. In recent years, many efforts have been done to assess the posterior distribution of a given problem with reasonable computational costs through inversion techniques such as McMC. The derived posterior distribution is always dependent on the prior distribution. However, most of the studies ignore modeling the prior with realistic uncertainty. In this paper, we propose a workflow to assess the uncertainty of inversion of hydraulic heads data through the addition of electrical resistivity tomography (ERT) constraining data. The workflow is divided in three successive steps: 1) Construction of prior: we generate multiple alternative geological scenarios from literature data (architecture of facies) as well as site specific data (proportions of facies). Spatial uncertainty within each scenario is integrated hierarchically through geostatistics (multiple-point statistics simulation of facies constrained by ERT data as soft data). 2) Validation of prior scenarios: we transform prior facies scenarios into resistivity distribution scenarios through forward and inverse modeling. The scenarios are validated by comparison with field ERT data. The comparison is made through distance calculation and projection into a low dimensional space to calculate the probability of each scenario given field ERT data. 3) Matching dynamical data: we use the probability perturbation method, within each scenario, to integrate hydraulic heads to our models. We account for scenario probabilities, calculated in 2, in determining how many models per scenario we have to consider for building a reliable posterior distribution. As an illustration, the method is applied on a field case study in an alluvial aquifer (Belgium) where we consider prior uncertainty related to the type of elements (gravel channels or bars) and to their size. This study shows the importance of considering the uncertainty of the prior in inverse problems as it has a strong influence on model predictions and decision-making problems.
As many regions of Western Europe, the Provence area is characterized by slow deformation rates, ... more As many regions of Western Europe, the Provence area is characterized by slow deformation rates, substantial erosion rates, dense vegetation coverage and human activity. These combined factors affect the detection of geomorphological evidence of slow active faults and consequently the selection of trench sites for paleoseismological studies. However, in terms of seismic hazard, these regions have to be studied carefully. For example, the most important instrumental seismic event in France history, the Lambesc earthquake (M=6), occured in Provence in 1909. A review of recent combined studies (Chardon et al., Bull. Soc. Géol. France, t. 174, no. 5, 2003) allows constraining the tectonic and geomorphic evolution of the Trévaresse ridge anticline and associated thrust ramp that obviously ruptured during the 1909 event. In this context, the use of combined geophysical methods can be interesting to determine i) their ability to detect directly, from physico-mechanical contrasts, the locat...
Symposium on the Application of Geophysics to Engineering and Environmental Problems 2011, 2011
Abstract:[en] Image appraisal is a problem frequently encountered in electrical resistivity tomog... more Abstract:[en] Image appraisal is a problem frequently encountered in electrical resistivity tomography (ERT), and more generally in non-linear geophysical inversion. It may include several aspects such as the identification of the geometry of buried structures, the ...
Groundwater resources are increasingly used around the world as geothermal systems. Understanding... more Groundwater resources are increasingly used around the world as geothermal systems. Understanding physical processes and quantification of parameters determining heat transport in porous media is therefore important. Geophysical methods may be useful in order to yield additional information with greater coverage than conventional wells. We report a heat transport study during a shallow heat injection and storage field test. Heated water (about 50°C) was injected for 6 days at the rate of 80 l/h in a 10.5°C aquifer. Since bulk electric resistivity variations can bring important information on temperature changes in aquifers (water electric conductivity increases about 2%/°C around 25°C), we monitored the test with surface electric resistivity tomography and demonstrate its ability to monitor spatially temperature variations. Time-lapse electric images clearly show the decrease and then the increase in bulk electric resistivity of the plume of heated water, during respectively the injection and the storage phase. This information enabled to calibrate the conceptual flow and heat model used to simulate the test. Inverted resistivity values are validated with borehole electromagnetic measurements (EM39) and are in agreement with the temperature logs used to calibrate the parameters of the thermohydrogeological model for the injection phase. The short term evolution of the ERT-derived temperature and the temperature logs is coherent for both a qualitative and quantitative use of ERT-derived temperature. However, the mid-and long-term evolution need to account for other phenomena such as variations of TDS content as a function of temperature to quantitatively use ERT estimates as temperature proxy. This field work demonstrates that surface electric resistivity tomography can monitor heat injection and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems or the use of heated water to replace salt water in tracer tests.
Water Resources Research, 2014
Electrical resistivity tomography (ERT) can be used to constrain seawater intrusion models becaus... more Electrical resistivity tomography (ERT) can be used to constrain seawater intrusion models because of its high sensitivity to total dissolved solid contents (TDS) in groundwater and its relatively high lateral coverage. However, the spatial variability of resolution in electrical imaging may prevent the correct recovery of the desired hydrochemical properties such as salt mass fraction. This paper presents a sequential approach to evaluate the feasibility of identifying hydraulic conductivity and dispersivity in densitydependent flow and transport models from surface ERT-derived mass fraction. In the course of this study, geophysical inversion was performed by using a smoothness constraint Tikhonov approach, whereas the hydrological inversion was performed using a gradient-based Levenberg-Marquardt algorithm. Two synthetic benchmarks were tested. They represent a pumping experiment in a homogeneous and heterogeneous coastal aquifer, respectively. These simulations demonstrated that only the lower salt mass fraction of the seawater-freshwater transition zone can be recovered for different times. This ability has here been quantified in terms of cumulative sensitivity and our study has further demonstrated that the mismatch between the targeted and the recovered salt mass fraction occurs from a certain threshold. We were additionally able to explore the capability of sensitivity-filtered ERT images using ground surface data only to recover (in both synthetic cases) the hydraulic conductivity while the dispersivity is more difficult to estimate. We attribute the latter mainly to the lack of ERT-derived data at depth (where resolution is poorer) as well as to the smoothing effect of the ERT inversion.
Near Surface Geoscience 2013, 2013
ABSTRACT The aim of this study is to monitor (bio) physical processes occurring in a landfill. Th... more ABSTRACT The aim of this study is to monitor (bio) physical processes occurring in a landfill. The experiment consists in injecting leachate towards a drain in unsaturated and not yet digested waste to reactivate (or activate) waste biodegradation. The target is the first 15 meters of the studied landfill subsurface. The visualisation of the wet front arrival (short term effect) is crucial because we want to ensure that waste is entirely humidified to allow the reactivation of waste digestion. The second process is a long term effect consisting in the increase of the internal temperature of the landfill which is synonymous of the reactivation of biodegradation processes. We currently use 3D time-lapse ERT on a monthly basis to capture the decrease of electrical resistivity related to the increasing temperature. We also collect ground truth data, including distributed temperatures in a borehole to validate results. For short term effects, we monitored the wet front arrival with three 2D ERT profiles during an entire day. Preliminary results, corroborated by ground truth data, show that leachate flow is anisotropic (more rapid horizontally than vertically). So far, waste was completely humidified but only slight changes of temperature occurred.
Near Surface Geoscience 2013, 2013
ABSTRACT In the context of landfill mining, we conducted electrical resistivity tomography (ERT),... more ABSTRACT In the context of landfill mining, we conducted electrical resistivity tomography (ERT), borehole electromagnetics (EM39), and magnetometry campaigns to characterise the subsurface of a waste deposit. The electrical resistivity structures in the landfill appear tabular with a first layer (12-15 m) characterised by higher resistivities (> 10 ohm.m; with spots > 30 ohm.m) whereas the saturated zone (below 15 m) presents lower resistivities (< 4 ohm.m). Based on ERT images, we positioned 6 drillings, collected waste samples, and recorded in-situ measurements (distributed temperature and EM39) to confirm the ERT interpretation and bring out some limitations of the method. Magnetic field data were recorded in the same area. We noticed a clear distinction between the inside (high magnetic field gradient) and the outside (low variation) of the landfill. The large scale variation of the total magnetic field is compatible with the existence of a clinker dam below the landfill bottom membrane (known from historical information). Therefore, magnetometry could be used to detect variation in the waste composition (clinker, metallic drums, or municipal waste) but is also a cheap and powerful tool to map unknown landfill boundaries. Both aspects are crucial in the context of landfill mining.
Near Surface Geoscience 2013, 2013
We performed laboratory measurements on fully saturated sand samples in the context of deriving r... more We performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in monitoring an increase of temperature in sand samples with electrical resistivity measurements. We neglected the effect of surface conductivity since experiments showed two orders of magnitude between surface and fluid conductivities. We show that using simple linear relationship between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates. Indeed, chemical analyses highlight the importance of accounting chemical reactions occurring when temperature changes, including dissolution/precipitation processes. We performed two experiments based on typical in-situ conditions. We first simulated the injection of a less conductive tap water and second, the injection of heated formation water. In the second case, minerals solubility decreases and precipitation occurs, leading to an increase of bulk resistivity. This mechanism competes with dissolution of minerals when tap water is injected, since tap water is not in equilibrium with the medium. In any case, further research is needed to fully understand the mechanisms and to develop a fully integrated law to derive better temperature estimates.
Near Surface Geoscience 2013, 2013
ABSTRACT Several techniques are available to estimate the depth of investigation (DOI) or to iden... more ABSTRACT Several techniques are available to estimate the depth of investigation (DOI) or to identify possible artifacts in resistivity and IP surveys. Commonly, the DOI is mainly estimated using an arbitrarily chosen cut-off value on a selected resolution indicator (resolution, sensitivity or DOI index). Small changes in threshold values may induce strong variations in the estimated DOI. To overcome this problem, we developed a new statistical method to estimate the DOI based on a modified DOI index approach. Three inversions are performed using three strongly different resistivity reference models. We found that the cumulative distribution function of the DOI index values is well fitted by the sum of two normal distributions. We then focused on the evaluation of the mean and standard deviation of the normal distribution linked to the statistically well-constrained cells. We introduced two reliability indexes RI2σ and RI3σ based on confidence intervals, respectively 2σ and 3σ. They are used as alpha transparency values when plotting resistivity and chargeability models to discriminate between well- and poorly-constrained cells. The efficiency of the proposed methodology is assessed on synthetic data. Based on synthetic benchmark analysis, we demonstrated that the selected well-constrained cells are well-reconstructed in size, shape and resistivity.
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Papers by Frederic Nguyen