References of "Nguyen, Frédéric"
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See detail4D electrical resistivity tomography (ERT) for aquifer thermal energy storage monitoring
Lesparre, Nolwenn; Robert, Tanguy ULiege; Nguyen, Frédéric ULiege et al

in Geothermics (2019), 77

In the context of aquifer thermal energy storage, we conducted a hydrogeophysical experiment emulating the functioning of a groundwater heat pump for heat storage into an aquifer. This experiment allowed ... [more ▼]

In the context of aquifer thermal energy storage, we conducted a hydrogeophysical experiment emulating the functioning of a groundwater heat pump for heat storage into an aquifer. This experiment allowed the assessment of surface electrical resistivity tomography (ERT) ability to monitor the 3D development over time of the aquifer thermally affected zone. The resistivity images were converted into temperature. The images reliability was evaluated using synthetic tests and the temperature estimates were compared to direct temperature measurements. Results showed the capacity of surface ERT to characterize the thermal plume and to reveal the spatial variability of the aquifer hydraulic properties, not captured from borehole measurements. A simulation of the experiment was also performed using a groundwater flow and heat transport model calibrated with a larger set-up. Comparisons of the simulation with measurements highlighted the presence of smaller heterogeneities that strongly influenced the groundwater flow and heat transport. [less ▲]

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See detailTowards a subsurface predictive-model environment to simulate aquifer thermal energy storage for demand-side management applications
Robert, Tanguy ULiege; Hermans, Thomas ULiege; Lesparre, Nolwenn et al

in Proceedings of SSB 2018, 10th International Conference on System Simulation in Buildings (2018, December 12)

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See detailUpdating structural uncertainty through dimension reduction of geophysical data
Nguyen, Frédéric ULiege; Lopez Alvis, Jorge ULiege; Hermans, Thomas

Conference (2018, December)

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See detailUse and challenges of electrical resistivity tomography to study processes in agro-ecosystems
Garré, Sarah ULiege; Javaux, Mathie; Manhaeghe, Thibault et al

Scientific conference (2018, September 25)

Sustainable management of agro-ecosystems requires a thorough understanding of the interaction between physical, chemical and biological processes at play. In addition, processes at pore scale are linked ... [more ▼]

Sustainable management of agro-ecosystems requires a thorough understanding of the interaction between physical, chemical and biological processes at play. In addition, processes at pore scale are linked to field scale phenomena, but this connection is often poorly understood. Electrical resistivity tomography (ERT) is increasingly used in the context of agriculture since the measured resistivity distribution can be linked to soil moisture, soil structural characteristics or pore water salinity. Due to its minimally invasive character, its spatial coverage and its monitoring abilities, ERT can be used to study field heterogeneity and competition between plants, quantify water fluxes throughout a growing season or distinguish preferential flow pathways in soils. Is resolution is well-below classical soil imaging techniques such as X-ray CT or MRI, but its spatial coverage much larger. This highlights the potential of ERT to link our knowledge obtained from pore scale data to field scale processes. Nevertheless, a lot of challenges still remain. A Tikhonov-type regularization approach is often used to solve the ill-posed, inverse problem linked to ERT, resulting in a smoothed resistivity distribution. However, in reality strong contrasts can exist due to e.g. compacted soil layers due to ploughing, water infiltration fronts, etc. and in that case other operators have been proposed to regularize the inversion. Taking into account spatial heterogeneity of petrophysical characteristics and providing a realistic uncertainty estimation are additional challenges, which can be addressed using stochastic approaches. Monitoring data provides further elements to constrain the inverse problem: data can be replaced by data difference and regularization may incorporate the temporal dimension for instance. However, such constraints require their compatibility with the studied temporal process, which is not always straightforward. Several alternative strategies are being developed, such as coupled hydrogeophysical inversion, or stochastic approaches using a prior falsification/validation method following a Popper-Bayes philosophy. In this talk, we will address some of these challenges and give some recent applications in the field of agro-geophysics. [less ▲]

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See detailImproving the understanding and conceptualization of regional groundwater flow patterns in fractured rocks with self-potential data
Robert, Tanguy ULiege; Molron, Justine; De Schepper, Guillaume et al

Conference (2018, September 11)

This collection of geophysical case studies demonstrates the positive impact of self-potential (SP) data on the understanding and conceptualization of groundwater flow patterns in fractured rocks of ... [more ▼]

This collection of geophysical case studies demonstrates the positive impact of self-potential (SP) data on the understanding and conceptualization of groundwater flow patterns in fractured rocks of Wallonia, Belgium. We first identify the SP signature of a preferential groundwater flow path in Cambrian fractured quartzite as a negative anomaly of 15 mV in the raw SP signal. This result allowed the delineation of hydraulically-active fractured rocks as well as the refinement of groundwater flow conceptual models with zones of higher hydraulic conductivities. We then demonstrate that the apparent electrokinetic coupling coefficient C_w that links SP and hydraulic gradients is often positive for Carboniferous limestone (although its exact value is site dependent) while it is often presented as negative in the literature (mostly focusing on silicates). With this result in mind, we illustrate several methodologies to quickly and almost inexpensively investigate Carboniferous limestone valleys/synclines qualitatively and quantitatively such as SP mapping and profiling. These allowed us to demonstrate the presence of two perpendicular hydraulic gradients in such typical synclines. In a monitoring mode, SP profiling can follow the increase (and then the decrease) of the hydraulic gradient during high (low) groundwater level periods, offering an interesting potentiality to densely monitor complex hydrogeological systems. Combined with classical piezometric heads measured in wells, SP profiling (or mapping) can become quantitative once C_w is correctly estimated, allowing either the reconstruction of a smooth piezometric profile (or map) or the amplitude of hydraulic gradients along different directions, as proven by our data which were able to image the drawdown in an aquifer submitted to dewatering in a limestone quarry. This work also shows promising perspectives by considering SP measurements as hard data for groundwater flow modeling. [less ▲]

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See detailCan short-term hydrogeological experiments predict the long-term behavior of subsurface reservoirs? An example from shallow geothermy
Hermans, Thomas; Nguyen, Frédéric ULiege; Lesparre, Nolwenn et al

Conference (2018, September 10)

Aquifer thermal energy storage (ATES) systems can potentially store and recover thermal energy seasonally. In practice, the increase of energy efficiency is often lower than expected from simulations due ... [more ▼]

Aquifer thermal energy storage (ATES) systems can potentially store and recover thermal energy seasonally. In practice, the increase of energy efficiency is often lower than expected from simulations due to spatial heterogeneity or non-favorable conditions. In many cases, the lack of available data leads the modeler to consider homogeneous layered conceptual models to forecast the long-term behavior of geothermal systems. Ignoring spatial heterogeneity bears the risk of misleading decisions based on the prediction of those models. The proper design of ATES systems should always consider the uncertainty about subsurface parameters. In practice, classical hydrogeological tests including geophysical surveys are performed to gain knowledge on subsurface parameters. One question remains: are these tests sufficiently informative to predict with realistic uncertainty the long-term behavior of reservoirs? We investigate how short-term heat tracing and storage experiments can predict the long-term behavior of ATES system. We combine field experiments with a probabilistic modeling approach called Bayesian Evidential Learning (BEL) to assess the information content of our data set(s). BEL relies on a set of surrogate models of the subsurface representing prior uncertainty. It uses a global sensitivity analysis to identify sensitive parameters for long-term heat storage and short-term experimental data and can validate the use of short-term experiments to generate informative data sets. In addition, this approach allows a direct estimate of the uncertainty range of the prediction from the observed experimental data, without explicit inverse modeling. The methodology therefore allows us to tests experimental hypothesis that can be further validated with field data. Here, we use the approach to compare the information content of different data acquisition schemes: tracing vs. storage/push-pull experiments, standard vs. multi-cycle experiments, very-short vs. long experiments, single-hole vs. multi-borehole and geophysical measurements. Finally, we illustrate and validate the proposed framework with field data. [less ▲]

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See detailExperimental assessment and prediction of short-term aquifer thermal energy storage for energy demand-side management applications
Robert, Tanguy ULiege; Hermans, Thomas; Lesparre, Nolwenn et al

Conference (2018, September 10)

Groundwater heat pump (GWHP) systems are now widely used for space heating and cooling and domestic hot water production. On one hand, GWHP systems are considered as a renewable energy and on the other ... [more ▼]

Groundwater heat pump (GWHP) systems are now widely used for space heating and cooling and domestic hot water production. On one hand, GWHP systems are considered as a renewable energy and on the other hand, their increasing use impacts electrical grid balancing. Since the coupling of electrically-driven heating, ventilation, and air-conditioning (HVAC) systems with thermal energy storage (TES) is seen as a promising tool for demand-side management (DSM) in the low-voltage grid, experimental validation of aquifer thermal energy storage (ATES) capabilities at demand side management (DSM) frequencies (real time, intraday, interday, and interseasonal) is needed for the sector to adopt it. We demonstrated here that hourly to daily ATES can be efficient in terms of energy recovery rate and exergy whereas weekly to seasonal ATES bears the risk of lower recovery rates and presents almost always low exergy. Moreover, energy recovery rates are improved with the increasing use of storage and recovery cycles. To formulate this demonstration, we emulated the operating conditions of GWHP systems with ATES in several well-characterized experimental pilot sites in Wallonia. The standardized experiment we used to estimate the different hydrodynamic parameters and energy recovery at the pilot sites was based on push/pull tests with the injection of heated water, its storage for different DSM periods, and finally its recovery. Fluxes were either measured in-situ by means of the finite volume point dilution method or estimated with the local hydraulic conductivity and gradient. We report here a direct relationship between the energy recovery rate and natural groundwater fluxes. [less ▲]

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See detailA new method for characterizing the complex electrical properties of root segments
Ehosioke, Solomon ULiege; Garré, Sarah ULiege; Kremer, Thomas ULiege et al

Conference (2018, July 09)

The application of geophysical methods to root investigation is increasing in recent years because of the limitations associated with the use of traditional methods (root excavation, monoliths ... [more ▼]

The application of geophysical methods to root investigation is increasing in recent years because of the limitations associated with the use of traditional methods (root excavation, monoliths, minirhizotron etc.). Point sampling is only partially satisfactory as a result of the spatial variability and dynamics of the roots zone. Geophysical methods address these limitations by offering high resolution and non-invasive approaches to root investigation due to their ability to infer properties and structures of the subsurface as well as the flow and transport processes occurring in the shallow subsurface, at various spatial scales. Recent studies (Weigand and Kemna 2017; Mary et al. 2017) have reported a low frequency polarization of root systems and have shown that SIP/EIT holds a promising future for root system characterization. Despite these significant improvements, there is still a knowledge gap regarding the electrical response of fine roots at the segment scale which is essential to enable us to account for the effect of roots in the estimation of soil moisture content of rooted soils or to exploit electromagnetic methods to characterize certain root system characteristics. We hereby propose a method for the characterization of electrical properties of single root segments of various plants resulting from an externally applied electrical field. A sample holder was designed and tested on ideal resistors and root segments, and was found to be suitable for assessing the electrical properties of root segments of 1-5 cm length and 2 mm diameter at a frequency range of 1Hz – 45 KHz. The system was then used to obtain complex electrical responses of the root segments of the target plants in the laboratory. This enabled us to study different root properties and their effects on the root electrical signals. Our results suggest that fine roots could be differentiated from soils because they show lower resistivity and polarizes more than soils. [less ▲]

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See detailCan short-term hydrogeological experiments predict the long-term behavior of subsurface reservoirs? An example from shallow geothermy
Hermans, Thomas ULiege; Nguyen, Frédéric ULiege; Robert, Tanguy ULiege et al

Conference (2018, June 01)

Aquifer thermal energy storage (ATES) systems can potentially store and recover thermal energy seasonally. In practice, the increase of energy efficiency is often lower than expected from simulations due ... [more ▼]

Aquifer thermal energy storage (ATES) systems can potentially store and recover thermal energy seasonally. In practice, the increase of energy efficiency is often lower than expected from simulations due to spatial heterogeneity or non-favorable conditions. In many cases, the lack of available data leads the modeler to consider homogeneous layered conceptual models to forecast the long-term behavior of geothermal systems. Ignoring spatial heterogeneity bears the risk of misleading decisions based on the prediction of those models. The proper design of ATES systems should always consider the uncertainty about subsurface parameters. In practice, classical hydrogeological tests including geophysical surveys are performed to gain knowledge on subsurface parameters. One question remains: are these tests sufficiently informative to predict with realistic uncertainty the long-term behavior of reservoirs? We investigate how short-term heat tracing and storage experiments can predict the long-term behavior of ATES system. We combine field experiments with a probabilistic modeling approach called Bayesian Evidential Learning (BEL) to assess the information content of our data set(s). BEL relies on a set of surrogate models of the subsurface representing prior uncertainty. It uses a global sensitivity analysis to identify sensitive parameters for long-term heat storage and short-term experimental data and can validate the use of short-term experiments to generate informative data sets. In addition, this approach allows a direct estimate of the uncertainty range of the prediction from the observed experimental data, without explicit inverse modeling. The methodology therefore allows us to tests experimental hypothesis that can be further validated with field data. Here, we use the approach to compare the information content of different data acquisition schemes: tracing vs. storage/push-pull experiments, standard vs. multi-cycle experiments, very-short vs. long experiments, single-hole vs. multi-borehole and geophysical measurements. Finally, we illustrate and validate the proposed framework with field data. [less ▲]

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See detailUpdating prior geologic uncertainty with GPR traveltime tomographic data
Lopez Alvis, Jorge ULiege; Nguyen, Frédéric ULiege; Hermans, Thomas

Conference (2018, June)

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See detailCan short-term hydrogeological experiments predict the long-term behavior of subsurface reservoirs?
Hermans, Thomas; Nguyen, Frédéric ULiege; Robert, Tanguy ULiege et al

Conference (2018, April 12)

In theory, aquifer thermal energy storage (ATES) systems can recover in winter the heat stored in the aquifer during summer to increase the energy efficiency of the system. In practice, the energy ... [more ▼]

In theory, aquifer thermal energy storage (ATES) systems can recover in winter the heat stored in the aquifer during summer to increase the energy efficiency of the system. In practice, the energy efficiency is often lower than expected from simulations due to spatial heterogeneity of hydraulic properties or non-favorable hydrogeological conditions. In many cases, the lack of available data leads the modeler to consider homogeneous layered conceptual models to forecast the long-term behavior of geothermal systems. Ignoring spatial heterogeneity bears the risk of misleading decisions based on the prediction of those models. The proper design of ATES systems should always consider the uncertainty about subsurface parameters. In practice, in-situ tests such as push/pull tests, heat storage experiments, heat tracer tests or other specific tests can be performed to gain knowledge on subsurface parameters. One question remains: are these tests sufficiently informative to predict with realistic uncertainty the behavior of the reservoirs for long-term use? In this contribution, we investigate how we can predict the long-term behavior of ATES systems using short-term heat tracing and storage experiments. We combine field experiments with a probabilistic modeling approach called Bayesian Evidential Learning (BEL) to gain knowledge on the information content of our data sets. BEL relies on a set of surrogate models of the subsurface representing prior uncertainty. It uses a global sensitivity analysis to identify sensitive parameters for long-term heat storage and short-term experimental data and can validate the use of short-term heat tracing experiments to generate informative data sets. In addition, we show that the approach can be used to directly estimate the uncertainty range of the prediction from the observed experimental data, without explicit inverse modeling. The methodology therefore allows us to tests experimental hypothesis that can be further validated with field data. We use the approach to compare the information content of different data acquisition schemes: tracing vs. storage/push-pull experiments, standard vs. multi-cycle experiments, very-short vs. long experiments, single-hole vs. multi-borehole and geophysical measurements. Finally, we illustrate the proposed framework with field data. [less ▲]

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See detailUse and challenges of geophysics to study processes in agro-ecosystems (invited)
Garré, Sarah ULiege; Javaux, Mathieu; Dumont, Gaël ULiege et al

Conference (2018, April 05)

Electrical resistivity tomography (ERT) is increasingly used in the context of agriculture since the measured resistivity distribution can be linked to soil moisture, soil structure or pore water salinity ... [more ▼]

Electrical resistivity tomography (ERT) is increasingly used in the context of agriculture since the measured resistivity distribution can be linked to soil moisture, soil structure or pore water salinity. Due to its minimally invasive character, its spatial coverage and its monitoring abilities, ERT can be used to study field heterogeneity and competition between plants, quantify water fluxes throughout a growing season or distinguish preferential flow pathways in soils. Nevertheless, a lot of challenges still remain. From a mathematical point of view, the inverse problem linked to ERT is ill-posed. To solve it, the inverse problem is often regularized with a Tikhonov-type approach resulting in a smoothed resistivity distribution. However, in reality strong contrasts can exist due to e.g. compacted soil layers due to ploughing, water infiltration fronts, etc. and in that case other operators have been proposed to regularize the inversion. Taking into account spatial heterogeneity of petrophysical characteristics and providing a realistic uncertainty estimation are additional challenges, which can be addressed using stochastic approaches. Monitoring data provides further elements to constrain the inverse problem: data can be replaced by data difference and regularization may incorporate the temporal dimension for instance. However, such constraints require their compatibility with the studied temporal process, which is not always straightforward. Several alternative strategies are being developed, such as coupled hydrogeophysical inversion, or stochastic approaches using a prior falsification/validation method following a Popper-Bayes philosophy. In this presentation, we will illustrate the mentioned challenges and some recent developments in the context of agrogeophysics. [less ▲]

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See detailUncertainty quantification of medium‐term heat storage from short‐term geophysical experiments using bayesian evidential learning
Hermans, Thomas ULiege; Nguyen, Frédéric ULiege; Klepikova, Maria et al

in Water Resources Research (2018), 54

In theory, aquifer thermal energy storage (ATES) systems can recover in winter the heat stored in the aquifer during summer to increase the energy efficiency of the system. In practice, the energy ... [more ▼]

In theory, aquifer thermal energy storage (ATES) systems can recover in winter the heat stored in the aquifer during summer to increase the energy efficiency of the system. In practice, the energy efficiency is often lower than expected from simulations due to spatial heterogeneity of hydraulic properties or non‐favorable hydrogeological conditions. A proper design of ATES systems should therefore consider the uncertainty of the prediction related to those parameters. We use a novel framework called Bayesian Evidential Learning (BEL) to estimate the heat storage capacity of an alluvial aquifer using a heat tracing experiment. BEL is based on two main stages: pre‐ and postfield data acquisition. Before data acquisition, Monte Carlo simulations and global sensitivity analysis are used to assess the information content of the data to reduce the uncertainty of the prediction. After data acquisition, prior falsification and machine learning based on the same Monte Carlo are used to directly assess uncertainty on key prediction variables from observations. The result is a full quantification of the posterior distribution of the prediction conditioned to observed data, without any explicit full model inversion. We demonstrate the methodology in field conditions and validate the framework using independent measurements. [less ▲]

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See detailThe effect of initial water distribution and spatial resolution on the interpretation of ERT monitoring of water infiltration in a landfill cover
Dumont, Gael; Pilaswki, Tamara; Hermans, Thomas et al

E-print/Working paper (2018)

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See detailElectrical resistivity tomography and distributed temperature sensing monitoring to assess the efficiency of horizontal recirculation drains on retrofit bioreactor landfills
Dumont, Gaël ULiege; Robert, Tanguy ULiege; Nguyen, Frédéric ULiege

in Geophysics (2018), 83(2), 13-23

In bioreactor landfills, the recirculation of water can accelerate biodegradation and increase gas production. The dedicated infrastructure aims at increasing wastewater content over a wide area, with a ... [more ▼]

In bioreactor landfills, the recirculation of water can accelerate biodegradation and increase gas production. The dedicated infrastructure aims at increasing wastewater content over a wide area, with a long-lasting effect. To assess the efficiency of horizontal drains in bioreactor landfills, we use electrical resistivity tomography (ERT) and distributed temperature sensing (DTS) to monitor two injection experiments. The first monitoring experiment focuses on image resolution and takes advantage of a pseudo 3D ERT data set. This technique successfully highlights the waste horizontal anisotropy and the crucial role of existing gas wells, acting as vertical preferential flow paths. The observations are supported by borehole temperature logging. The second monitoring experiment focuses on temporal resolution and requires repeated 2D ERT measurements. The hourly acquisition frequency offers better insight on the water-flow dynamics, such as the flow direction and velocity and the water retention trough time. Temperature logging along the horizontal drain indicates that the injected water is distributed over the entire drain length. Altogether, the two recirculation experiments inform us on the suitability of large horizontal drains for water recirculation on bioreactor landfills. In conclusion, the two geophysical tools provide essential information to determine the most appropriate water-injection protocol in terms of frequency, volume, and flow rate. © 2018 Society of Exploration Geophysicists. [less ▲]

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See detailImproving the accuracy of 1D SNMR surveys using the multi-central-loop configuration
Kremer, Thomas ULiege; Michel, Hadrien; Muller-Petke, Mike et al

Poster (2018, March)

A multi-central loop configuration has been studied through forward and inverse modelling of synthetics and real data. This set-up takes advantage of the multichannel features of the NMR device and ... [more ▼]

A multi-central loop configuration has been studied through forward and inverse modelling of synthetics and real data. This set-up takes advantage of the multichannel features of the NMR device and consists of using several (2 to 3) additional receiver loops displayed concentrically with the main transmitter/receiver loop, which all record the NMR signal simultaneously within a single acquisition. If the loop diameters are chosen appropriately, the kernel sensitivity distributions for each receiver loop can show complementary features. Inverting simultaneously the data sets obtained through each different receiver loop can then enhance the accuracy of the final model. To do so, a 1D QT inversion scheme in the frequency domain dedicated to the inversion of multiple data sets is being used. One challenging feature is to adapt the regularization of the inverse process so as to handle correctly the noise originating from different data sets. The efficiency of this multi-central loop acquisition set-up and procedure is being assessed through the forward and inverse modelling of several scenarios implying varying aquifer characteristics. Finally a field case is being presented that was conducted on a low noise level site located in Germany, where conditions were favourable to the implementation and testing of circular multi-central loop configurations.We also introduce a new method for determining NMR parameters, named the prediction-focused-approach (PFA), that is based on statistical analysis of a large number of simple models. We observe, using synthetic examples, that the effciency of the method benefits from the use of the multi-central-loop configurations. [less ▲]

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See detailManaging past landfills for future site development: A review of the contribution of geophysical methods
Nguyen, Frédéric ULiege; Ghose, Ranajit; Isunza Manrique, Itzel et al

in Proceedings of the 4th International Symposium on Enhanced Landfill Mining (2018)

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See detailGeophysical Investigation of the Pb–Zn Deposit of Lontzen–Poppelsberg, Belgium
Evrard, Maxime ULiege; Dumont, Gael; Hermans, Thomas et al

in Minerals (2018), 8(6 233),

The drillhole information from the Lontzen–Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface ... [more ▼]

The drillhole information from the Lontzen–Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface. The localisation of the lodes makes them excellent targets for further exploration with geophysics. This deposit is classified as a Mississippi Valley Type (MVT) deposit. It consists mainly of Pb–Zn–Fe sulphides that display contrasting values in resistivity, chargeability, density, and magnetic susceptibility, with regards to the sedimentary host rocks. The dipole–dipole direct current (DC) resistivity and induce polarization (IP) profiles have been collected and inverted to successfully delineate the Pb–Zn mineralization and the geological structures. Short-spacing EM34 electromagnetic conductivity data were collected mainly on the top of Poppelsberg East lode and have revealed a conductive body matching with the geologically modelled mineralization. Gravity profiles have been carried out perpendicularly to the lode orientation; they show a strong structural anomaly. High resolution ground magnetic data were collected over the study area, but they showed no anomaly over the ore deposits. The geophysical inversion results are complementary to the model based on drill information, and allow us to refine the delineation of the mineralization. The identification of the geophysical signatures of this deposit permits targeting new possible mineralization in the area. [less ▲]

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See detailDevelopment of new processing schemes for the removal of harmonic noise from SNMR acquisition signals in complex, multi-frequency noise environments.
Kremer, Thomas ULiege; Larsen, Jakob Juul; Nguyen, Frédéric ULiege

Conference (2018)

In SNMR surveys, the electromagnetic (EM) perturbations originating from powerlines infrastructures is often the main EM noise source that contaminates the SNMR acquisition signal. The removal of these ... [more ▼]

In SNMR surveys, the electromagnetic (EM) perturbations originating from powerlines infrastructures is often the main EM noise source that contaminates the SNMR acquisition signal. The removal of these harmonic signals is therefore critical to extract the data and carry out the study. To date, some techniques exist that give satisfying results in many cases, provided that the assumptions of a harmonic noise based on a single and constant fundamental frequency are valid. However, when these conditions are not met and the characteristics of the harmonic noise become more complex, these methods will fail to remove it correctly. In this paper, we show how such complex conditions will affect the frequency spectra of the SNMR acquisition signal. Then, we address the issue of a harmonic noise composition based on two different fundamental frequencies, by solving the non-linear optimization problem simultaneously for the two frequency values, using a 2D grid-search. Then we investigate the case where the fundamental frequency of the harmonic signal is not constant but variates within the signal duration. We expose a new approach based on the Nyman and Gaiser estimator, which, associated with a signal bootstrapping strategy, efficiently handles this situation. [less ▲]

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