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... The HydroGeophysics Group, Department of Earth Sciences, University of Aarhus ... With the resolution capabilities and the equivalence problem of resistivity data in mind it is important to know not only the... more
... The HydroGeophysics Group, Department of Earth Sciences, University of Aarhus ... With the resolution capabilities and the equivalence problem of resistivity data in mind it is important to know not only the geological/hydrogeophysical model but also the probability of the model. ...
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Tunisie, entre Mahdia et La Chebba, a proximite de Ksour-Essaf| c'est l'antique Sullecthum. Les AA. renvoient a la presentation exhaustive qu'ils publieront dans le volume " Sullecthum I ", Cahiers des Etudes... more
Tunisie, entre Mahdia et La Chebba, a proximite de Ksour-Essaf| c'est l'antique Sullecthum. Les AA. renvoient a la presentation exhaustive qu'ils publieront dans le volume " Sullecthum I ", Cahiers des Etudes Anciennes, fasc. XXII, a paraitre prochainement. Ces catacombes chretiennes sont exceptionnellement bien conservees| l'article ne donne pas encore de donnees chronologiques
Research Interests: Humanities and Art
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Processing of potential field data is commonly done by spectral methods because of their low computational complexity. However, we have studied some geostatistical methods to process the potential field data, and we find the advantages of... more
Processing of potential field data is commonly done by spectral methods because of their low computational complexity. However, we have studied some geostatistical methods to process the potential field data, and we find the advantages of using these spatial methods. First, we investigate transformation of data by kriging using a gravimetric model of covariance, we compare this approach with the spectral method, and we find its advantage when the data were sparse and not on a regular grid using a synthetic example as well as a field data example. Then, we use factorial kriging for noise reduction and separation of the regional and residual components. This method does not have some of the practical limitations that the spectral-based methods encounter. Finally, we determine the flexibility of interpolation using nonstationary covariances.
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In geophysical inverse problems, an a priori structured mesh is often used for inversion and mesh refinement is applied if needed by the user after observation of inversion results. We have developed a new intelligent self-adaptive... more
In geophysical inverse problems, an a priori structured mesh is often used for inversion and mesh refinement is applied if needed by the user after observation of inversion results. We have developed a new intelligent self-adaptive unstructured finite-element meshing technique for electrical resistivity tomography inverse problems. This new approach uses Harris corner-and-edge detectors that are based on the local autocorrelation function of 2D distribution of pixels. This meshing technique optimizes the size of the inverse problem by refining areas where variations in the physical property structure are sensed to be important. The meshing technique also generates a more appropriate and optimum mesh for the inverse problem that is dependent on the problem itself. Tests on modeled data have demonstrated that the proposed intelligent meshing technique can reduce data misfit, produce a better reconstruction of the true physical properties, and minimize the size of the inverse problem. The synthetic model consists of a conductive dike in a resistive medium. By applying the proposed intelligent meshing technique, the inverse model of the dike is very similar to the inverse model produced using fine meshes, and it is also better reconstructed than the inverse model produced using conventional meshes. We have also applied the intelligent meshing technique to survey data collected for groundwater-saltwater mapping and characterizing the subsurface conductive structure with topography included. Our results indicate that the new meshing technique can produce solutions that are comparable with standard meshing and fine meshing techniques, while optimizing the size of the inverse problem.
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Field distortions caused by topography hamper the interpretation of magnetotelluric (MT) data. Topographic features that can be simulated by two‐dimensional models seriously affect the H-polarization results. A technique to reduce those... more
Field distortions caused by topography hamper the interpretation of magnetotelluric (MT) data. Topographic features that can be simulated by two‐dimensional models seriously affect the H-polarization results. A technique to reduce those effects in MT data uses a finite‐element program to compute correction coefficients. After correction, the resulting data can be interpreted as if they were obtained over a flat surface and depended only on the subsurface structure. The technique is applied to four examples representative of MT survey targets in high‐relief terrain. Results indicate that terrain correction removes the misleading topographic anomalies and improves the quality of subsurface interpretation in regions where the surface relief is two‐dimensional. The correction technique yields some geometrical distortion of the original subsurface structure, but the distortion is usually of small importance. In practice, telluric dipoles of realistic length do not smooth out topographic effects having wavelengths longer than the telluric dipole. A practical procedure derived from the proposed technique allows terrain correction when the relief is approximately two‐dimensional.
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In the spring of 1996, a direct current (dc) resistivity and induced polarization (IP) borehole survey was carried out at the Casa Berardi gold mine in northwestern Quebec to study the spatial extent of the economic disseminated zone of... more
In the spring of 1996, a direct current (dc) resistivity and induced polarization (IP) borehole survey was carried out at the Casa Berardi gold mine in northwestern Quebec to study the spatial extent of the economic disseminated zone of an auriferous quartz vein type orebody. Crosshole pole‐pole and single‐hole pole‐dipole configurations were used to delineate the geometry of the body associated with the Casa Berardi fault system. Since the spatial data sampling was insufficient for 3D inversion, the interpretation has been done using 3D dc and IP forward modeling. Model changes were applied iteratively to match synthetic with field data. Sensitivities provide information on how to alter the models efficiently. Furthermore, they indicate the significant regions of the model, giving evidence on where the model is meaningful. A model study using a simple prismatic block structure is shown to enhance understanding of the physical response associated with the two types of borehole survey. The algorithms used for the interpretation offer a so‐called grid‐independent electrode positioning technique, which is a helpful modus operandi to significantly facilitate the simulation process. The result is a resistivity and chargeability model that produces the observed physical response and incorporates all known geological a priori information. Particularly, the IP response carries detailed information on the appearance of the orebody, whereas the potential response reflects the resistivity contrast between metavolcanic and metasedimentary rocks at the Casa Berardi fault.
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Core Ideas 3D time‐lapse ERT is used to monitor water infiltration for mining environmental issues. Geoelectrical images provide information where no hydrogeological data is available. Water resistivity must be taken into account to... more
Core Ideas 3D time‐lapse ERT is used to monitor water infiltration for mining environmental issues. Geoelectrical images provide information where no hydrogeological data is available. Water resistivity must be taken into account to understand bulk resistivity variations. Electrical resistivity of water is used as a tracer to reconstruct water infiltration. Infiltration model integrating both hydrogeological and geophysical data is proposed. Open‐pit mines often generate large quantities of waste rocks that are usually stored in waste rock piles (WRPs). When the waste rocks contain reactive minerals (mainly sulfides), water and air circulation can lead to the generation of contaminated drainage. An experimental WRP was built at the Lac Tio mine (Canada) to validate a new disposal method that aims to limit water infiltration into reactive waste rocks. More specifically, a flow control layer was placed on top of the pile, which represents a typical bench level, to divert water toward ...
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Applications of the spectral induced polarization (SIP) method to mineral exploration are limited by our knowledge of the relationships among rock texture, mineral composition, and electrical properties. Laboratory SIP responses were... more
Applications of the spectral induced polarization (SIP) method to mineral exploration are limited by our knowledge of the relationships among rock texture, mineral composition, and electrical properties. Laboratory SIP responses were measured on rock samples from the Canadian Malartic gold deposit. Field SIP responses were also measured at the outcrop scale, along a profile that intersects a well-studied mineralized zone. The mineralogy and the texture of sedimentary rocks from this deposit were quantitatively determined with mineral liberation analysis. A systematic decrease (Pearson [Formula: see text]) in total chargeability with increasing fraction of the sulfide mineral interfaces associated with feldspar minerals (namely, K-feldspar and albite) was observed. On the other hand, total chargeability increased with the fraction of sulfide mineral interfaces associated with carbonates and micas (Pearson [Formula: see text]). At Canadian Malartic, proximal alteration in the minerali...
Research Interests: Geology, Geochemistry, Geophysics, Mineralogy, Pyrite, and 5 moreBiotite, Mineral, Feldspar, Pyrrhotite, and albite
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An analysis of geophysical and geological data from the Gallen test site, Quebec, Canada Richard S. Smith, Fugro, Li Zhen Cheng, UQAT, Michel Allard, Falconbridge, Michel Chouteau, Ecole Poly-technique, Pierre Keating, GSC, Jean Lemieux,... more
An analysis of geophysical and geological data from the Gallen test site, Quebec, Canada Richard S. Smith, Fugro, Li Zhen Cheng, UQAT, Michel Allard, Falconbridge, Michel Chouteau, Ecole Poly-technique, Pierre Keating, GSC, Jean Lemieux, Marc A. Vallee and David ...
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Fixed-wing airborne electromagnetic (FWAEM) systems are asymmetric so the response will vary when the line direction reverses (even though the targets are symmetric). This can complicate interpretation. In a recent survey, each traverse... more
Fixed-wing airborne electromagnetic (FWAEM) systems are asymmetric so the response will vary when the line direction reverses (even though the targets are symmetric). This can complicate interpretation. In a recent survey, each traverse line was flown in both line directions, allowing the data to be combined to give a “virtual symmetric array” (VSA) response. Images of the VSA data are easy to interpret, are free of flight direction artifacts, show good resolution, good trending, and improved signal to noise. Dip can also be inferred from these images. In cases when data cannot be collected in both line directions, an interpolated VSA image can be created using the reverse line direction data from adjacent lines.
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Core Ideas A new technique for estimating the saturated hydraulic conductivity is developed. It uses a combination of hydrogeophysical and numerical methods. Electrical resistivity tomography and the instantaneous profile method are used.... more
Core Ideas A new technique for estimating the saturated hydraulic conductivity is developed. It uses a combination of hydrogeophysical and numerical methods. Electrical resistivity tomography and the instantaneous profile method are used. The technique is validated using simple and complex numerical hydrogeological models. Many hydrogeological and geophysical tools have been developed to determine subsoil properties, but they are often limited by sparse datasets and by the portability of the method from one site to another and often underestimate the complexity of the medium. We present a saturated hydraulic conductivity (Ks) estimation scheme, named the KES method, based on hydrogeophysical and numerical methods. The targeted medium of investigation is an unsaturated and heterogeneous soil. Estimation of Ks is accomplished by estimating the position of the wetting front and the distribution and velocity of flow lines during an infiltration test. Using numerical modeling, Ks is determined by minimizing the velocity difference between the measured flow lines and the modeled flow lines. Surface and buried electrodes are used as part of the electrical resistivity survey in determining the position of the wetting front. An instantaneous profile method is used to determine the water retention curve of the medium. The KES method has been tested and validated using data produced from simple and more complex geological models from published case studies. We obtained good reconstruction of the saturated hydraulic conductivity. We have found that the estimated value of Ks in log scale has a mean error <2.5%. Error increases along the boundaries of different hydrofacies.
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We have investigated seepage in Les Cedres embankment dam (Valleyfield, Canada) by combining self-potential tomography (SPT), electrical resistance tomography (ERT), thermometry, electromagnetic (EM) conductivity and magnetic... more
We have investigated seepage in Les Cedres embankment dam (Valleyfield, Canada) by combining self-potential tomography (SPT), electrical resistance tomography (ERT), thermometry, electromagnetic (EM) conductivity and magnetic measurements. SPT consists of inverting self-potential data to retrieve the source current density distribution associated with water flow pathways (streaming current density) in embankment dams. The SPT inverse problem relies on the resistivity model of the dam that is obtained by 3-D ERT. Our 3-D SPT code is based on Occam’s inversion. The forward problem is solved using the finite-volume scheme. The investigated embankment dam is used to channel water from the Saint-Lawrence River to a hydroelectric plant. It separates in its upstream and downstream sides Les Cedres and St-Timothee reservoirs respectively. St-Timothee reservoir is emptied during the winter and filled during the summer. Temperature monitoring was done in a borehole installed in the middle of the survey zone. To build a better understanding of water flow through the dam, it is important to separate the part of the source current density caused by the electrokinetic effect from the other sources (principally electro-chemical). In order to achieve that, ERT, EM31, magnetic and thermometric measurements have been used in the interpretation. EM conductivity maps allowed identifying two linear anomalies caused by metal-shielded electrical cables. The magnetic survey shows an important anomaly zone that is probably related to a metallic object. Therefore, all measurements near these zones were discarded from inversion. SPT shows a few seepage sources on the upstream dam side at a depth between 4 and 5 m. Two of them are confirmed by geotechnical testing. The water flow through the dam appears complex. It is partly controlled by a permeable zone that is well identified in the resistivity model. In the vicinity of the vertical temperature profile the SPT shows that the water flows parallel to the dam orientation and not through the borehole used for thermometry. This is why there is no clear indication of water seepage in temperature measurements. Finally, all observable seepage outlets on the downstream side can be related to the SPT anomalies and are observed as conductive zones in the resistivity model.
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The flexibility of geostatistical inversions in geophysics is limited by the use of stationary covariances, which, implicitly and mostly for mathematical convenience, assumes statistical homogeneity of the studied field. For fields... more
The flexibility of geostatistical inversions in geophysics is limited by the use of stationary covariances, which, implicitly and mostly for mathematical convenience, assumes statistical homogeneity of the studied field. For fields showing sharp contrasts due, for example, to faults or folds, an approach based on the use of nonstationary covariances for cokriging inversion was developed. The approach was tested on two synthetic cases and one real data set. Inversion results based on the nonstationary covariance were compared to the results from the stationary covariance for two synthetic models. The nonstationary covariance better recovered the known synthetic models. With the real data set, the nonstationary assumption resulted in a better match with the known surface geology.
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The stochastic gravity inversion on unstructured meshes is presented. The unstructured meshes are used because they provide the flexibility required to closely approximate complicated geological structures. Sharp topographic relief and... more
The stochastic gravity inversion on unstructured meshes is presented. The unstructured meshes are used because they provide the flexibility required to closely approximate complicated geological structures. Sharp topographic relief and geological bodies of complex shapes are usually more accurately described by unstructured meshes rather than with regular grids. The forward method has a closed form solution and gives accurate results. A geostatistical method is applied to invert the gravity data.
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Vallee et al. (1992) remark on the sensitivity of airborne ratio measuring VLF instruments to platform attitude stability. The authors also remind the users of VLF total field amplitude data, as produced by instruments such as the Herz... more
Vallee et al. (1992) remark on the sensitivity of airborne ratio measuring VLF instruments to platform attitude stability. The authors also remind the users of VLF total field amplitude data, as produced by instruments such as the Herz TOTEM, of two problems associated with this type of data: spatial and temporal fluctuations of the VLF primary field. They recommend the use of a dense network of VLF monitoring stations and numerical modeling of field propagation to cope with these problems. These two recommendations are briefly discussed here and one alternative recommendation is made.
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Mining operations generate large amounts of wastes which are usually stored into large-scale storage facilities which pose major environmental concerns and must be properly monitored to manage the risk of catastrophic failures and also to... more
Mining operations generate large amounts of wastes which are usually stored into large-scale storage facilities which pose major environmental concerns and must be properly monitored to manage the risk of catastrophic failures and also to control the generation of contaminated mine drainage. In this context, non-invasive monitoring techniques such as time-lapse electrical resistivity tomography (TL-ERT) are promising since they provide large-scale subsurface information that complements surface observations (walkover, aerial photogrammetry or remote sensing) and traditional monitoring tools, which often sample a tiny proportion of the mining waste storage facilities. The purposes of this review are as follows: (i) to understand the current state of research on TL-ERT for various applications; (ii) to create a reference library for future research on TL-ERT and geoelectrical monitoring mining waste; and (iii) to identify promising areas of development and future research needs on thi...
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Seismic data can play an important role in mineral exploration, however processing and interpretation of seismic data is difficult due to complexity of hard-rock geology. On the other hand, the recovered model from potential field methods... more
Seismic data can play an important role in mineral exploration, however processing and interpretation of seismic data is difficult due to complexity of hard-rock geology. On the other hand, the recovered model from potential field methods is affected by inherent non uniqueness caused by the nature of the physics and by underdetermination of the problem. Joint inversion of seismic and potential field data can mitigate weakness of separate inversion of these methods.
Research Interests: Geology and Seismology
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Mineral exploration in Canada is increasingly focused on concealed and deeply buried targets, requiring more effective tools to detect large-scale ore-forming systems and to vector from their most distal margins to their high grade cores.... more
Mineral exploration in Canada is increasingly focused on concealed and deeply buried targets, requiring more effective tools to detect large-scale ore-forming systems and to vector from their most distal margins to their high grade cores. A new generation of ore system models is required to achieve this. The Mineral Exploration Footprints Research Network is a consortium of 70 faculty, research associates, and students from 20 Canadian universities working with 30 mining, mineral exploration, and mining service providers to develop new approaches to ore system modelling based on more effective integration and visualization of multi-parameter geologicalstructural-mineralogical-lithogeochemical-petrophysical-geophysical exploration data. The Network is developing the next generation ore system models and exploration strategies at three sites based on integrated data visualization using self-consistent 3D Common Earth Models and geostatistical/machine learning technologies. Thus far ov...
The northern flank of the Galine anticline of the Matagami camp has been well known for decades for hosting polymetallic VMS deposits (mostly copper and zinc). In such an already explored area, sophisticated exploration tools must be used... more
The northern flank of the Galine anticline of the Matagami camp has been well known for decades for hosting polymetallic VMS deposits (mostly copper and zinc). In such an already explored area, sophisticated exploration tools must be used in the hope of generating new information. By integrating geophysical, petrophysical, and geological data into a three-dimensional (3D) common earth model to constrain gravity and magnetic inversions, our objective was to validate and improve the geological interpretations of the area and to highlight geophysical anomalies unexplained in the current geological model. Both 3D magnetic and gravity data inversions confirm the surface geometry and the steep dip of the geological units. The inverted gravity models indicate that units observable at the surface of the northern flank extend subvertically to several kilometres in depth. The pyroxenitic phase of the Bell River Complex is most certainly dipping steeply to the south, contrary to some surface m...