Olivier Kaufmann

3D geological modelling from boreholes, cross-sections and geological maps, application over former natural gas storages in coal mines

In a wide range of applications involving geological modelling, geological data available at low cost usually consist of documents such as cross-sections or geological maps and punctual data like borehole logs or outcrop descriptions. In order to build accurate 3D geological models based on this information, it is necessary to develop a methodology that takes into account the variety of available data. Such models, of the geometry of geological bodies, should also be easy to edit and update to integrate new data. This kind of model should produce a consistent representation of subsurface geology that may be a support for modelling other subsoil characteristics such as hydrogeologic or geothermic properties of the geological bodies. This paper presents a methodology developed to process geological information in this context. The aims of this methodology are comprehensive data description, effective data validation and easier model updates. Thus, special attention has been given to data structures and processing flows. The adopted methodology is implemented on a system architecture formed by a geographic information system, a geomodeler and a database communicating by file transfers. An application of this methodology, to build a 3D geological model of the subsoil over former coalmines used to store natural gas, is then presented. This model integrates the geological information available and is representative of the geological context. It is a support to the environmental follow-up needed after the end of gas-storage operations.

Geohazard map of cover-collapse sinkholes in the 'Tournaisis' area, southern Belgium

Abstract This paper reports the methodology developed to draw up a geohazard map of cover-collapse sinkhole occurrences in the ‘Tournaisis’ area. In this area, Carboniferous limestones are overlain by a Mesocenozoic cover, mainly consisting of marls, sand and clay. The thickness of this cover ranges from a few meters to more than 100 m. The surficial morphology of the area does not show any karstic evidence except for the occurrence of these collapses. From a paleogeographical point of view, a developed quaternary karst is not conceivable in the area. Recent works suggested that the collapses are set off from reactivated paleokarsts. The paleokarsts studied in the area proved to be the result of a particular weathering of the limestone. The organization of these paleokarsts seems very low and mainly guided by the limestone fracturing. As for most induced sinkholes, the reactivation of these paleokarsts is linked to the lowering of piezometric heads. In most of the area, a thick cover and intensive land use mask potential surface hints of the buried paleokarsts and of the fracturing of the bedrock. Aerial photographs and remote sensing techniques have therefore shown little results in delineating collapse hazard zones up to now. The study of the surficial morphology is also of little help. In order to draw up the geohazard map in such a difficult context, hydrogeological data and geological mapping information could only be used. These informations are based on a limited number of boreholes and piezometers and are thus, only valid on a regional scale. Records of former collapses were also available. These records were of great interest since sinkhole distribution is obviously clustered in the area. Bedrock roof and cover formation floor altitudes were digitized and adapted to produce digital thematic maps. Piezometric heads were imported from a calibrated groundwater model of the aquifer. These data and a digital elevation model of the area were integrated into a geographical information system (GIS) to produce a coherent 3-D description of the area on a regional scale. Parameters such as the dewatering of the limestone and the thickness of the cover formation where sinkholes occurred were then estimated. Density of former collapses was also computed. This showed that zones of high sinkhole occurrence coincide with zones of heavy lowering of piezometric heads. Combining the density of former collapses with the dewatering of the limestone enabled us to delineate zones of low, moderate and high collapse hazard.

Cover-collapse sinkholes in the “Tournaisis” area, southern Belgium

Abstract In the Tournaisis area (southern Belgium), cover-collapse sinkholes have been reported since the beginning of this century. In this area, the relatively flat lying carboniferous limestone is overlain by an unconformable mesocenozoic cover. Recent research has shown that these sinkholes developed from reactivated paleokarsts. The intensive pumping for domestic and industrial water supply combined with the dewatering due to deep limestone quarries operations resulted in the lowering of groundwater levels. This triggered the reactivation of paleokarstic systems and thus the occurrence of cover-collapse sinkholes. These sudden and unexpected collapses have already had major impacts in the region. In 1977, about thirty sinkholes opened up along the Escaut River downstream from the city of Tournai. As a consequence, the groundwater was polluted by an extensive loss of river water into the Carboniferous Aquifer. More recently, a house and industrial facilities were severely damaged by such cover-collapse sinkholes. In this context, a geoharzard map would be a valuable tool as far as regional development is concerned. This points at the importance to develop methods to locate and characterize hidden paleokarsts. A study site has been chosen in an area where sinkholes have been opening up for about ten years. A geophysical survey of the site which included electrical resistance mapping of the area has shown “weakness” lineaments. These lineaments are parallel to a main fault affecting the region.

Improving the delineation of hydrocarbon-impacted soils and water through induced polarization (IP) tomographies: a field study at an industrial waste land.

Without a good estimation of samples representativeness, the delineation of the contaminated plume extent and the evaluation of volumes of hydrocarbon-impacted soils may remain difficult. To contribute to this question, a time domain induced polarization (IP) field experiment was conducted on an industrial waste land. Boreholes were drilled to specify the local geological context. Cross-hole seismic tomographies were performed to extend borehole logs and to draw an interpreted geological cross-section. Soil samples taken during drillings were analysed in laboratory. A preliminary survey was conducted to locate the IP profile. The polarization signatures linked to the presence of clayey sediments were filtered out from the data set. Chargeability and resistivity depth soundings were computed and compared to mean concentrations of total organic products to overcome the data support issue between the geophysical models and the spot samples of soils. A logarithmic relation between chargeabilities and smoothed hydrocarbon concentrations in soils was found. Taking into account contaminants concentration thresholds defined in local codes and regulations allows defining chargeability classes to delineate hotspots on this site. This showed that IP tomography can be an accurate screening methodology. A statistical methodology is proposed to assess the efficiency of the investigation strategy.

Modelling the Source of Blasting for the Numerical Simulation of Blast-Induced Ground Vibrations: A Review

The mining and construction industries have long been faced with considerable attention and criticism in regard to the effects of blasting. The generation of ground vibrations is one of the most significant factors associated with blasting and is becoming increasingly important as mining sites are now regularly located near urban areas. This is of concern to not only the operators of the mine but also residents. Mining sites are subjected to an inevitable compromise: a production blast is designed to fragment the utmost amount of rock possible; however, any increase in the blast can generate ground vibrations which can propagate great distances and cause structural damage or discomfort to residents in surrounding urban areas. To accurately predict the propagation of ground vibrations near these sensitive areas, the blasting process and surrounding environment must be characterised and understood. As an initial step, an accurate model of the source of blast-induced vibrations is required. This paper presents a comprehensive review of the approaches to model the blasting source in order to critically evaluate developments in the field. An overview of the blasting process and description of the various factors which influence the blast performance and subsequent ground vibrations are also presented. Several approaches to analytically model explosives are discussed. Ground vibration prediction methods focused on seed waveform and charge weight scaling techniques are presented. Finally, numerical simulations of the blasting source are discussed, including methods to estimate blasthole wall pressure time-history, and hydrodynamic codes.

Reprint of 3D geological modelling from boreholes, cross-sections and geological maps, application over former natural gas storages in coal mines [Comput. Geosci. 34 (2008) 278-290]

In a wide range of applications involving geological modelling, geological data available at low cost usually consist of documents such as cross-sections or geological maps and punctual data like borehole logs or outcrop descriptions. In order to build accurate 3D geological models based on this information, it is necessary to develop a methodology that takes into account the variety of available data. Such models, of the geometry of geological bodies, should also be easy to edit and update to integrate new data. This kind of model should produce a consistent representation of subsurface geology that may be a support for modelling other subsoil characteristics such as hydrogeologic or geothermic properties of the geological bodies. This paper presents a methodology developed to process geological information in this context. The aims of this methodology are comprehensive data description, effective data validation and easier model updates. Thus, special attention has been given to data structures and processing flows. The adopted methodology is implemented on a system architecture formed by a geographic information system, a geomodeler and a database communicating by file transfers. An application of this methodology, to build a 3D geological model of the subsoil over former coalmines used to store natural gas, is then presented. This model integrates the geological information available and is representative of the geological context. It is a support to the environmental follow-up needed after the end of gas-storage operations.

A 3D Resistivity Tomography Study of a LNAPL Plume Near a Gas Station at Brugelette (Belgium)

A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site located near a gas station where a tank had leaked its content. In this area, a dolomitic bedrock is overlain by 5to9m of clayey sands. In order to delineate the plume and measure piezometric heads and depths to bedrock, ten boreholes were drilled and four cone penetration tests were conducted. Soil and water samples were collected and analyzed. Although these samplings highlight the presence of hydrocarbons in some boreholes, the plume is poorly outlined due to the limited number of drillings. To predict the response of hydrocarbon-contaminated areas in this specific context, a laboratory study was undertaken. It shows that an increase in resistivities of about 40% should be found in highly polluted areas. To assess the contribution of geo-electrical investigations in delineating hydrocarbon-contaminated areas, a 3D cross-diagonal resistivity survey was performed using a roll-along technique. The electrical d...

A New Method to Quantify Carbonate Rock Weathering

The structure and composition of carbonate rocks are modified greatly when they are subjected to phenomena that lead to their weathering. These processes result in the production of residual alterite whose petrophysical, mechanical, and hydrological properties differ completely from those of the unweathered rock. From a geotechnical perspective, it is important that such changes are fully understood as they affect reservoir behavior and rock mass stability. This paper presents a quantitative method of calculating a weathering index for carbonate rock samples based on a range of petrophysical models. In total, four models are proposed, each of which incorporates one or more of the processes involved in carbonate rock weathering (calcite dissolution, gravitational compaction, and the incorporation of inputs). The selected weathering processes are defined for each model along with theoretical laws that describe the development of the rock properties. Based on these laws, common properties such as rock density, porosity, and calcite carbonate content are estimated from the specific carbonate rock weathering index of the model. The propagation of measurement uncertainties through the calculations has been computed for each model in order to estimate their effects on the calculated weathering index. A new methodology is then proposed to determine the weathering index for carbonate rock samples taken from across a weathered feature and to constrain the most probable weathering scenario. This protocol is applied to a field dataset to illustrate how these petrophysical models can be used to quantify the weathering and to better understand the underlying weathering processes.

The Role of Geophysical Investigations in Physical Planning over Covered Karstic Areas in Southern Belgium

Since 1999, maps (at 1:10,000 scale) of zones of karstic constraints have been established in the Walloon Region of Belgium (the southern part). These maps sort areas in terms of karstic hazards (negligible, low, moderate and high). They were drawn based on geological settings, field observations, background knowledge and geostatistical forecasts. Nowadays, these maps are used as background documents in physical planning over covered karstic areas. Indeed, the local government now requires to conduct geophysical and/or geotechnical investigations previous to building construction over moderate and high karstic hazard areas. In this context, we conducted several 2D, quasi-3D and 3D resistivity tomography surveys using a total number of electrodes ranging from 100 to 1000. 3D resistivities were measured using a cross-diagonal survey with a combination of dipole-dipole, pole-dipole and equatorial dipole arrays. The inversions were computed within Res2DInv and Res3DInv. The results of these investigations allowed pointing out resistivity anomalies interpreted as effects of karstic features. When possible, boreholes are then drilled over and away from the detected anomalies to confront geophysical results. Finally, geophysical interpretations are used to adapt construction projects.

Correlation between Inverted Chargeabilities and Organic Compounds Concentrations in Soils – A Field Experiment

Resistivity and induced polarization tomographies were performed along profiles on an industrial waste land to detect and delineate hydrocarbon-impacted areas. To confront geophysical results and to conduct cross-hole measurements, 4 boreholes were drilled. Soil samples were taken and analysed. Concentrations of organic compounds in soils were then compared to inversed chargeabilities obtained from surface measurements. This comparison shows that chargeabilities are linked to contaminant concentrations in soil samples except from areas where clays were found. A filtering technique based on normalized chargeability values was implemented and validated in order to discriminate chargeabilities linked to the presence of clayey sediments and contaminant concentrations in soil samples.