Flemming Jørgensen

Nitrate reduction in geologically heterogeneous catchments--a framework for assessing the scale of predictive capability of hydrological models.

In order to fulfil the requirements of the EU Water Framework Directive nitrate load from agricultural areas to surface water in Denmark needs to be reduced by about 40%. The regulations imposed until now have been uniform, i.e. the same restrictions for all areas independent of the subsurface conditions. Studies have shown that on a national basis about 2/3 of the nitrate leaching from the root zone is reduced naturally, through denitrification, in the subsurface before reaching the streams. Therefore, it is more cost-effective to identify robust areas, where nitrate leaching through the root zone is reduced in the saturated zone before reaching the streams, and vulnerable areas, where no subsurface reduction takes place, and then only impose regulations/restrictions on the vulnerable areas. Distributed hydrological models can make predictions at grid scale, i.e. at much smaller scale than the entire catchment. However, as distributed models often do not include local scale hydrogeological heterogeneities, they are typically not able to make accurate predictions at scales smaller than they are calibrated. We present a framework for assessing nitrate reduction in the subsurface and for assessing at which spatial scales modelling tools have predictive capabilities. A new instrument has been developed for airborne geophysical measurements, Mini-SkyTEM, dedicated to identifying geological structures and heterogeneities with horizontal and lateral resolutions of 30-50 m and 2m, respectively, in the upper 30 m. The geological heterogeneity and uncertainty are further analysed by use of the geostatistical software TProGS by generating stochastic geological realisations that are soft conditioned against the geophysical data. Finally, the flow paths within the catchment are simulated by use of the MIKE SHE hydrological modelling system for each of the geological models generated by TProGS and the prediction uncertainty is characterised by the variance between the predictions of the different models.

Transition probability- based stochastic geological modeling using airborne geophysical data and borehole data

Geological heterogeneity is a very important factor to consider when developing geological models for hydrological purposes. Using statistically based stochastic geological simulations, the spatial heterogeneity in such models can be accounted for. However, various types of uncertainties are associated with both the geostatistical method and the observation data. In the present study, TProGS is used as the geostatistical modeling tool to simulate structural heterogeneity for glacial deposits in a head water catchment in Denmark. The focus is on how the observation data uncertainty can be incorporated in the stochastic simulation process. The study uses two types of observation data: borehole data and airborne geophysical data. It is commonly acknowledged that the density of the borehole data is usually too sparse to characterize the horizontal heterogeneity. The use of geophysical data gives an unprecedented opportunity to obtain high-resolution information and thus to identify geostatistical properties more accurately especially in the horizontal direction. However, since such data are not a direct measurement of the lithology, larger uncertainty of point estimates can be expected as compared to the use of borehole data. We have proposed a histogram probability matching method in order to link the information on resistivity to hydrofacies, while considering the data uncertainty at the same time. Transition probabilities and Markov Chain models are established using the transformed geophysical data. It is shown that such transformation is in fact practical; however, the cutoff value for dividing the resistivity data into facies is difficult to determine. The simulated geological realizations indicate significant differences of spatial structure depending on the type of conditioning data selected. It is to our knowledge the first time that grid-to-grid airborne geophysical data including the data uncertainty are used in conditional geostatistical simulations in TProGS. Therefore, it provides valuable insights regarding the advantages and challenges of using such comprehensive data.

A method for cognitive 3D geological voxel modelling of AEM data

Airborne electromagnetic (AEM) data have proven successful for the purpose of near-surface geological mapping and are increasingly being collected worldwide. However, conversion of data from measured resistivity to lithology is not a straightforward task. Therefore, it is still challenging to make full use of these data. Many limitations must be considered before a successful geological interpretation can be performed and a reasonable 3D geological model constructed. In this paper, we propose a method for 3D geological modelling of AEM data in which the limitations are jointly considered together with a cognitive and knowledge-driven data interpretation. The modelling is performed iteratively by using voxel modelling techniques with tools developed for this exact purpose. Based on 3D resistivity grids, the tools allow the geologist to select voxel groups that define any desirable volumetric shape in the 3D model. Recent developments in octree modelling ensure exact modelling with a limited number of voxels.

Integrated management and utilization of hydrogeophysical data on a national scale

Development of more time-efficient and airborne geophysical data acquisition systems during the past decades have made large-scale mapping attractive and affordable in the planning and administration nof e.g., groundwater resources or raw material deposits. The handling and optimized use of large geophysical data sets covering large geographic areas requires a system that allows data to be easily stored, extracted, interpreted, combined and used one time after another with different purposes. nSuch an integrated system for management and utilization of hydrogeophysical data on a national scale has been developed during the past decade in Denmark. nThis data handling system includes a comprehensive national geophysical data base (the GERDA data base), a national data base for borehole information (the Jupiter data base), a program npackage for processing, interpretation and visualization of electrical and electromagnetic data as well as preparation of these data for upload to the geophysical data base (the Aarhus Workbench) and finally a 3D visualization and modelling tool used for geological modelling and data quality control. The Aarhus Workbench program package allows visualization and analysis of subsets of data from the geophysical data base, which may include data from many individual nmapping campaigns. The 3D visualization and modelling tool uses data from the geophysical and the borehole data bases directly; moreover, it handles maps and grids produced in the Aarhus Workbench. nThe integrated system for management of hydrogeophysical data allows management of large amounts of data collected over several years in different mapping campaigns, of different consultant companies and with different geophysical methods and instrumentation. It is now used by all partners ninvolved in the groundwater mapping in Denmark. The system promotes reuse of geophysical data and models in future mapping projects, as well as easing and promoting the use of geophysical data in the geological modelling. The integrated system secures transfer of documentation all the way from data acquisition over processing and inversion of the geophysical data to geological modelling nthrough storage of data acquisition parameters, data processing parameters, inversion parameters nand uncertainties on data and models in the geophysical data base. nThe benefits of the large amount of geophysical data gathered in the national geophysical data base and utilized by the two program packages are invaluable for all future groundwater planning and administration.

Combining 3D geological modelling techniques to address variations in geology, data type and density - An example from Southern Denmark

The very complex near-surface geology in Denmark is a big challenge when constructing 3D geological models. Borehole data alone are normally insufficient for proper 3D modelling because data are too widespread. Therefore, Airborne ElectroMagnetic (AEM) techniques are often used to obtain supplementary information on the spatial distribution and composition of the geology.A large-scale AEM survey and high-resolution seismic data along with both new and existing borehole data and seismic data from hydrocarbon exploration were available for the construction of a detailed 3D geological model in our study area. The data are unevenly distributed, and only part of the study area was covered by the AEM survey. Cross-cutting tunnel valleys, erosional unconformities, delta units and a large glaciotectonic complex are among the geological features identified in the area. The geological complexity varies significantly across the model area.A broad geological overview and understanding of the area was obtained by joint cognitive interpretation of the geophysical and the geological data. To address the geological complexity and the very high level of detail gained from the AEM data, the model was constructed as a voxel model with lithofacies attributes supplemented by a number of bounding surfaces. In areas where the geology is not too detailed and complex, the model was constructed manually, whereas automated methods were used to populate voxels in areas with a high complexity. The automated methods comprised clay fraction modelling, which was used where AEM data are available, and stochastic modelling, which was used outside the area covered by AEM data.Our study shows that it is advantageous to combine several modelling methods in areas with varying geological complexity and data density. The choice of modelling methods should depend on the character and coverage of available data and on variations in geology throughout the model area. We present a 3D geological model combined by three different techniques.The choice of modelling technique is guided by geology and data type/density.An interactive manual approach is preferable where the geology is simple.Automated approaches are preferable where the geology is complex.Results from the three models were merged into a final voxel model.

Flawed processing of airborne EM data affecting hydrogeological interpretation.

Airborne electromagnetics (AEMs) is increasingly being used across the globe as a tool for groundwater and environmental management. Focus is on ensuring the quality of the source data, their processing and modeling, and the integration of results with ancillary information to generate accurate and relevant products. Accurate processing and editing of raw AEM data, the topic of this article, is one of the crucial steps in obtaining quantitative information for groundwater modeling and management. In this article, we examine the consequences that different levels of processing of helicopter transient electromagnetic method data have on the resulting electrical models and subsequently on hydrogeological models. We focus on different approaches used in the industry for processing of the raw data and show how the electrical resistivity-depth models, which is the end geophysical product (after data inversion) of an AEM survey, change with different levels of processing of the raw data. We then extend the study to show the impact on some of the hydrogeological parameters or models, which can be derived from the geophysical results. The consequences of improper handling of raw data to groundwater and environmental management can be significant and expensive.

Substratum control on tunnel-valley formation in Denmark

Abstract Tunnel valleys formed by meltwater erosion underneath the margins of the Pleistocene ice sheets are present in high numbers in the Danish onshore area. The geographical distribution of the buried tunnel valleys is uneven, but when comparing with the substrata lithology we find a large number of valleys in areas dominated by low-permeable sediment and a smaller number in areas with highly permeable substrata. The observations point to the drainage capacity of the ice-sheet substratum as an important factor controlling tunnel-valley formation. Tunnel-valley formation appears to be favoured in areas with low-permeable substrata because meltwater drainage through the sediments is impeded, leading to the formation of a channelized subglacial drainage system. The high transmissivity in areas dominated by permeable substrata facilitates drainage of a part of the meltwater as groundwater. This causes a lowering of the subglacial meltwater pressures, and tunnel-valley formation is less likely. Once formed and filled, the tunnel valleys cause a change of the hydraulic properties of the substratum and if subglacial water pressures underneath a subsequent ice advance are sufficiently high, old tunnel valleys will be prone to reactivation.

The Impact on Geological and Hydrogeological Mapping Results of Moving from Ground to Airborne TEM

In the past three decades, airborne electromagnetic (AEM) systems have been used for many groundwater exploration purposes. This contribution of airborne geophysics for both groundwater resource mapping and water quality evaluations and management has increased dramatically over the past ten years, proving how these systems are appropriate for large-scale and efficient groundwater surveying. One of the major reasons for its popularity is the time and cost efficiency in producing spatially extensive datasets that can be applied to multiple purposes. In this paper, we carry out a simple, yet rigorous, simulation showing the impact of an AEM dataset towards hydrogeological mapping, comparing it to having only a ground-based transient electromagnetic (TEM) dataset (even if large and dense), and to having only boreholes. We start from an AEM survey and then simulate two different ground TEM datasets: a high resolution survey and a reconnaissance survey. The electrical resistivity model, which is the final geophysical product after data processing and inversion, changes with different levels of data density. We then extend the study to describe the impact on the geological and hydrogeological output models, which can be derived from these different geophysical results, and the potential consequences for groundwater management. Different data density results in significant differences not only in the spatial resolution of the output resistivity model, but also in the model uncertainty, the accuracy of geological interpretations and, in turn, the appropriateness of groundwater management decisions. The AEM dataset provides high resolution results and well-connected geological interpretations, which result in a more detailed and confident description of all of the existing geological structures. In contrast, a low density dataset from a ground-based TEM survey yields low resolution resistivity models, and an uncertain description of the geological setting.

A synthetic study of geophysics-based modelling of groundwater flow in catchments with a buried valley

Groundwater models simulating flow in buried valleys interacting with regional aquifers are often based on hydrogeological models interpreted from dense geophysical datasets and scarce borehole data. For three simple synthetic cases, it is demonstrated that alternative methods of inversion of transient electro-magnetic (TEM) data can lead to very different interpretations of the hydrogeology inside and surrounding a buried valley. The alternative interpreted hydrogeological models are used in numerical modelling of groundwater flow to a pumping well. It is demonstrated that the alternative models result in quite different groundwater-model predictions of capture zone, recharge area, and groundwater age for the pumping well. It is briefly demonstrated that model calibration against hydraulic head data is not likely to improve the predictions or to identify the structural error of the interpreted hydrogeological models. It is therefore concluded that when TEM-based resistivity models are interpreted to construct the hydrogeological framework of a groundwater model, it must be done cautiously with support from deep borehole information. Too much reliance on geophysical mapping can lead to seriously wrong hydrogeological models and correspondingly wrong groundwater-model predictions.RésuméLes modèles simulant l’écoulement dans des vallées enfouies interagissant avec des aquifères régionaux sont souvent basés sur des modèles hydrogéologiques construits à partir d’ensembles denses de données géophysiques et de rares données de forage. Dans trois cas synthétiques et simples, il est démontré que les méthodes alternatives d’inversion des données électromagnétiques transitoires (TEM) peuvent conduire à des interprétations très différentes de l’hydrogéologie à l’intérieur et dans l’encaissant d’une vallée enfouie. Les modèles alternatifs d’interprétation hydrogéologique sont utilisés en modélisation numérique de l’écoulement souterrain vers un puits de pompage. Il est démontré que les modèles alternatifs conduisent à des prévisions tout à fait différentes pour la zone de captage, l’aire de recharge, et l’âge de l’eau souterraine pour le puits de pompage. Il est démontré brièvement que le calage du modèle sur des données piézométriques ne va vraisemblablement pas améliorer la réponse ni identifier l’erreur de structure des modèles d’interprétation hydrogéologique. On conclu que lorsque des modèles de résistivité basés sur la méthode TEM sont utilisés pour construire le cadre hydrogéologique d’un modèle de nappe souterraine, cela doit être fait prudemment avec l’appui de l’information de forage profond. Une trop grande confiance en la cartographie géophysique peut mener à des modèles hydrogéologiques sérieusement erronés et par conséquent à des prédictions erronées de modèle de nappe.ResumenLos modelos de de agua subterránea que simulan el flujo en valles enterrados que interactúan con acuíferos regionales están a menudo basados en modelos hidrogeológicos interpretados a partir de densos conjuntos de datos geofísicos y escasos datos de perforaciones. Se demuestra para tres casos sintéticos simples, que los métodos alternativos de inversión de datos transitorios electromagnéticos (TEM) pueden conducir a muy diferentes interpretaciones dentro de la hidrogeología y los alrededores de un valle enterrado. Los modelos hidrogeológicos alternativos interpretados se usan en la modelación numérica del flujo subterráneo hacia pozos de bombeo. Se demuestra que los modelos alternativos dan como resultado muy diferentes predicciones de la modelación de agua subterránea de la zona de captura, el área de recarga y de la edad del agua para los pozos de bombeo. Se demuestra brevemente que no es probable la calibración del modelo en función de los datos de carga hidráulica para mejorar las predicciones o identificar el error estructural de los modelos hidrogeológicos interpretados. Además se concluye que cuando los modelos de resistividad basados en TEM son interpretados para construir el marco hidrogeológico de un modelo de agua subterránea, debe hacerse cautamente con el apoyo de la información de pozos profundos. Demasiada confianza en los mapeos geofísicos puede conducir a modelos hidrogeológicos seriamente equivocados y correspondientes a predicciones erróneas de los modelos de agua subterránea.摘要模拟和区域含水层之间有相互作用的地下河谷地下水流的模型通常是基于水文地质模型的,这些水文地质模型是根据大量的地球物理数据和稀少的钻孔数据解译的。三个简单的综合分析例子表明瞬变电磁(TEM)数据反演的替代方法可以得出地下河谷内部和周围水文地质条件的不同解释。这个替代解释水文地质模型常常用于抽水井的地下水流数值模拟。结果表明替代模型得出完全不同的地下水模型预测出的抽水井的捕获区,补给区和地下水年龄。它简单的说明了依据水头数据来校正模型不大可能改善预测结果或者识别出解释水文地质模型的结构性的错误。因此得出的结论是,当利用基于TEM的电阻率模型来建立地下水模型的水文地质框架时,在有深部钻孔信息的帮助下,我们必须慎重对待。过多的依赖地球物理测绘会得出有严重错误的水文地质模型以及相应错误的地下水模型预测结果。ResumoOs modelos de simulação de fluxo subterrâneo em vales enterrados que interagem com aquíferos regionais baseiam-se frequentemente em modelos hidrogeológicos interpretados a partir de vastos conjuntos de dados geofísicos e escassa informação de logs de sondagem. Demonstra-se, para três casos sintéticos simples, que a utilização de métodos alternativos de inversão de dados electromagnéticos transientes (TEM) pode levar a interpretações muito diferentes da hidrogeologia da zona interior e envolvente do vale enterrado. Utilizam-se os modelos conceptuais alternativos de interpretação hidrogeológica na modelação numérica do escoamento subterrâneo em direcção a um furo de extracção. Demonstra-se que as interpretações alternativas resultam em previsões do modelo muito diferentes da zona de captura, da área de recarga e da idade da água subterrânea para o furo de extracção. É demonstrado de forma sucinta que não é provável que a calibração dos modelos com dados de potencial hidráulico resulte em melhorias de previsão ou na identificação do erro estrutural dos modelos hidrogeológicos interpretados. Conclui-se por esta razão que, quando os modelos de resistividade baseados no TEM são interpretados para construir o enquadramento hidrogeológico de um modelo de fluxo subterrâneo, tal deve ser feito de forma cautelosa e com o apoio de informação de sondagens profundas. A confiança em demasia no mapeamento geofísico pode levar a modelos conceptuais hidrogeológicos seriamente errados e consequentemente a previsões erradas pelo modelo de fluxo subterrâneo.

3D modeling of buried valley geology using airborne electromagnetic data

Buried valleys are important hydrogeologic features of glaciated terrains. They often contain valuable groundwater resources; however, they can remain undetected by borehole-based hydrogeologic mapping or prospecting campaigns. Airborne electromagnetic (AEM) surveys provide high-density information that can allow detailed features of buried valleys to be efficiently mapped over large geographic areas. Using AEM data for the Spiritwood Valley Aquifer system in Manitoba, Canada, we developed a 3D electric property model and a geologic model of the buried valley network. The 3D models were derived from voxel-based segmentation of electric resistivity obtained via spatially constrained inversion of two separate helicopter time-domain electromagnetic data sets (AeroTEM and versatile time-domain electromagnetic [VTEM]) collected over the survey area. Because the electric resistivity do not provide unequivocal information on subsurface lithology, we have used a cognitive procedure to interpret the electric property models of the aquifer complex, while simultaneously incorporating supporting information for the assignment of lithology in the 3D geologic model. For the Spiritwood model, supporting information included seismic reflection data and borehole records. These data constrained valley geometry and provided lithologic benchmarks at specific borehole sites and along seismic transects. The large-scale AeroTEM survey provided the basis for modeling the regional extent and connectivity of the Spiritwood Valley Aquifer system, whereas the local-scale VTEM survey provided higher near-surface resolution and insight into a detailed shallow architecture of individual buried valleys and their fill.