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In the past few years, small‐scale (2 m) prepolarized surface nuclear magnetic resonance (SNMR) has gained increasing interest in the research community. As recent studies demonstrated, the application of a strong prepolarization field enhances the SNMR signal of coils with a footprint

Upscaling permeability of grid blocks is crucial for groundwater models. A novel upscaling method for three-dimensional fractured porous rocks is presented. The objective of the study was to compare this method with the commonly used Oda upscaling method and the volume averaging method. First, the multiple boundary method and its computational framework were defined for three-dimensional stochastic fracture networks. Then, the different upscaling methods were compared for a set of rotated fractures, for tortuous fractures, and for two discrete fracture networks. The results computed by the multiple boundary method are comparable with those of the other two methods and fit best the analytical solution for a set of rotated fractures. The errors in flow rate of the equivalent fracture model decrease when using the multiple boundary method. Furthermore, the errors of the equivalent fracture models increase from well-connected fracture networks to poorly connected ones. Finally, the diagonal components of the equivalent permeability tensors tend to follow a normal or log-normal distribution for the well-connected fracture network model with infinite fracture size. By contrast, they exhibit a power-law distribution for the poorly connected fracture network with multiple scale fractures. The study demonstrates the accuracy and the flexibility of the multiple boundary upscaling concept. This makes it attractive for being incorporated into any existing flow-based upscaling procedures, which helps in reducing the uncertainty of groundwater models.

The impact, caused by a construction of a dam site on the Birs river close to Basel (Switzerland), on the evolution of a gypsum-karst aquifer in the vicinity of the hydraulic building, is studied. Several sinkholes provoked subsidence of the dam and the highway nearby. Extensive technical measures had to be conducted in order to prevent further karstification. The numerous geophysical and geological field studies executed in the area, together with a 3D hydrogeological model of the aquifer, provide a very detailed information about the boundary conditions, and the local properties determining the karstification. In this work, we present a 2D karst evolution model of the aquifer in the vicinity of the dam structure. In contrast to older studies, this time the focus of the research is not the basic processes governing the karst evolution, but modeling the temporal development of the real aquifer. Using the large amount of information about the location, we demonstrate that a detailed knowledge of the local properties of the rock (hydraulic conductivity, solubility) is of crucial importance when modeling real aquifers. From a wide range of possible evolution scenarios, we deduce a warning that solutional features, such as sinkholes, can develop far away from the hydraulic structure and endanger facilities at the surface. Our model is able to reproduce and successfully explain the main geological features revealed by field studies. We suggest a workflow to combine the data from field observations, groundwater modeling, and karst evolution modeling and to study the karstification of real aquifers. We propose a scenario for the evolution of the aquifer and a reasonable range for the values of the basic parameters governing the karstification.

We present a three-dimensional model describing the evolution of a gypsum karst aquifer along the Birs river near Basel in Switzerland. Here, a small dam-site was built in 1890 to generate hydro-electric power. After subsidence of a nearby highway, geophysical and geological field studies have been carried out and a detailed hydrogeological model has been developed. These observations identified a karstified weathered zone as the reason for the subsidence. To prevent any further damage, remedial construction measures have been carried out. We employ our numerical model KARSTAQUIFER, which simulates the temporal evolution of a karst system by dissolution, increasing the porosity in the aquifer. We used results from geological and geophysical investigations as a priori information for implementing the initial boundary conditions into our three-dimensional numerical model. Our model accounts for topography, precipitation and geology and especially for the anisotropy of the local bedrock. It can successfully reproduce the weathered zone in its current horizontal and vertical extent. We cm also simulate the possible evolution of the aquifer following a remedial construction phase. Therefore, taken together with the geophysical and geological observations and the groundwater model, our three-dimensional karst aquifer evolution model enhances the understanding of the development of this heterogeneous karst aquifer system. (C) 2012 Elsevier B.V. All rights reserved. Hydrogeologie der Nordwestschweiz, Grundwasser

Summary Dam-sites and reservoirs located above soluble rock are often damaged by increased leakage through the sub-surface within the life-time of the structure. The high hydraulic gradients driving the water through the fracture and fissure system of the bedrock have a strong impact on the aquifer evolution. The increased permeability, if not prevented, leads to an imminent danger of high leakage rates (breakthrough) as well. As a result, the structural safety of the dam-site itself is at risk. Past experience has shown that this may have large environmental and economical consequences. For a better understanding of the evolution of karst aquifer systems in the vicinity of dam-sites, a three-dimensional conceptual model is presented. We show the evolution of the karst aquifer for simple three-dimensional dam-site setups. Keeping the symmetry and simplicity of the models we can relate our results to the two- and one-dimensional scenarios presented in the past. Implementing a statistical fracture network and topographic information to this basic setup we show that these complex three-dimensional properties of the real aquifers, have a significant influence on the karstification, and cannot always be addressed by two -and one-dimensional models.

The measurements of nuclear magnetic resonance (NMR) parameters to investigate petrophysical properties related to fluid (e.g., water) storage and transport processes provide unique insights compared to other geophysical methods and have become a very useful tool for geophysicists during the last decades at the laboratory scale and as a borehole tool. We investigated, at a groundwater test site in the desert of Abu Dhabi, the reliability and limitation of surface NMR, a new but establishing technique that measures the NMR parameter from the surface by comparing its results to borehole NMR logs. Surface NMR or magnetic resonance sounding measurements (MRS) were conducted along a profile, close to several boreholes. The available borehole NMR logs were used to i) evaluate the potential of surface NMR derived results comparing them with borehole NMR measurements and to ii) extend the hydrogeological knowledge of the groundwater site. Firstly, we show how to carefully handle short relaxation signals of surface NMR data. The most significant steps during this process are: i) broad-band filtering to preserve the short decaying NMR signals, ii) correction for relaxation during pulse effects and iii) QT-inversion to extract reliable subsurface parameter distribution. By comparing surface NMR results with borehole NMR logs we found the following limitations: i) surface NMR is not able to detect borehole NMR measured water content related to T2 decay times lower than ≈80 ms T2 decay time. This reduced detectable water content of surface NMR is due to an instrumental dead time of 40 ms, measured T2* relaxation times and a lower Larmor frequency of 2 kHz and ii) borehole NMR has significantly higher vertical resolution. Taking this into account, surface NMR is in good agreement with borehole NMR. Secondly, on a profile of 1.3 km length 11 MRS measurements were carried out to map the lateral aquifer structure. The obtained results show that surface NMR provides unique lateral information of demanded aquifer properties complementary to e.g., transient electromagnetic.

We have developed a numerical model able to describe the karstification of aquifers in fractured rocks containing soluble (limestone or gypsum) and insoluble layers. When water is flowing along fractures crossing the soluble layers, it is able to dissolve the material there, to increase the aperture width of the conduit, and consequently to increase the local hydraulic conductivity. Depending on the thickness and the distribution of these layers, the dissolution can be active only for limited periods, or during the whole evolution time. Fractures located in insoluble layers do not change at all. We are interested in the integral effect of these local processes and study four simplified scenarios of karstification along a prominent wide conduit crossing a fractured limestone block. We keep the initial and the boundary conditions the same for all scenarios and vary only in the amount and the distribution of the soluble material. We demonstrate that aquifers in 100% limestone, without any insoluble layers, develop along areas with high hydraulic conductivities and high hydraulic gradients, creating channel like pathways. On the other hand aquifers containing soluble layers with limited thickness develop faster and exhibit diffuse patterns determined by the chemical properties of the rock. The second part of the paper is a step towards modeling of real karst systems. We present the evolution of an aquifer located in the vicinity of a large hydraulic structure. All initial and boundary conditions, except the amount and the distribution of the soluble rock, remain the same for all scenarios. As a material example for the bedrock, we chose Gipskeuper from an aquifer along the Birs river in Switzerland. This rock consists of soluble gypsum layers and insoluble clays and marls, with typical layer thickness in the range of millimeters to centimeters. The basic processes discussed in the first part of the paper remain valid. We demonstrate that large insoluble zones can impair the karstification process and even completely block it, while areas with thin soluble layers can provide a preferential pathway and decrease the evolution times considerably. Finally we show that the evolution of the leakage rates and the head distribution within the aquifer can sometimes reveal misleading information about the stage of karstification and the safeness of the dam. Our model can be used not only to study simplified geological settings and basic processes, but also to address some of the complications arising when modeling real aquifers.

Summary We have developed the program package KARSTAQUIFER, which simulates flow and transport in a three-dimensional transient karst aquifer consisting of fractures and matrix elements. The fractures can be enlarged by chemical dissolution of calcite, increasing the secondary permeability of the karst aquifer. Boundary conditions for our model setup are guided by published benchmark models for karst aquifer evolution in two dimensions (length and width), with a 100 m high hydraulic head difference across the model domain responsible for relatively fast evolution of the aquifer. We are interested in the effect of the third dimension on the evolution of the karst aquifer, especially the effect of matrix flow, with a matrix defined either as porous medium or as fine fracture system, or both. We first discuss our model results using a pseudo-3D setup to be able to directly compare results to the 2D benchmark scenarios published. We then discuss real-3D models with a prominent fault located in the central part of the aquifer, connecting input and output regions. Finally, we replace the prominent fault with a statistical fracture diameter distribution. Our results compare well with existing 2D scenarios, and the additional third dimension offers new insights into the evolution of karst aquifers. Especially the role of matrix flow under fixed-head boundary conditions in their early phases can be studied in more detail.

We present a 3D numerical karst evolution model describing mechanisms governing the evolution of collapse dolines. The results confirm the current state of the art of the knowledge about the conditions necessary for a collapse doline to grow. We demonstrate that these geological features develop in karst aquifers, above subsurface rivers flowing through active cave systems or mechanically unstable zones. The mechanisms of growth of single/isolated collapse dolines are generally two dimensional and can be modeled with very dense numerical models. On the other hand, our results demonstrate that there can be more than one location, within a karst system, where the conditions allow the development of a collapse doline. For such complex scenarios a 3D modeling approach and a detailed knowledge of the hydro-geological situation is required in order to correctly predict and describe the development of collapse dolines.