Laurent Tacher

Geological uncertainties associated with 3-D subsurface models

Subsurface models are generally built from both subjective interpretation and mathematical interpolation/extrapolation techniques. These models are therefore uncertain, but their uncertainty is rarely expressed in a geological forecast. In this paper, an evaluation method of geological uncertainties related to 3-D subsurface models is proposed and tested on a real case. This method is based on the subsurface model, which is considered the most probable prediction (best guess). The various geological interfaces are handled as Gaussian random fields to which a model of spatial variability describing possible fluctuations around the best guess is applied. Several structural constraints, such as the shape of folds and thickness of layers are accounted for in the model. At this point, the local variance can be estimated throughout the study area by application of the simple kriging technique. Finally, the variability is converted into probabilities of occurrence of the various rock masses present in the study area. The probabilities are calculated according to intersection rules governing the stratigraphic sequence of the subsurface model. They enable one to probabilistically model subsurface structures in the form of a three-dimensional (3-D) probability field.

Numerical modelling of the hydrogeological and geomechanical behaviour of a large slope movement: The Triesenberg landslide (Liechtenstein)

Using advanced hydrogeological and geomechanical finite element modelling, it has been possible to model the mechanical behaviour of a large slope movement, the Triesenberg landslide. This slope is located along the Rhine valley in the Principality of Liechtenstein and covers an area of around 5 km 2 , which includes two villages. Pore-water pressure fields calculated by the hydrogeological model were used as input for the geomechanical model. The results obtained through numerical simulation agree fairly well with field measurements of peak velocity, spatial and temporal distribution of velocity, and total displacements. Such results were obtained using a modified Cam-Clay elastoplastic constitutive model for which the required material parameters were obtained through careful geotechnical tests. These finite element models were carried out in two and three dimensions to gradually improve the understanding of the physical phenomena governing the hydrogeological conditions and the movements of the slope.

Generation of granular media

A discrete reduced distance method to generate 2-D and 3-D granular porous media is presented. The main property of the method is to produce heterogeneous and/or anisotropic packed beds of joined grains with arbitrary shapes and optimum fitting (i.e., minimum porosity). The iterative generation process starts with the coarsest grain and adjusts the size and location of the next ones depending on the updated available space. Hence, grain size distribution cannot be specified directly but is merely the consequence of user defined input parameters. The latter consists of a set of randomly distributed initial points, a few typical predefined grain shapes as well as the minimum and maximum grain diameters. The simulated granular media can readily be processed by an appropriate mesh generator to allow for subsequent numerical solutions of differential equations.

Understanding cave genesis along favourable bedding planes. The role of the primary rock permeability

Recent studies on the complex 3D geometry of large cave systems around the World allowed us to get statistical evidence of the inception horizon hypothesis. It clearly confirmed the idea that the development of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons. An inception horizon is a part of a rock succession that can favour the earliest cave forming processes (LOWE 1992).

Methodology for delineating groundwater protection areas against persistent contaminants

This study proposes a general methodology, applied in Switzerland, for the delineation of protection areas for abstracted groundwater against persistent chemical contaminants in order to establish: (1) remediation programmes in the event of contamination and (2) prevention programmes, in a general way. This new approach is based on the complete groundwater flow cycle, from the surface of the catchment zone to the groundwater source itself; areas with the highest contribution rate to the groundwater source will be preferentially protected. The method under consideration thus provides a tool for sustainable development and land management. To date, four actual cases have been treated in order to test the methods applicability. The results are convincing in various porous or fractured aquifers. One case is presented to illustrate an application of the methodology.

Automatic nodes generation in N-dimensional space

Reference GEOLEP-ARTICLE-1996-004doi:10.1002/(SICI)1099-0887(199604)12:4 3.0.CO;2-R Record created on 2008-02-13, modified on 2016-08-08