HydroGIS 96: Application of Geographic Information Systems in Hydrology and Water Resources
Management (Proceedings of the Vienna Conference, April 1996). IAHS Publ. no. 235, 1996. 5 9 3
GeoFEST: an integrated GIS and visualization
environment for the development of three-dimensional
hydrogeological models
MARK D. WILLIAMS, CHARLES R. COLE, MICHAEL G. FOLEY & SIGNE K. WURSTNER
Pacific Northwest National Laboratory1, PO Box 999, Richland,
Washington 99352, USA
Abstract GeoFEST provides an integrated GIS, groundwater flow and transport modelling, and scientific visualization environment to aid in developing three-dimensional hydrogeological models. A key concept in the design of GeoFEST was to keep characterization data (e.g. geology) separate from model-specific data (e.g. finite element mesh, hydraulic properties, boundary conditions). This minimizes the impact of modifica-tions in the model and allows characterization data to be used for constructing models at different scales. A GIS is used to digitize, contour, and grid maps of characterization data and for contouring plan-view results of the numerical model. Scientific visualization software provides for interactively viewing and dissecting the three-dimensional hydrogeological model and for visualizing modelling results. GeoFEST is currently being used in the development of three-dimensional hydrogeological models at different scales in the West Siberian basin and at the Hanford Site in Washington State.
INTRODUCTION
GeoFEST (Geologic Finite Element Synthesis Tool) was developed to aid in constructing three-dimensional hydrogeological models. A key objective in developing GeoFEST was to keep the site characterization data independent of the model parameters to minimize the impact of changes in the model and to use the site characterization data for models of different scales. Modelling is an iterative process, therefore the model description should be easily modified to provide for refinements, feedback from simulations calibration runs and sensitivity analysis.
Figure 1 illustrates the main components and linkages of the GeoFEST system. GeoFEST provides the linkages to integrate a geographic information system used in digitizing, contouring, and gridding site characterization data (ARC/INFO — a register-ed trademark of Environmental Systems Research Institute, Inc., Rregister-edlands, California); a groundwater flow and solute transport code (CFEST - Gupta et al., 1987); and an object-oriented scientific visualization program (AVS - a registered trademark of Advanced Visual Systems, Inc., Irvine, California) into a complete system for hydrogeological model development.
1 Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the US
594 Mark D. Williams et al. I Arc/Info Facies and Isopachs / Geologic Structure Description/ Facies Translation Table Finite Element Grid Boundary Conditions Material Properties Table
Fig. 1 GeoFEST data flow diagram showing major components and linkages between the programs used in constructing, visualizing, and conducting numerical simulations of hydrogeological models.
This report briefly describes the site characterization and model parameters used in GeoFEST (a more detailed description can be found in Foley et al, 1995). The examples used to illustrate this system are from a hydrogeological model of the West Siberian basin, the largest platformal basin in the world (see Fig. 2) with a total area of 3.5 million km2. This large-scale model was developed to provide boundary conditions for smaller-scale models within the basin. GeoFEST is also being used for developing a three-dimensional hydrogeological model of the US Department of Energy's Hanford Site in Washington State, USA.
SITE CHARACTERIZATION
GeoFEST: an integrated GIS and visualization environment 595
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Fig. 2 CFEST finite element grids for the West Siberian basin regional model (a) and the Tomsk Local model (b).
file, a text file that lists the GIS files describing the spatial variations in rock faciès type and thickness or top-of-layer elevation (i.e. isopach or structure contours) for each layer in the hydrogeological system. Figure 3 shows the facies for the 13 layers used in the West Siberian Basin Model. The layering order is defined by the order of file names in the geological structure description file, starting with the top layer. An option is provided for specifying constant facies, thickness, or elevation of a layer. The upper elevation of the system is described by a digital elevation model (DEM). An optional DEM of the water table can also be specified. The facies, isopach, structure contour, and DEM files are in ARC/INFO gridded ASCII format.
Although currently not a direct input to the GeoFEST system, rivers in the area need to be identified with the spatial coordinates along with river mile and elevation using topographic maps or other sources (DEMs are not adequate for compiling these data). These data are required for identifying nodes that are located on rivers and for specifying the appropriate boundary conditions.
HYDROGEOLOGICAL MODEL PARAMETERS
The main hydrogeological model parameters used by GeoFEST are the finite element mesh, translation table, material property table, and boundary conditions. These data are specified in text files and are described below.
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The finite element mesh required for GeoFEST is two-dimensional and is composed of a node file and an element file. The node file consists of a list of node numbers with corresponding x and y coordinates. The element file consists of an element number and a list of node numbers, in counterclockwise order, that make up the surface element. In regional models, the boundaries of the finite element mesh are typically defined by drainage basin boundaries (where practical) and the interior elements and nodes are aligned with rivers. The current approach for developing a regional finite element model is iterative. A coarse grid is created that is aligned along the external and internal model boundaries, the rivers, and other important internal features. Next the number of nodes in this original grid is doubled (one element is split into four elements) through a utility program. Newly created nodes near these boundaries, rivers, and features are then interactively aligned to provide a more accurate representation of their geometry. This re-doubling and interactive alignment process is then repeated until the desired mesh size (resolution) is achieved. This approach allows large finite element meshes with relatively uniform element areas to be quickly generated. The Tomsk finite element mesh, shown in Fig. 2(b), was developed using this method.
Material Property Assignment
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Translation Table
Fig. 4 Conceptual diagram of the process for translating facies to materials used in the
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The isopach/structure contour grids specified in the geological structure description file are stacked onto the two-dimensional finite element grid (see Figs 2 and 3), creating a three-dimensional finite element mesh. The gridded files are sampled at the node locations. The variable boundaries of the layers can result in pinched elements, where not all the nodes in an element exist in a layer. The upper elevation of the system is specified by the surface elevation DEM and the layers are stacked from the top down. If a water table DEM is specified, nodes are not created within an element until the elevation of the node is below the water table.
Hydrostratigraphic interpretation and hydraulic property assignment logic is shown in Fig. 4. This process is accomplished through the use of two text files: a translation table and a material property table. The use of these two separate files provides the capability to keep the characterization data separate from the hydrostratigraphic interpretation. The translation table permits the user to group similar facies types into a smaller number of zones with the same hydraulic properties. In the West Siberian basin, for example, 70 facies zones exist across the 13-layer system. These facies are lumped into 17 categories with distinct hydraulic properties through the translation table. The material property table contains the hydraulic conductivity in the x, y, and z direction; porosity; specific storage; and longitudinal and transverse dispersivity. This process allows for new hydrostratigraphic interpretations of the facies and easy modification of hydraulic properties for calibration and sensitivity runs.
Currently, most of the logic for handling boundary conditions occurs outside of the main GeoFEST system. Utility programs and networks within ARC/INFO are used to specify Dirichlet (prescribed head) boundary conditions on nodes located on surface water features. The resulting boundary condition file contains a combination of characterization and model data (node numbers and elevation), requiring an additional step to regenerate this file whenever changes in the finite element mesh influence node locations. Other types of boundaries (e.g. recharge and other sources and sinks) are currently handled outside of the GeoFEST system.
ANALYSING RESULTS
The output of GeoFEST consists of an AVS UCD (unstructured cell data) file for visualization, a CFEST input file for numerical modelling of groundwater flow and transport, and files containing summary information on the nodes and elements in the models. Additional programs have been developed for combining CFEST results (calculated hydraulic head, fluid velocity, concentration, temperature, and fluid density) with the AVS UCD file for three-dimensional visualization of results and for building ARC/INFO TINs for contouring results in plan view.
Figure 5 shows the AVS UCD file for the West Siberian basin hydrogeological model. The model is colour coded based on material properties. The entire model can be dissected interactively for viewing and individual materials can be viewed selectively or removed within AVS. Cross-sections and fence diagrams can be extracted using programs outside AVS based on a list of surface nodes or by layers. To check our hydro-stratigraphic interpretation of the West Siberian basin, a cross-section was generated along a similar transect of a Russian hydrostratigraphic interpretation published in the literature (Nudner, 1970). This comparison showed that our coarse-grained, more
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permeable units corresponded to transmissive layers in the Russian interpretation and our finer grained, less permeable units corresponded to confining layers.
Streamlines were generated with AVS using the UCD file combined with the velocities within the elements calculated from a numerical simulation, as shown in Fig. 6. The velocity field was extracted from the three-dimensional model along a cross-section so the streamlines are also three-dimensional. Generating numerous streamlines and projecting them on a two-dimensional cross-section yields a trace of the flow directions along the cross-section. A number of simulations were conducted using different hydraulic properties in the materials (isotropic, anisotropic, and homogeneous hydraulic conductivity) and the resulting streamline patterns were compared to each other for sensitivity analysis. Velocity vectors can also be plotted to evaluate flow paths. The three-dimensional UCD file can be colour-coded and/or contoured within AVS based on calculated hydraulic heads, temperatures, or concentrations.
Acknowledgements This work was sponsored by the US Department of Energy, Office of Technology Development, Characterization, Monitoring, and Sensor Technology Cross-Cutting Program.
REFERENCES
Foley, M. G., Bradley, D. J., Cole, C. R., Hanson, J. P., Hoover, K. A., Perkins, W. A. & Williams, M. D. (1995) Hydrogeology of the West Siberian basin and Tomsk region. PNL-10585, Pacific Northwest Laboratory, Richland, Washington.
Gupta, S. K., Cole, C. R., Kincaid, C. T. & Monti, A. M. (1987) Coupled fluid, energy, and solute transport (CFEST) model: formulation and user's manual. Battelle Memorial Institute Report BMUONWI-660, Columbus, Ohio. Nudner, V. A. (ed.) (1970) Gidrogeologiya SSSR; torn XVI, Zapadno-Sibirskaya Ravnina (Tyumeskaya, Omskaya,
NovosibirskayaiTomskayaoblasti) (in Russian) (Hydrogeology of the USSR; vol. 16, West Siberian Plain, including Tuymen, Omsk, Novosibirsk, and Tomsk regions.) Ministry of Geology, Moscow.
Peterson, J. A. & Clarke, J. W. (1991) Geology and Hydrocarbon Habitat of the West Siberian Basin. AAPG Studies in Geology 32, Am. Ass. of Petroleum Geologists, Tulsa, Oklahoma.
