conceptual model development for modflow or...
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Conceptual Model Development forMODFLOW or FEFLOW models
FEFLOW ConferenceSeptember 2009
Wayne HeschSchlumberger Water Services
OutlineIntroduction
What is a conceptual modelGroundwater modeling workflows
Numerical modelingConceptual modeling
Benefits of Conceptual ModelingFuture DevelopmentQuestions
IntroductionIn order for a groundwater model to be accurate, reliable, and robust, it requires a tremendous amount of information and understanding of the aquifer. The first step in developing a groundwater model, and perhaps the most important, involves the design of a conceptual modelConceptual modeling is often overlooked => modelers constrained by selected simulator, and/or a specific numerical grid or mesh Conceptual modeling can lead to more efficient model development, and opportunity for multiple interpretations and multiple discretizations.
Build a Conceptual Modela conceptual model is a hydrogeologist’s mental representation of the groundwater flow systemalways sketch the system and augment this representation with:
distribution of hydrogeologic layers,location of boundaries,2D/3D representation of the domain,plan vs. cross-sections,tables of parameter input values,
Conceptual Model: Definitions“A conceptual model is a simplified, high-level representation of the site to be modeled…”“The conceptual model represents our best idea of how the aquifer works.”“A conceptual model is a basic graphical representation of a complex natural aquifer system that can more easily be adjusted prior to dedicating the effort in developing the numerical model.“
Why Create Conceptual Models?Simplify the field problemOrganize field data so that the system can be analyzed more easilyThe closer the conceptual model approximates the field situation, the more accurate is the numerical modelStrive for parsimony – simplest is best, but retain enough complexity to adequately reproduce the system behaviorFailure of numerical models to make accurate predictions can often be attributed to errors in the conceptual model
Numerical Model Developmentthe conceptual hydrogeologic model is the most important step in groundwater model processit forms the basis for developing the numerical modelan increased level of effort in creating the conceptual model reduces the effort calibrating the numerical model
Level of EffortConceptualModel
NumericalModel
“Everything should be made as simple as possible, but not simpler.” – Albert Einstein
Developing a good conceptual model requires you to compile detailed information on
geologic formationsgroundwater flow directionshydrologic boundaries (recharge, rivers, lakes, wetlands, …)hydrogeologic parameters (conductivity, storage, porosity, …)extraction or injection from wells (location, depth, screens, rates), andobservations of groundwater head and water quality
Conceptual Model
vast amounts of data generated from numerous sources in a variety of formats
Field, Analytic, SpatialEg: GIS, CAD, Gridded files, spreadsheets, databases
added complexity of multiple projects and changing conditions over timedetermining which data is needed for the groundwater modelgathering the required data from other applications in the correct format to import into the modeling software package
Conceptual Model Challenge
The Groundwater Modeling Process
Build Conceptual Model
Assign Model Parameters
Collect Data
Design Model Grid/Mesh
Assign Boundary Conditions
Define Objectives
Yes
No
Predictive Simulations
Post Audit?
Calibrate and Validate Model
Sensitivity Analysis
Suitable?Yes
NoSuitable?
Yes
NoSuitable?
Yes
NoSuitable?
After Anderson & Woessner (1982)
Conceptual Modeling
Numerical Modeling
Traditional Approach - Numerical Modelingwith numerical modeling designing the grid/mesh is the first stepthe disadvantages of this approach include:
The correct grid/mesh must be generated before assigning properties, boundaries, wells, etc. If the grid/mesh is modified after other inputsare defined, you will need to “check andre-work” those input elements, to see thatthey are still in the appropriate locationGenerally the input elements are not easilymodified, typically you need to delete them and then re-assign them
Numerical Modeling Workflow (FEFLOW)
model?
Another
Define Numerical Model• Develop mesh• Define the Property Zones• Define Boundaries (rivers, wells,, …)
Define SuperElement Mesh Define 2D Mesh
Define Slice Elevations
Input Data• Import shapes, wells, surfaces,
XYZ points, cross-sections• Digitize new GIS layers
Define Property Zones
Define Flow Boundaries
Run Simulation &Analyze Results• Run FEFLOW• Check (visualize) results
Conceptual Modelingwith conceptual modeling, designing the gridor mesh is the last step
Advantages:define the conceptual model boundary, and model inputs independent of any numerical grid or meshprovides the freedom to design multiple conceptualizations of your site, and easily change your conceptual modeldefine multiple grids or mesh types, each with different resolution and size, and choose the most appropriate onetransfer the conceptual model, and the desired numerical grid/mesh, to the numerical modelAbility to change the simulator, based on the project needs
all model inputs including properties, wells, and boundary conditions are assigned to the selected grid/mesh automaticallyresulting MODFLOW or FEFLOW input files are generated
Conceptual Modelingother advantages:
if you are not happy with the grid/mesh, you can design a new one and re-generate a new numerical model using this new gridthis flexibility is not possible with classical numerical modeling, as it would require you to build and manage multiple numerical models
Easily change your model after it is createdraw data are left in tact and grid/mesh-independentEasily expand size of the model domain, vertical discretization, and the model inputs can be easily regenerated from the conceptual objectsif the project objectives change, a new numerical model can be easily generated, or existing ones updated, from the conceptual objectsit allows for translating the conceptual model to FEFLOW or MODFLOW, with vertical layers that follow the geology or are layer-independent
Workflow: Data → Conceptual → Numerical
Run Simulation &Analyze Results• Load the files into VMOD/FEFLOW to
run the simulation• Load results into Hydro GeoBuilder for
visualization and interpretation
Numerical Model (MODFLOW/FEFLOW)• Apply a grid/mesh• Assign the conceptual model to the grid• Create input files for the simulator
(MODFLOW/FEFLOW)
Finite Differences
Finite Elements
Input Data• Import shapes, wells, surfaces,
XYZ points, cross-sections• Digitize new GIS layers
Structure
Define Conceptual Model• Define the Geology: Coverage and Horizons• Define the Property Zones• Define Boundaries (recharge, pumping wells)
Properties Boundary Conditions
Define Model Domain• Define the region where you want to
run a model simulation
Conceptual Model Structure
define horizons from surfaces horizon truncation rule determines hierarchy; in case of intersections, which will be pushed up/down, or be truncated by surfaces above/belowseveral horizons types accommodate various geological conditions (pinchouts, discontinuous layers)
Conceptual Model: Generating Geologic ModelDefine surfaces
by interpolating XYZ pointsfrom well unit contactsfrom cross-sectionsImporting .DEM, .GRD, etc.
Convert to horizons
Conceptual Model: Generating Geologic Modelload fence diagrams, cross-sectionsinterpolate contact points to create surfaces
Conceptual Model Structure: Benefits
Model AreaEasily modify the size of the model=>Re-generate superelement mesh and slices=>Re-translate .FEM file input.
HorizonsUse native file formats to define surfaces, and resulting horizons (.XLS, XYZ points, ESRI .GRD, Surfer .GRD, cross-sections)Horizon rules simplifies modeling of complex geology
Conceptual Model: Property Zonesuse shapefiles (*.SHP) or CAD polygons to define property zonesseveral methods for defining property zone values:
constant value (by layer)Use shapefile attributes2D interpolated surface (2D Grid)use 3D Gridded Data
Conceptual Model Properties: Benefits
Flexible units for flow materialsVarious methods for defining inputNot assigned to a mesh/grid
If mesh changes, can easily re-generate FEFLOW input from conceptual model
Conceptual Model: Boundary Conditionsuse shapefiles (*.SHP) or CAD polygons/polylines to define boundary geometry and attributesseveral methods for defining boundary conditions:
constant valueuse Surface (river stage from DEM)use time scheduleuse shapefile attributes
Assign values to entire zone or vertices on lines (eg. River gauging stations)Assign geometry to side faces of model domain
Conceptual Model Boundary Conditions: Benefits
Flexible units for flow rates, heads, etcVarious methods for defining inputWork with combination of data objects and operations minimized pre-processing in GISNot assigned to a mesh/grid
If mesh changes, can easily re-generate FEFLOW input from conceptual modelCan move boundary objects (eg. Groundwater divide)
Pumping wellsScreen locations and pumping rates are mesh-independent: if mesh changes, FEFLOW input can be easily re-generatedDuring translation to .FEM file:
well screens are assigned between appropriate slicesflow rates are distributed accordingly for multi-layered wells(no need to assign wells on layer-by-layer basis)
Define Numerical ModelSelect simulator and define appropriate grid or meshMODFLOW
Define horizontal grid resolution, rotationRefine grid, or define local grids Define vertical layers
Use HorizonsIndependent of geology
FEFLOWDefine superelement meshDefine 2D Horizontal meshDefine 3D Slice elevations
Using HorizonsIndependent of geology
Benefits of Grid/Mesh Generation
Deformed layer elevations automatically taken from conceptual modelGenerate model layers independent of the geologic structureMin layer thickness enforced, in pinchout regionsAdvanced vertical refinementIterative approach
From Conceptual Model to Multiple Numerical grids with MODFLOW properties
Property zones in theconceptual model
Semi-uniform Grid•deformed top and bottom layers, uniform in middle•Useful for discontinuous layers (common in unconsolidated aquifers)
Uniform Grid•Flat layer top/bottoms•Fully respects FD assumptions•More layers, but useful for transport/density dependent simulations
Deformed Grid•Layers follow geology•Easy, few layers•Problems with pinch-outs and cell aspect ratios
From Conceptual Model to Multiple Meshes
Property zones in the conceptual model
Semi-uniform•deformed top and bottom layers•uniform in middle•Property upscaling is applied•Useful where Deformed mesh fails
Deformed Mesh•Layers follow geology•Easy, few layers•Convergence issues with tight geometry/water table fluctuations
Property TranslationWith numerical modeling, properties in pinch out layers have to be assigned manually.With conceptual modeling, properties are assigned to 3D Volumes. During translation, for layers that “pinch out”, the properties are automatically assigned from layers above/below (depending on minimum layer thickness and horizon rules)
Property Upscaling:
Algorithm to Satisfy Darcy’s Law on Element LevelFor each finite element
Calculate all property zones intersected by the element (even the thinnest ones are taken into account)Upscale horizontal conductivity using parallel connection rulesUpscale vertical conductivity using sequential connection rules – using a weighted average of zone values intersected by finite element
Numerical Property Upscaling
Zone lines
Grid lines
Zone=1
Zone=2
Zone=3
1 2 3
4 5 6
Elements 1, 2, 3 get zone values calculated at their centers.Elements 4, 5, 6 use properties upscaled from all intersected zones (1, 2, and 3)
Conductivity Upscaling
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Deformed Mesh – 5 layers Semi-Uniform Mesh – 10 Layers
Simple Budget Analyzer: Comparing Meshes
2.75% difference….more in future work
Future DevelopmentFully conceptual, simulator-independent approach to building a groundwater model Current implementation supports USGS MODFLOW and FEFLOWFEFLOW: supports 3D mesh design, flow materials, and pumping wells
Future support for Type 1,2,3 boundary conditionsAdditional Analytical modelsAdditional Finite Difference/Finite Element modelsIntegration with surface water modelsSupport for Linked simulations using OpenMI technology
Summarythe classical approach to numerical modeling starts with a grid or mesh and then assigns model properties and boundaries
for better local modeling the grid is refined over a number of iterations, which requires you to re-work property zones and boundariesthis can be a time-consuming/frustrating process
SummaryA conceptual model improves the efficiencies of these iterations, by housing all data, and providing a visual environment
It helps with the up-front design of the model; more detailed adjustments are done on numerical levelIt can be considered as the common “root” for a family of numerical models, so it can also be used as a version control for modeling projects
the use of a conceptual model builder allows you to define mesh and grid-independent model location, flow properties, and boundary conditions
the model grid/mesh is assigned after these have been designedthis allows more flexibility in choosing grid orientation and discretizationgrid refinement is easy to apply to conceptual objectsit supports multiple conceptual models for determining the best approach to simulating a specific groundwater environment
AcknowledgmentsCo-authors
Serguei Chmakov, Petr Sychev, Collin Tu, Marconi Lima, Schlumberger Water Services
DHI-WASY: Peter Schatzl and Support TeamThe workflow based approach was strongly motivated by powerful Schlumberger seismic to simulation workflows in the Petrel software (http://www.slb.com/content/services/software/geo/petrel/index.asp?)
References
Anderson, M.P. and W.W. Woessner (1992) “Applied Groundwater Modeling: Simulation of Flow and Advective Transport”. Academic Press, Inc. New York.Visual MODFLOW 3D-Builder Users Manual: Schlumberger Water ServicesA New Generation of Waterloo Hydrogeologic Software. MODFLOW and More 2008: Ground Water and Public Policy - Conference Proceedings, Poeter, Hill, & Zheng -www.mines.edu/igwmc/ pp. 154-158http://www.twdb.state.tx.us/gam/GAM_GW_model.htmhttp://www.ce.utexas.edu/prof/maidment/GISHyd97/gms/gms.htmhttp://www.indygov.org/For more information on the OpenMI project, please refer to the extensive OpenMI website at www.openmi.orgFEFLOW. FEM File Format
Thank youQuestions?