FEFLOW_Conference_HydroGeoBuilder_final (HESCH 2009).pdf

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Conceptual Model Development forMODFLOW or FEFLOW modelsFEFLOW Conference

September 2009

Wayne Hesch

Schlumberger Water Services


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OutlineIntroduction

What is a conceptual model

Groundwater modeling workflows

Numerical modelingConceptual modeling

Benefits of Conceptual Modeling

Future Development

Questions


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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 perhapsthe most important, involves the design of a conceptual model

Conceptual modeling is often overlooked => modelers

constrained by selected simulator, and/or a specific numericalgrid or mesh

Conceptual modeling can lead to more efficient model

development, and opportunity for multiple interpretations and

multiple discretizations.


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Build a Conceptual Modela conceptual model is a hydrogeologists mental representation

of the groundwater flow system

always 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,


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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 thatcan more easily be

adjusted prior to dedicating the effort in developing thenumerical model.


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Why Create Conceptual Models?

Simplify the field problem

Organize field data so that the system can be analyzed more

easily

The closer the conceptual model approximates the field

situation, the more accurate is the numerical model

Strive 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


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Numerical Model Development

the conceptual hydrogeologic model is the most important step

in groundwater model process

it forms the basis for developing the numerical model

an increased level of effort in creating the conceptual modelreduces the effort calibrating the numerical model

Level of Effortonceptual

Model

Numerical

Model

Everything should be made as simple as possible but not simpler.

Albert Einstein


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Developing a good conceptual model requires you to

compile detailed information ongeologic formations

groundwater flow directionshydrologic boundaries (recharge, rivers, lakes, wetlands, )

hydrogeologic parameters (conductivity, storage, porosity, )

extraction or injection from wells (location, depth, screens,

rates), and

observations of groundwater head and water quality

Conceptual Model


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vast amounts of data generated from numerous

sources in a variety of formatsField, Analytic, Spatial

Eg: GIS, CAD, Gridded files, spreadsheets, databases

added complexity of multiple projects and changing

conditions over time

determining which data is needed for the groundwater

model

gathering the required data from other applications in

the correct format to import into the modeling software

package

Conceptual Model Challenge


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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

No

Suitable?

Yes

No

Suitable?

Yes

No

Suitable?

Af ter Anderson & Woessner (1982)

Conceptual

Modeling

Numerical

Modeling


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Traditional Approach - Numerical Modeling

with numerical modeling designing the grid/mesh is the first

step

the 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 inputs

are defined, you will need to check and

re-work those input elements, to see thatthey are still in the appropriate location

Generally the input elements are not easily

modified, typically you need to delete them

and then re-assign them


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Numerical Modeling Workflow (FEFLOW)

model?

Ano

ther

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


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Conceptual Modeling

with conceptual modeling, designing the gridor mesh is the last step

Advantages:

define the conceptual model boundary, and model inputs independent ofany 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, tothe 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


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Conceptual Modeling

other 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 grid

this flexibility is not possible with classical numerical modeling, as itwould 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-independent

Easily expand size of the model domain, vertical discretization, and the

model inputs can be easily regenerated from the conceptual objects

if the project objectives change, a new numerical model can be easily

generated, or existing ones updated, from the conceptual objects

it allows for translating the conceptual model to FEFLOW or MODFLOW,

with vertical layers that follow the geology or are layer-independent


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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


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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/below

several horizons types accommodate

various geological conditions(pinchouts, discontinuous layers)


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Conceptual Model: Generating Geologic Model

Define surfaces

by interpolating XYZ points

from well unit contacts

from cross-sectionsImporting .DEM, .GRD, etc.

Convert to horizons


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Conceptual Model: Generating Geologic Model

load fence diagrams, cross-sections

interpolate contact points to create surfaces


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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


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Conceptual Model: Property Zones

use shapefiles (*.SHP) or CAD polygons to define property zones

several methods for defining property zone values:constant value (by layer)

Use shapefile attributes2D interpolated surface (2D Grid)

use 3D Gridded Data


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Conceptual Model Properties: Benefits

Flexible units for flow materials

Various methods for defining input

Not assigned to a mesh/gridIf mesh changes, can easily re-generate FEFLOW input fromconceptual model


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Conceptual Model: Boundary Conditions

use shapefiles (*.SHP) or CAD polygons/polylines to define

boundary geometry and attributes

several methods for defining boundary conditions:

constant valueuse Surface (river stage from DEM)

use time schedule

use shapefile attributes

Assign values to entire zone or vertices on lines (eg. Rivergauging stations)

Assign geometry to side faces of model domain


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Conceptual Model Boundary Conditions: Benefits

Flexible units for flow rates, heads, etc

Various methods for defining input

Work with combination of data objects and operationsminimized pre-processing in GIS

Not assigned to a mesh/gridIf 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-generated

During translation to .FEM file:well screens are assigned between appropriate slices

flow rates are distributed accordingly for multi-layered wells

(no need to assign wells on layer-by-layer basis)


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Define Numerical ModelSelect simulator and define appropriate grid or meshMODFLOW

Define horizontal grid resolution, rotationRefine grid, or define local gridsDefine vertical layers

Use HorizonsIndependent of geology

FEFLOWDefine superelement meshDefine 2D Horizontal meshDefine 3D Slice elevations

Using Horizons

Independent of geology


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Benefits of Grid/Mesh Generation

Deformed layer elevations automatically taken from

conceptual model

Generate model layers independent of the geologic structureMin layer thickness enforced, in pinchout regions

Advanced vertical refinement

Iterative approach


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From Conceptual Model to Multiple Numerical grids with

MODFLOW properties

Property zones in the

conceptual 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-outsand cell aspect ratios


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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


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Property Translation

With 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)


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Property Upscaling:

Algorithm to Satisfy Darcys Law on Element Level

For each finite elementCalculate all property zones intersected by the element (even the thinnest

ones are taken into account)Upscale horizontal conductivity using parallel connection rules

Upscale vertical conductivity using sequential connection rules using a

weighted average of zone values intersected by finite element


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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)


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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


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Future Development

Fully conceptual, simulator-independent approach to building a

groundwater model

Current implementation supports USGS MODFLOW and FEFLOW

FEFLOW: supports 3D mesh design, flow materials, and pumpingwells

Future support for Type 1,2,3 boundary conditions

Additional Analytical models

Additional Finite Difference/Finite Element modelsIntegration with surface water models

Support for Linked simulations using OpenMI technology


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Summary

the classical approach to numerical modeling starts with a grid

or mesh and then assigns model properties and boundariesfor 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


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Summary

A conceptual model improves the efficiencies of these iterations,

by housing all data, and providing a visual environmentIt 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, andboundary conditionsthe model grid/mesh is assigned afterthese have been designed

this allows more flexibility in choosing grid orientation and discretization

grid refinement is easy to apply to conceptual objects

it supports multiple conceptual models for determining the best approach

to simulating a specific groundwater environment


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Acknowledgments

Co-authorsSerguei 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?)


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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 Services

A 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. 154-158

http://www.twdb.state.tx.us/gam/GAM_GW_model.htm

http://www.ce.utexas.edu/prof/maidment/GISHyd97/gms/gms.htm

http://www.indygov.org/

For more information on the OpenMI project, please refer to the extensive OpenMI website atwww.openmi.org

FEFLOW. FEM File Format


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Thank you

Questions?

FEFLOW_Conference_HydroGeoBuilder_final (HESCH 2009).pdf

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