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Modelling Groundwater Flow in Hard Rock
Environments
Alexander Renz, DHI, Munich, Germany

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Part 1
Representing Complex Geology
© DHI

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© DHI
Complex

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© DHI

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© DHI
3D Groundwater Model

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3D Groundwater Model
© DHI
Multi-Faulted Regional Groundwater Model, Switzerland

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3D Groundwater Model
© DHI
• Continuous Layers
• Vertical Walls

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Unstructured Meshes
© DHI

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Example
of teaser
Follow the guide from the Branding SharePoint page to learn how to change the pictures
© DHI

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© DHI

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All Problems solved?

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Traditional Layered Meshes
Creating Computation Meshes
© DHI

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Mathematical Approach
© DHI
• Finite Differences Method
• Finite Element Method

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© DHI

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2.5 D Mesh (Layered Mesh)
© DHI
2 Triangles
1 Quadrangle

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Element Deactivation
© DHI
• Create Caveats in Mesh, e.g.
for pit mines or tunnels
• Permanent or temporary

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Element Deactivation
© DHI
• Remove unwanted layers
(Top / bottom only)

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Steep Interfaces
© DHI
• moderate inclination:
Layer Elevation
• steep inclination:
step layout

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Layered Meshes with local remeshing
Creating Computation Meshes
© DHI

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Pinching
© DHI
Topography

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Pinching
© DHI

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Local Re-Meshing
© DHI

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Non-Layered (Fully Unstructured) Meshes
Creating Computation Meshes
© DHI

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© DHI

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© DHI

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Non-Layered Meshed
© DHI
• Geological contacts (volumes)
• Insets such as well locations

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Non-Layered Meshed
© DHI
Legacy Approach
Structural Geologic Model
Computation mesh for numerical
model

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Interfaces of geologic units
Non-Layered Mesh
© DHI

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Interfaces and geologic units
Non-Layered Mesh
© DHI

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Tetrahedral-meshed geologic units
Non-Layered Mesh
© DHI

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Clipped tetrahedrons
Non-Layered Mesh
© DHI

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Non-Layered Mesh
© DHI
Full tetrahedral mesh

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Challenges
© DHI

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Piecewise Linear Complexes (PLC)
© DHI
http://wias-berlin.de/software/tetgen/plc.html

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The Problem of Self-intersecting Surfaces
© DHI

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Closed and Consistent Volumes
© DHI
Piecewise Linear Complexes (PLCs)

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DHI In-House Solution with TetGen
Example Case
© DHI

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Consideration of Fault Offsets

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Consideration of Fault Offsets
© DHI

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Consideration of Fault Offsets
© DHI
• Case 1: Offset at isolated fault
• Solution: Record and sort all elevation data at faults
+
applicable for isolated faults
-
Not applicable for isolated faults

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Consideration of Fault Offsets
© DHI
• Case 2: Intersecting Faults
Grouping of elements to sectors
+
can represent more generell complex settings

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Consideration of Pinchouts
© DHI
• Pinchouts and outer model hull

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Closing of Remaining Gaps
© DHI
• Ensure closed volumes

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Regional Markers
© DHI
• Bookkeeping for later numerical model

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Entwicklung der Methodik
© DHI
• Result: Set of 3D face objects consistently enclosing all separable
geologic volumes
• ~ 700.000 points
• ~ 500.000 faces

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Geologic Units
© DHI
VIDEO

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Geologic Units
© DHI
VIDEO

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Face Objects
© DHI
• red
: Outer Border
• Light green
: Faults
• blue
: pinch-outs
• Other colors: geologic contacts

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Resulting Mesh
© DHI

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Carbonate Layer at Depth
© DHI

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CGAL Meshing Generator
Computational Geometry Algorithms Library CGAL
© DHI

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CGAL Library
© DHI
• Computational Geometry Algorithms Library CGAL
• Open Source
(www.cgal.org)
• Integrated in Geomodeller (Intrepid Geophysics)

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Pilot Model of Lusitian Over-Thrust
© DHI
27 May, 2019
#53
• Geological model created in
3D Geomodeller
• Incl. Major geologic units
• Incl.
vertical
faults
and
Lusatian overthrust

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Unstructured mesh generated
Tetrahedron mesh created using CGAL
© DHI
27 May, 2019
#54

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© DHI
27 May, 2019
#55
Faults require a finite thickness
• Conduit
• Barrier
Handling of Faults
Local refinement

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Handling of Faults
© DHI
27 May, 2019
#56
Nodal connections
Test24.fem

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Mesh Refinement
Accuracy vs. Computational Speed
© DHI
27 May, 2019
#57
more refinement
more refinement
Elements: 669.910
Nodes: 114.274
Elements: 723.628
Nodes: 124.077
Elements: 1’201.954
Nodes: 201.930

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Optimum for Geometric Representation
© DHI
27 May, 2019
#58

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Optimum for Computation
27 May, 2019
© DHI
#59

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Finally: What are the Benefits?
© DHI

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Flexibility
© DHI

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© DHI

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© DHI

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Thank you!
are@dhigroup.com

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DHI are the first people you should call when you have a tough
challenge to solve in a water environment.
In the world of water, our knowledge is second-to-none, and we strive
to make it globally accessible to clients and partners.
So whether you need to save water, share it fairly, improve its quality,
quantify its impact or manage its flow, we can help. Our knowledge,
combined with our team’s expertise and the power of our technology,
hold the key to unlocking the right solution.
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