User Guide
NetworkGT consists of 5 seperate steps that involve:
- Digitising and correcting an interpretation to produce a topologically consistent fracture network.
- Sampling strategy to analyse a fracture network.
- Geometrical analysis of a fracture network fracture length distributions, orientations, sets and line frequency.
- Topological analysis of a fracture networks topology, clusters, blocks and connectivity.
- Flow assessment of a fracture network including aperture, permeability, percoloation, 1D and 2D flow simulation.
NetworkGT workflow from digitising, sampling, geometrical analysis, topological analysis and fluid flow assessment
Make sure to first install the NetworkGT plugin for QGIS which is explained in detail here.
To use the NetworkGT tools, a fracture network needs to be digitzed either manually as shown in this tutorial here or by more advanced automated options. It is important that the dataset is in a projected coordinate system (e.g. UTM) in order to properly analyze geometrical and topological parameters. This tutorial is designed to show the main tools and general workflow of the NetworkGT plugin using the Hartland Fracture network dataset.
Note - It is important to realize that the tutorial does not cover all the available tools (listed above) and does not discuss the theoretical background and considerations that should be used in selecting the different values used in each algorithm.
If you encounter an issue, please report those issues here after checking the already resolved/closed issues.
For this example, we will use the Hartland Fracture network dataset available for download here. This dataset contains two files, 1) the fracture network delineations that map the fracture lines as individual polylines, and 2) the delineations showing the extent of the observable fractures.
Once downloaded, drag and drop the contents within the QGIS map. You may be asked to transform the datasets on import, select OK.
A good habit in analyzing any new dataset is to get an initial overview of the entire study region by analyzing the geometry and topology. To do this we will first explore the geometry of the Hartland Fracture Network:
The first logical geometrical parameter to analyze is the distribution analysis. This plots the cumulative line frequency against fracture length using different size distributions (e.g., negative exponential, power-law, normal, log-normal). Open the 'Distribution Analysis' tool found within the 3. Geometry toolbox and use the default settings:
1. Fracture Network -> Fracture Network
2. Click Run
In the Hartland Fracture Network, we observe a power-law distribution in the size of fractures. Note that in the QGIS dialog window we also receive summary statistics for that fracture network.
Another important parameter to analyze is the Rose Diagram that may provide further insightful to classify sets of different fracture lines. In this example, open the 'Rose Diagram' tool found within the 3. Geometry toolbox and select the following options:
1. Fracture Network -> Fracture Network
2. Equal Area -> checked
3. Percentage -> checked
4. Click Run
In this initial overview, we can perhaps deduce that the Hartland Fracture Network has at least two sets with the main set occuring in the 145 degree bin. These sets may be defined as an attribute to each fracture line by using the 'Define Sets' tool.
To prepare for our contour grid analyses below, we will look at the 'Line Frequency' plots. The line frequency is a cumulative measure of how frequently a fracture line intersects a sample line. The first step in creating a line frequency plot is to create a line grid to sample against the Hartland Fracture Network. To achieve this, open the 'Line Grid' tool found within the 2. Sampling toolbox and select the following options:
1. Fracture Network or Interpretation Boundary -> Fracture Network
2. Spacing -> 500
3. Rotation -> 55
4. Click Run
This will create a series of lines at a 55 degree angle at a 500m spacing. The 55 degree angle was chosen considering it is perpendicular to the 145 degree bin observed from the rose diagram plot.
Next we will plot the actual fracture frequency along those grid lines by using the 'Line Frequency' tool found within the 3. Geometry toolbox. Use the following parameters:
1. Line Grid -> Line Grid
2. Fracture Network -> Fracture Network
3. Combine Plots -> Checked
4. Click Run
This will create one line frequency plot at each line. Note that sample #4 running through the center of the map has a mean spacing of roughly 160meters.
Now that we have established an inital set of geometrical analyses, we will analyse the topology of the fracture network.
The topology of a fracture network shows information about the arrangment of fractures within a network and its connectivity. To create this information, we first need to split the dataset into a series of branches and nodes. This is achived by the 'Branches and Nodes' tool found within the 4. Topology toolbox. Use the following options:
1. Network -> Fracture Network
2. Sample Area -> Interpretation Boundary
3. Interpretation Boundary -> Interpretation Boundary
4. Run
The sample area lets the algorithm know which area to analyze. The interpretation boundary shows the maximum observable extent of mapped fractures which is important in order to statistically account for the unknown information outside that region.
Note -If the tool runs but produces inconsistent topological analyses, then check out the topological processing section to pre-process your dataset.
Once the branches and nodes have been created, we can analyze the statistics and plot the information on ternary plots using the 'Topology Parameters' tool found within the 4. Topology toolbox. Use the following options:
1. Nodes -> Nodes
2. Branches -> Branches
3. Sample Areas -> Interpretation Boundary
4. Plot -> Checked
Great now we have an initial topological analysis of the Hartland Fracture Network with statistics available in the attribute table of the new vector layer. An initial view of the ternary plots show that the connectivity of the fracture network is rather low (~1 connections/branch). We would expect a fracture network to be fully connected at roughly 1.56.
While the steps above helped us to get an understanding of the study region, we may be interested in more detail on spatial distribution of geometrical and topological parameters within a fracture network. This is also an important step towards simulating fluid flow across a fracture network.
The contour grid creates an equally spaced rectangular grid for visualization with each grid containing search radius from it centroid point. This search radius is used to create a circular overlapping sampling area to measure the spatial geometrical and topological variability within a fracture network.
To create this grid, use the 'Contour Grid' tool found under the '2. Sampling' toolbox. Use the following options:
1. Fracture Network or Interpretation Boundary -> Fracture Network
2. Spacing -> 100
3. Radius -> 320
4. Grid Within Interpretation Boundary -> Checked
5. Run
This says that each grid will be 100mx100m with a search radius from each centroid of 320m which is 2x the fracture network spacing calculated above.
Note - We recommend starting with a search radius that is 2x the fracture network spacing although this needs to be adjusted on a case to case basis.
Next we will recalculate the branches and nodes for the region using the contour grid as our sampling area. Find the 'Branches and Nodes' tool under the 4. Topology toolbox and use the following options:
1. Network -> Fracture Network
2. Sample Area -> Grid
3. Interpretation Boundary -> Interpretation Boundary
4. Run
Rerun the 'Topology Parameters' tool using the contour grid as the sampling area. Find the 'Topology Parmeters' tool under the 4. Topology toolbox and use the following options:
1. Nodes -> Grid Nodes
2. Branches -> Grid Branches
3. Sample Areas -> Grid
4. Run
Now we have a contour grid which displays the spatial distribution of a wide variety of parameters including the number counts of different node and branch types, connections per branch, connections per line, average branch length, average line length, fracture intensity, dimensionless intensity. The initial display shows the connections per branch which is a good indicator for the connectivty of the fracture network (red - high, green - low). For the Hartland Fracture Network, we see that the connectivity is rather poor.
Under construction...
This section contains a list of external scripts or methodology that build on the NetworkGT plugin. We are not the developers of these packages and cannot guarantee that they are updated or work as intended.
- Georef Fracture Traces - Project 3D fracture networks onto a 2D georeferenced plane for use in NetworkGT.
- Fracture Analysis 2D - Cacluate cross-cutting and abutting relationships from the results obtained by NetworkGT.