NetworkGT: A GIS tool for geometric and topological analysis of two-dimensional fracture networks

Abstract

Fractures rarely occur individually but more usually as networks of numerous fractures whose arrangement, abundance, and interaction control the mechanical and transport properties of rock masses. Of particular importance are the distributions and spatial variations of different geometric (locations, orientation, length, etc.) and topological (intersections, connectivity, etc.) attributes of fractures in a network. Geographical Information Systems (GIS) provide a means to map and digitize two-dimensional fracture networks from a variety of field and remote sensing data and to display the results in the form of quality maps. We introduce NetworkGT, an open-source toolbox for ArcGIS capable of efficient sampling, analysis, and spatial mapping of geometric and topological attributes of two-dimensional fracture networks. The toolbox helps to extract and plot geometric and topological information from a given two-dimensional fracture network including: rose diagrams, plots of frequency distribution and topology, and maps of topological parameters. Using a fracture network example from offshore NW Devon, United Kingdom, we illustrate the practicality and effectiveness of the toolbox. This includes computing a contour grid with 1326 subsampled regions within the fracture network, which is used to demonstrate the quantitative capabilities of the toolbox and the ability to spatially map important network properties. The toolbox will help to facilitate the increasing application of geometry and topology in the analysis and comparison of fracture networks at a range of scales. Furthermore, the integration of the NetworkGT toolbox into ArcGIS allows two-dimensional fracture networks to be interpreted, mapped, and fully analyzed within the same software package.

Using network topology to constrain fracture network permeability

“…This was then stitched into a single image and imported into ArcMAP in order to digitise the fracture network. The NetworkGT toolkit developed by Nyberg et al (2018) provides an automated method for developing the nodal topology of a digitised fracture network, reducing the time and error in digitising node and branch types manually. Using this toolkit, a 50 cm sampling grid was generated, and the full range of topological parameters discussed in Sanderson and Nixon (2015) was generated for each grid square across the Castle Cove Fault fracture network, allowing for spatial analysis of topological variation within the fracture network.…”

“…Both in assessing the networks as an analogue for potential to percolate fluids, and in applying network topology to constrain the nature and extent of a fault damage zone. The NetworkGT tool of Nyberg et al (2018) allows for the generation of a high-resolution cross-sectional analysis of spatial variation of network topology parameters for a fault damage zone. This method allows relatively quick and easy production of large datasets of fracture populations, topology, and geometry.…”

Mapping the fracture network in the Lilstock pavement, Bristol Channel, UK: manual versus automatic

“…Several sampling techniques have been developed to extract data about fracture sets that do not necessarily require a complete mapping of the whole area, e.g., line sampling (Priest and Hudson, 1981), polygon or areal sampling (Wu and Pollard, 1995), circular scan-line sampling (Mauldon et al, 2001;Rohrbaugh et al, 2002;Watkins et al, 2015), or rectangular window sampling (Pahl, 1981).…”

The role of pre-existing jointing on damage zone evolution and faulting style of thin competent layers in mechanically stratified sequences: a case study from the Limestone Coal Formation at Spireslack Surface Coal Mine

“…Define sets: Six 'Interpretation boxes' were added as shape files to the ArcGIS (three along the 123 dip-slope and three along the high wall) and the orientation of faults and the fractures within 124 them analysed. Length-weighted rose diagrams with 5° bin widths were used to interpret the 125 'orientation sets' in the network using NetworkGT (Nyberg et al, 2018). The digitised fault and 126 fracture data sets were then combined using the merge function in ArcGIS, and all three 127 investigated separately.…”

“…128 2. Branch & Nodes: The topology of the network was extracted using the 'Branch and Node' tool, 129 which splits the fracture trace poly-line file into individual branches, and assigns nodes as a 130 separate point-files (Nyberg et al, 2018). The resulting network was visually checked for errors 131 (e.g.…”