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

Abstract

Abstract. The 100 000 m2 wave-cut pavement in the Bristol Channel near Lilstock, UK, is a world-class outcrop, perfectly exposing a very large fracture network in several thin limestone layers. We present an analysis based on manual interpretation of fracture generations in selected domains and compare it with automated fracture tracing. Our dataset of high-resolution aerial photographs of the complete outcrop was acquired by an unmanned aerial vehicle, using a survey altitude optimized to resolve all fractures. We map fractures and identify fracture generations based on abutting and overprinting criteria, and we present the fracture networks of five selected representative domains. Each domain is also mapped automatically using ridge detection based on the complex shearlet transform method. The automatic fracture detection technique provides results close to the manually traced fracture networks in shorter time but with a bias towards closely spaced Y over X nodes. The assignment of fractures into generations cannot yet be done automatically, because the fracture traces extracted by the automatic method are segmented at the nodes, unlike the manual interpretation in which fractures are traced as a path from fracture tip to fracture tip and consist of several connected segments. This segmentation makes an interpretation of relative age impossible, because the identification of correct abutting relationships requires the investigation of the complete fracture trace by following a clearly defined set of rules. Generations 1 and 2 are long fractures that traverse all domains. Generation 3 is only present in the southwestern domains. Generation 4 follows an ENE–WSW striking trend, is suborthogonal to generations 1 and 2, and abuts on them and generation 3, if present. Generations 5 is the youngest fracture set with a range of orientations, creating polygonal patterns by abutting at all other fracture generations. Our mapping results show that the northeastern domains only contain four fracture generations; thus, the five generations of the outcrop identified in the southwestern domains are either not all present in each of the five domains or vary locally in their geometry, preventing the interpreter from linking the fractures to their respective generation over several spatially separate mapping domains. Fracture intensities differ between domains where the lowest is in the NE with 7.3 m−1 and the highest is in the SW with 10 m−1, coinciding with different fracture orientations and distributions of abutting relationships. Each domain has slightly different fracture network characteristics, and greater connectivity occurs where the development of later shorter fractures is not affected by the stress shadowing of pre-existing longer fractures.

Combined Methodology for Rockfall Susceptibility Mapping Using UAV Imagery Data

“…The choice of method for determining cracks as well as tracing is based on a compromise between the required accuracy and detail and the absence of determination errors and processing costs [41,43]. Manual techniques are reasonable when the size of the study site is relatively small or high accuracy is required, while using them on large areas is time-consuming and impractical [44,45]. Thiele et al [46] compared the accuracy of manual fracture tracing, semi-automatic, and automatic techniques.…”

The influence of subseismic-scale fracture interconnectivity on fluid flow in fracture corridors of the Brejões carbonate karst system, Brazil

“…Using the open-source software QGIS 3.12v, fractures were identified and digitized on all outcrops and traced as polylines from tip to tip while keeping their original geometry (Weismüller et al, 2020). As a result, each fracture is represented by a trace, from where spatial parameters, such as orientation, trace length, intensity and density, can be measured.…”

The joint sets on the Lilstock Benches, UK. Observations based on mapping a full resolution UAV-based image

“…Ground truthing was done against sub-mm resolution photographs of selected locations on the surface to validate our identification of all joints, which are enhanced in visibility by wave erosion. Further details on the method used are published in Weismüller et al, (2020a) and the original orthomosaic is available in Weismüller et al, (2020b) .…”

“…The present paper is part of three publications using the dataset (Weismüller et al, 2020a, b). Weismüller et al (2020a) compares complete fracture maps from manual and automatic tracing methods, analyses geometry and topology of the fracture networks and provides an evolutionary model based on age relationships similar to the ones presented here. Weismüller et al, (2020b-open access) presents the orthomosaic we used for joint interpretation to allow verification of our results.…”