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 this with automated fracture tracing. Our dataset of high-resolution aerial photographs of the complete outcrop was acquired by 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 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 mapped fracture networks, however, with a bias towards closely spaced Y over X nodes. The assignment of fractures into generations cannot be done automatically yet. Generations one and two are long fractures that traverse our domains. Generation three is only present in the southwestern domains. Generation four follows an ENE-WSW striking trend, is sub-orthogonal to generations one and two and abuts on them and generation 3 if present. Generations five is the youngest fracture set with diffuse orientation, creating polygonal patterns by abutting on all older 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 to link the fractures to their respective generation over several spatially separate mapping domains. The domains have different P21 fracture intensities which is lowest in the NE with 7.31 m/m2 and highest in the SW with 10.01 m/m2, coinciding with different fracture orientations, and distributions of abutting relationships. Each domain has slightly different fracture network characteristics. The network analysis shows that connectivity increases, depending on the evolving fracture generations.
Mapping the fracture network in the Lilstock pavement,
Bristol Channel, UK: manual versus automatic
