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