Spatial and layer-controlled variability in fracture networks

Abstract

14Topological sampling, based on 1) node counting and 2) circular sampling areas, is 15 used to measure fracture intensity in surface exposures of a layered limestone/shale 16 sequence in north Somerset, UK. This method provides similar levels of precision as 17 more traditional line samples, but is about 10 times quicker and allows characterization 18 of the network topology. Georeferencing of photographs of the sample sites allows 19 later analysis of trace lengths and orientations, and identification of joint set 20 development. 21ANOVA tests support a complex interaction of within-layer, between-layer and 22 between-location variability in fracture intensity, with the different layers showing 23 2 anomalous intensity at different locations. This variation is not simply due to bed 24 thickness, nor can it be related to any obvious compositional or textural variation 25 between the limestone beds. These results are used to assess approaches to the 26 spatial mapping of fracture intensity. 27

How do we see fractures? Quantifying subjective bias in fracture data collection

“…The fracture density, intensity, and an estimate of mean trace length for the scanline can be calculated from the n and m values (Mauldon et al, 2001). To be statistically valid the number of fracture end points (m) should exceed 30 (Rohrbaugh et al, 2002), however, values between 20 and 30 can also be considered reliable (Procter and Sanderson, 2017). This rule defines the radius of the scanline as a function of fracture density and limits the use of the technique in areas of poor exposure and low-density fracture networks.…”

“…Fracture topology describes a fault or fracture network as a series of branches and nodes (Manzocchi, 2002;Sanderson et al, 2018;Procter and Sanderson, 2017;Sanderson and Nixon, 2015;Laubach et al, 2018). A branch is a fracture trace with 30 a node at each end that can be classified as terminating into rock at i-nodes (unconnected terminations), abutting against another fracture at a y-node, or crossing another branch at an x-node.…”

“…the nodes present within the circular window and using the sum of i-and y-nodes as the number of trace end points (m-value) in the circle (Procter and Sanderson, 2017). The ratio of node types is plotted on a triangular diagram (Manzocchi, 2002;Sanderson and Nixon, 2015).…”

“…participants counted the number of intersections with the circumference (Nc). Following the methodology of Procter and Sanderson (2017), participants were asked to identify the number of i-, y-and x-nodes within the circles. Finally, window sampling was conducted by tracing out the fracture networks on photographs of the circular scanlines in the workshops.…”

“…For circular scanlines it is widely agreed that a minimum of 20-30 fracture terminations within a circle is appropriate to derive 20 fracture statistics or undertake topological sampling, and the circle radius must be adjusted to capture enough fractures or fracture terminations (Procter and Sanderson, 2017;Rohrbaugh et al, 2002). Figure 10 shows the proportions of valid (capturing >30 terminations) and invalid (capturing <20 terminations) results for the circular scanlines.…”