An integrated workflow for parametrization of fracture network geometry in digital outcrop models

Mesoscale fractures, with lengths between meters and tens of meters, cannot be effectively characterized in the subsurface, due to limitations of borehole and geophysical datasets. On the other hand, large quantitative structural datasets can be collected on outcrops by combining direct observations and remote sensing (digital outcrop models – DOMs). These data can be used to constrain geometrical models of subsurface fracture networks with the outcrop analogue approach. In this contribution we present a workflow that leverages DOMs with at least two perpendicular faces and combines multiple types of input data (point cloud, textured surfaces and orthophoto DOMs), to collect a suite of statistical parameters to be used as input in current stochastic 3D DFN (Discrete Fracture Network) models. Orientation data are collected with a semi-automatic procedure applied to point cloud DOMs of the vertical side of the outcrop to extract 2D polygonal facets. Fracture sets are defined with a clustering procedure and different orientation distributions are fitted and tested with goodness-of-fit tests. Fracture traces are digitized on textured surface or orthophoto DOMs. Topological parameters are calculated on the digitized fracture network on horizontal and vertical orthomosaics, also considering relationships between fractures and bedding. Trace length and height distributions are estimated with an innovative approach, accounting for the censoring bias with survival/reliability analysis. P21 (ratio between total fracture length and sampling area) is measured from traces digitized on the large horizontal outcrop, also allowing for the Representative Elementary Area (REA) to be assessed. Even if the height/length ratio cannot be measured on an outcrop by any means, we attempt to relate heights and lengths under the assumption that the two datasets are correlated, with the longest fractures being also the tallest. We discuss the applicability of our workflow on a large high-quality fractured limestone outcrop in the Murge Plateau near Altamura (Puglia, Italy).