Visualizing and Quantifying 3D Fault Geometries: Digital Outcrop Models of Faults in an Active Coal Mine

Three-dimensional (3D) seismic reflection data have revolutionized our knowledge of faults in sedimentary basins. However, the resolution of such data is limited faults with throws < 30 m are not visible in typical 3D seismic profiles. Consequently, there are significant uncertainties in predicting the subsurface location, geometry and connectivity of small-scale faults and hence the impact these structures have on reservoir performance. A new Joint Industry Project at Durham University is addressing this problem by capturing detailed (cm-resolution) 2.5D Digital Outcrop Models (DOMs) of selected outcrops using terrestrial laser scanning (TLS). TLS is being undertaken at the Dehli Opencast Site, an active coal mine near Newcastle, UK, where both seismic and subseismic scale faults are exposed. Repeated laser scans during active mining allow us to collect detailed topographic and geological data from successive sections through the faulted Late Carboniferous sandstone/shale/coal strata. The site will remain active for 2 years, providing a unique opportunity to capture and visualise the 3D architecture of fault networks on a seismic to subseismic scale. The point cloud data derived from TLS are globally registered using differential GPS enabling us to tie individual scans in a series of geospatially referenced DOMs. These models act as a 3D visualization tool to aid our geological understanding and interpretation of the structures exposed in the mine. In particular, the DOMs allow us to access outcrops that would otherwise be out of reach (e.g. faults exposed on >100m high cliff section). Furthermore, the ability to zoom, tilt and pan through the DOM significantly enhances our interpretations of the fault geometry made whilst in the field, revealing new quantitative information on the 3D geometries of fault branchlines, relays and damage zones. Interpretations of DOMs from the Dehli site will provide insights into how attributes such as fault spacing, connectivity, throw, damage zone width and fault rock distribution vary on different scales. These deterministic data will provide a fully 3D basis for investigating the impact of subseismic scale faults on reservoir performance.