Fault asperity and roughness, insight from faults in 3D reflection seismic data

Abstract

The complexity of faults and the bias in the gathered fault data from different sources such as outcrops, reflection seismic data and earthquakes causes uncertainty to the understanding of fault plane geometry at the seismic scale, among other fault geometric attributes. Fault plane roughness and irregularities at different scales are important geometric attributes caused by asperities, which are linked to the fault frictional behaviour and mechanics. Asperities are localized areas with higher strength on the fault plane, which resist the applied stress to a certain limit before the fault ruptures. Imaging faults’ real geometry can help us to gain more insight into the fault roughness and asperities, thereby enhancing our understanding of the mechanics of faulting and earthquakes. In this study, for the first time, we map fault asperities and roughness on seismic scale faults using their 3D structures extracted from reflection seismic data. We have studied 21 normal faults ranging in depth from about 0.5 to 3 km with clastic dominated lithology and some carbonates by extracting and characterizing their true fault plane geometry on 3D reflection seismic data using Gaussian filtered coherency attributes and measuring throw over the entire fault plane. The outline of the mapped asperities is the envelope of internal tip-points between the segments on the fault plane 3D structure. Our results show that in most cases, at the boundary of the asperities, the strike of fault internal segments changes and the throw increases. We have introduced two new attributes characterizing fault plane roughness at seismic scale, notably 2D roughness attribute and segment mean roughness. The variations of these two attributes, i.e. higher values near the asperity boundary, have close link to the locations of asperities on the fault planes.