Identifying Conventionally Sub-Seismic Faults in Polygonal Fault Systems: An Ultra-High-Resolution Geophysical Study

Polygonal Fault Systems (PFS) are non-tectonic faults prevalent in sedimentary basins globally and represent potential fluid pathways. However the characterization of these pathways is subject to the limitations of conventional 3D seismic imaging; only capable of resolving features on a decametre scale horizontally and metres scale vertically. While outcrop and core examples can identify smaller features, they are limited by the extent of the exposures. The disparity between these scales results in a resolution gap allowing smaller faults to be lost, leaving potential pathways unseen.

Polygonal faults were first identified in the London/Ieper Clay, within much of London it is a common bedrock that bears construction and infrastructure foundations. Within the Southern Bight of the North Sea, it approaches the seafloor, thereby allowing for direct analysis of PFS surface expressions. Through the use of high-resolution 1m bathymetric imaging in combination with boomer 2D seismic imaging fault structures have been identified, with additional support from an ultra-high-resolution 3D seismic volume.

Surface expressions of over 1400 faults within the London Clay have been identified, with additional faults identified within the seismic lines/volume. The combination of these datasets allowing for pseudo-3D as well as true 3D characterisation of the PFS at less than decametre scales. Within all datasets, the faults length range from 8m to 926m, with offsets of 0.5m to 2m, scales that would typically be sub-seismic for conventional basin seismic imaging. These fault networks are directly compared to conventional 3D seismic data of the Ieper Clay from the offshore Dutch sector where it exists approximately 700m below the seafloor. Here PFS present with typical attributes, lengths of hundreds of metres to kilometres with throws of tens of metres. A magnitude larger than those identified in the Southern Bight. The similar polygonal patterns within both locations indicates that the smaller faults exist within typical PFS structure but are invisible to conventional imaging techniques. These unseen faults could create additional unseen pathways that affect construction in London via ground stability and influence fluid migration within basins when the additional connectedness of these fault networks is considered.