Semi-automated characterisation of seabed pockmarks in the central North Sea

Pockmarks are indicators of focused fluid seepage, most notably gas such as methane, and can occur in vast numbers in many marine and even in lacustrine environments. The presence and distribution of pockmarks need to be considered in the development of any infrastructure at the seabed. However, manual mapping of these features can be extremely time-consuming and it is implicitly subjective. An extensive area in the central North Sea, where the seabed comprises a thick sequence of muds and sandy muds of the late glacial Witch Ground Formation, shows numerous inactive pockmarks, typically 20-100 m diameter and 3-4 m deep. Within this area a few larger, active pockmarks, 500 m diameter and up to 17 m deep are known. Modern site investigations in this area regularly include multibeam sonar mapping, a technique that collects large volumes of bathymetric data that can be used to produce digital depth models of the seafloor with sufficient resolution to characterize individual pockmarks. This paper presents a semi-automated method to recognize, spatially delineate and characterise morphometrically pockmarks at the seabed. The method comprises two scripts, Pockmark Mapping and Pockmark Characterization, that allow the systematic application of a sequence of well defined tools available within the ESRI ArcGIS toolbox. Almost 4150 pockmarks were mapped applying this method to 18 selected site surveys across the central North Sea. The mapping and morphometric characterization of such vast number of pockmarks allows the identification of certain trends reflecting the hydrodynamic regime, whereas the pockmarks density and spatial distribution appears to be attributable to differences in shallow gas availability and deeper geology controlling fluid migration pathways.