Automatic pockmark detection in the Norwegian Channel

Pockmarks are widely distributed in areas with fine-grained layered sediments. We utilised a large multibeam bathymetric dataset covering an area of c. 15,000 km2 of the Norwegian Channel (NC), the western slope and adjacent shelf outside western Norway. Pockmarks were extracted from the bathymetry data with two approaches: by identifying local minima in the bathymetry and by mapping landform types based on geomorphons. While the former approach yielded a point dataset indicating local minimum depths, the latter approach allowed to outline potential pockmarks as polygons based on the landform types of pits and valleys. To increase the reliability of the classification, only pockmark polygons that contained at least one local minimum were subsequently retained. This mainly removed artefacts at the edges of the classified area. Likewise, only those local minima that fell inside a pockmark polygon were retained. Finally, a limited number (<1%) of polygons incorrectly mapped as pockmarks was manually removed. 

Approximately 65,000 pockmarks were automatically detected inside the study area. The highest pockmark densities were located in the western slope of the NC. Here, an extensive pattern of elongated pockmarks was found, indicating strong bottom currents over the area.  

The study area is located in the Viking Graben area with the Øygarden Fault zone to the east. The stratigraphy comprises dipping Mesozoic and Cenozoic clastic sediments over a Paleozoic or crystalline basement. On top of these layers an Upper Regional Unconformity (URU) appears. Above the URU, which forms the base of the NC, flat lying units of glacial (till) and marine sediments are found. Above these layers of late-glacial and Holocene sediments up to a few tens of metres appear.   

The gigantic Troll hydrocarbon field is located in the northern part of the study area, and several studies have documented that there is no active fluid seepage today, so the pockmarks are thought to have been formed by gas hydrate dissociation under/after the last deglaciation.