Geomorphometric Characterization of Pockmarks by Using a GIS-Based Semi-Automated Toolbox

Abstract

Pockmarks are seabed depressions developed by fluid flow processes that can be found in vast numbers in many marine and lacustrine environments. Manual mapping of these features based on geophysical data is, however, extremely time-consuming and subjective. Here, we present results from a semi-automated mapping toolbox developed to allow more efficient and objective mapping of pockmarks. This ArcGIS-based toolbox recognizes, spatially delineates, and morphometrically describes pockmarks. Since it was first developed, the toolbox has helped to map and characterize several thousands of pockmarks on the UK continental shelf, especially within the central North Sea. This paper presents the latest developments in the functionality of the toolbox and its adaptability for application to other geographic areas (Barents Sea, Norway, and Malin Deep, Ireland) with varied pockmark and seabed morphologies, and in different geological settings. The morphometric characterization of vast numbers of pockmarks allows an unprecedented statistical analysis of their morphology. The outputs from the toolbox provide an objective, quantitative baseline for combining this information with the geological and oceanographical knowledge of individual areas, which can provide further insights into the processes responsible for their development and their influence on local seabed conditions and habitats.The variability of size, spatial distribution, and geometry results from their development depending on a variety of parameters such as fluid type, flow fluxes, thickness and nature of near bottom sediments, underlying structure, and lithology.Most of the initial descriptions of these features were based on low penetration seismic profiles (mainly Boomer system) and/or from sidescan sonar data (e.g., [1,4,8]). Presently, pockmarks are predominantly mapped manually using seabed digital terrain models (DTM) created from MBES data (e.g., [6]), generally a time-consuming task. In addition, delineating their individual boundaries is subjective and consistency of criterion is hard to achieve. To address this, the British Geological Survey (BGS) developed a semi-automated mapping toolbox. This ArcGIS-based BGS Seabed Mapping Toolbox recognizes, spatially delineates, and morphometrically describes seabed features including pockmarks [9], coral mounds [10], and other confined features. The toolbox is embedded in ESRI ® ArcGIS, a geographic information system (GIS) widely used in the field of marine geology, allowing users to work in a familiar and integrated mapping environment. Furthermore, the scripts developed use standard ESRI ® algorithms that increase the clarity of the steps taken during delineation and characterization processes. With this approach, human interaction and expert knowledge is still part of the mapping process but is limited to restricting criteria for feature mapping. This allows multiple mapping exercises to be performed with the same criterion, improving comparisons across different areas or quantification of seabed changes ove...

Distribution and classification of pockmarks on the seabed around western Scotland

“…These limits include: cut-off vertical relief, minimum vertical relief, minimum width, minimum width/length ratio, buffer distance. For a more detailed account of the BGS seabed mapping toolbox see Gafeira et al (2012Gafeira et al ( , 2018. Just as with the previous step, several passes were required in order to assess the correct thresholds that capture the majority of the pockmarks and delineate them most accurately.…”

Pockmarks in the Witch Ground Basin, Central North Sea

“…Three decades of extensive surveying and seafloor mapping in the Witch Ground Basin has revealed abundant “normal” pockmarks and multiple “unusually large” pockmarks. These pockmarks indicate significant flow of fluids from shallow marine sediments (Gafeira et al, ; Hovland & Sommerville, ; Judd et al, ; Pfannkuche, ). The term normal pockmark describes pockmarks that are more than 5 m in diameter and found in isolation where free gas pockets in the subsurface degas cyclically (Hovland et al, ).…”

“…The normal pockmarks formed in the soft, fine‐grained sediments of the Witch Ground Formation and have been identified across the Witch Ground Basin (Gafeira et al, ; Judd et al, ; Long, ; Sejrup et al, ; Stoker & Long, ). Their morphometry deviates strongly from the large pockmarks, as they are mostly less than 3 m deep and 20–40 m wide.…”

“…Their morphometry deviates strongly from the large pockmarks, as they are mostly less than 3 m deep and 20–40 m wide. The density ranges from less than 5 pockmarks per square kilometer at the outer parts of the Witch Ground Basin, to almost 30 pockmarks per square kilometer at the center, where water depth exceeds 150 m (Gafeira et al, ). There is seismic evidence that these normal pockmarks occur in tiers at distinct stratigraphic layers and not only at the surface (Figure a; Stoker & Long, ).…”

Fluids mobilization in Arabia Terra, Mars: Depth of pressurized reservoir from mounds self-similar clustering

“…TPI relies on the difference of elevation between a cell and the average elevation of its neighborhood returning an index which is dimensionless. TPI along with the cell slope value can be used to classify the cells into classes related to different specific morphologies(hills, narrow valleys, plains, etc., Jenness et al, 2006).For example, TPI-based techniques were used to automatically extract pockmarks in the Barents Sea(Gafeira et al, 2018), to map deep seafloor mounds in the Canary Islands basin(Sanchez- Guillamòn et al, 2018) and live biogenic reefs in Mingulay Reef Complex in Scotland(De Clippele et al, 2017). In all these cases the topography surrounding the analyzed features was relatively flat floored making them clearly distinguishable.…”