The significance of pockmarks to understanding fluid flow processes and geohazards

Abstract

Underwater gas and liquid escape from the seafloor has long been treated as a mere curiosity. It was only after the advent of the side‐scan sonar and the subsequent discovery of pockmarks that the scale of fluid escape and the moon‐like terrain on parts of the ocean floor became generally known. Today, pockmarks ranging in size from the ‘unit pockmark’ (1–10 m wide, < 0.6 m deep) to the normal pockmark (10–700 m wide, up to 45 m deep) are known to occur in most seas, oceans, lakes and in many diverse geological settings. In addition to indicating areas of the seabed that are ‘hydraulically active’, pockmarks are known to occur on continental slopes with gas hydrates and in association with slides and slumps. However, possibly their potentially greatest significance is as an indicator of deep fluid pressure build‐up prior to earthquakes. Whereas only a few locations containing active (bubbling) pockmarks are known, those that become active a few days prior to major earthquakes may be important precursors that have been overlooked. Pockmark fields and individual pockmarks need to be instrumented with temperature and pressure sensors, and monitoring should continue over years. The scale of such research calls for a multinational project in several pockmark fields in various geological settings.

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

“…They occur in different shapes e.g. as circular craters or as non-circular, 45 elongated forms (Hovland et al, 2002). Sizes generally range from 10 to 200 m with 46 depths up to 35 m, but also giant pockmarks with diameters of up to 1 km or more 47 have been reported (Cole et al, 2000;Ondréas et al, 2005).…”

“…It can be controlled by underlying 65 bedrock and lithological boundaries (Forwick et al, 2009), but can also be triggered 66 by extreme events like earthquakes and tsunamis, melting of grounded icebergs or 67 even by human influence (Hovland et al, 2002). 68…”

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

“…In Belfast Bay and Passamaquoddy Bay, pockmark chains occur at the abrupt thinning of Holocene sediment related to antecedent geology, glacial deposits and paleochannels. Hovland et al (2002) noted that pockmark chains often nucleate along zones of weakness where fluid flow focuses. The tens of meters of relief over a relatively short horizontal difference (e.g., 200 m) that typifies the regions' underlying topography may set up conditions for focused fluid expulsion capable of forming pockmarks.…”

Fabric Development and Pore-Throat Reduction in a Mass-Transport Deposit in the Jubilee Gas Field, Eastern Gulf of Mexico: Consequences for the Sealing Capacity of MTDs

“…Moreover, the presence of an interpreted gas chimney or pipe overlying one of the pockmarks is evidence of active or past fluid migration (Heggland, 2005) (Figure 4.12). Although the pockmarks at the Jubilee field are buried more than 2 km below sea floor, when near the surface they can be a geohazard indicator (Hovland et al, 2002). …”