Regional Processes and Geohazards from U.K. Deep-Atlantic Margin Surveys

Abstract

Abstract 30 kHz side-scan sonar, cores and photographic data, collected during more than 165 days at sea, have been used to characterise over 30,000 sq kilometers of seafloor in the Faeroe-Shetland Channel & Rockall Trough (UK continental margin) in water depths between 100 and 2,200 metres. In addition to the distinct community variations seen either side of the Wyville-Thomson Ridge, deep-water carbonate mounds, pock marks, landslides and contourite deposits were identified. Integrating surveys for surficial geology with environmental studies provides a powerful method for mapping habitats and geohazards over large areas. In this way regional processes that may affect habitats or seafloor stability can be identified and their potential impact upon seafloor developments assessed. Natural changes in deep-water environments can occur within the lifetime of oil-field developments. These changes may appear to be the result of the development activities even though they occur quite naturally. Regional mapping enables site specific studies to be placed into a context of the likelyhood of such changes occurring, thereby mitigating their impact on the operations. This work may be used to model deep water sedimentary processes for both geo-hazard and environmental studies. A CD-ROM of the side-scan sonar data, a selection of the underwater photographs and the analytical results from the AFEN surveys are available free of charge from www.geotek.co.uk/afen. The same strategy could be used to identify deep-water targets elsewhere, world-wide. Introduction Surficial geological hazards of interest to rig site surveyors often indicate sediment processes that are important in Environmental Impact Assessments (EIAs). The purpose of this paper is to describe environmental surveys conducted in the deep-water UKCS and bring them and their potential value to the attention of geologists concerned with geohazards. The Atlantic Margin Environmental Network (AFEN, a consortium including most of the UK deep water operators and government regulators) sponsored surveys using a 30 kHz deep-towed side-scan sonar. Images were obtained giving 100% coverage of all of the deep water areas licensed in the UKCS. In addition to this sediment cores and underwater photographs were obtained to observe and verify sea floor habitats. The extensive coverage provided by these surveys has given operators an unprecedented resource of information on potential hazards such as submarine landslides, canyons, mobile sands and pock marks. Background Increasingly, operators are being asked to demonstrate that their activities did not affect the environment. In other words, they need to demonstrate that, post drilling, there were no damaging impacts on the environment. Unfortunately, some natural processes in deep water can have such a dramatic effect on the benthic environments that, were they not identified as a natural process (or event), severe changes in benthic ecology could be blamed upon the operator. Processes that can do this usually involve sediment transport or rapid changes in water column structure. In addition to these, features such as pock marks, and cold-seep communities may be classified as Special Areas of Conservation (SAC). Such classification means that operators need to take special measures to ensure their protection.