International audienceThe presence of gas is a common feature in many seismic sections. However, the origin of the gas is often difficult to determine. Recently acquired very high resolution seismic profiles using an IKB Seistec boomer provide useful insight to the understanding of the gas origins in a range of environmental settings including sea lochs and coastal lagoons. The gas features are described both from a qualitative point of view through their acoustic facies, and quantitatively through the associated seismic signal (polarisation, amplitude). Acoustic facies include acoustic turbidity, gas "curtains" and "blankets" as well as "white fringes" and "black shadows". All the features encountered have been related to specific gas nature generated by different sources (organic matter degradation in paleo-valley infillings, waste material effluent)
1. The distribution and abundance of reefs of Serpula 6ermicularis was examined in Loch Creran, the only known remaining site of reefs of this species in Scotland. In view of the decline in populations elsewhere, the aims included assessment of the importance of the Scottish population in terms of conservation of the reef habitat and the establishment of a baseline against which future changes could be gauged. A further objective was to determine appropriate conservation management of the reefs by examining the influence of both natural and anthropogenic factors.2. Reefs were largely restricted to a depth range of 1 -13 m. Factors controlling the depth distribution are discussed.3. Serpula 6ermicularis reefs colonized a variety of substrata, but predominantly grew on lamellibranch shells on a seabed of muddy sand. There was some evidence that reef abundance was influenced by the availability of suitable substrata. 4. A belt of scattered reefs fringed much of the coastline of the loch with profuse reef development at two sites, where reef coverage exceeded 10% of the seabed. In terms of total abundance of Serpula 6ermicularis reefs, Loch Creran represents the major world site for reef development. It is concluded that the reefs of Loch Creran are of significant nature conservation importance and grounds for the establishment of conservation management are discussed.5. Reef development is apparently curtailed in areas of strong currents and high flushing rate. There is evidence that human activity has adversely influenced reef growth through the discharge of organic factory effluent and the physical disturbance caused by mooring ground tackle. Scallop dredging also represents a significant threat to the persistence of serpulid reefs. Methods of minimising damage to this rare and fragile feature are discussed.
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