Acoustic and physical characteristics of gassy sediments in the western Irish Sea

Abstract

High resolution sub-bottom profiles acquired in the western Irish Sea have revealed extensive areas of acoustic turbidity which are probably caused by a high content of gas in the fine grained Holocene sediments. These turbid sediments basically lie within two well-defined large areas, with a few smaller pockets in between, and with the upper surface being between 4 and 10 m below the seabed surface. The largest area measures about 40 km by 56 km and lies to the southwest of the Isle of Man. The seismo-acoustic characteristics of these sediments have been analysed in terms of seismic reflection patterns and seismic facies. The predominant feature can be defined as “acoustic blanking”. Several other acoustic features attributable to high gas concentration have been noted, including plumes, pockmarks and seabed doming, all of which are indicative of migrating gas. Acoustic logging of cores collected in the area has in one instance produced propagation velocities as low as 1200 m s −1. More detailed measurements carried out in a geophysically instrumented consolidation cell have shown that the compressional wave velocity can in fact fall off with depth due to the effect of gas. The geographic distribution of the gas bearing sediments can be explained in terms of local geological control, and present and former environmental conditions.