Distribution, morphology and acoustic characterisation of a gas pockmark field on the malin shelf, NW Ireland

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Monteys, Xavier۱; Hardy, David۱; Doyle, Eibhlیn۱; Garcia-Gil, Soledad۲
Holding Date 08 September 2008

High frequency acoustics and shallow geophysical data have been used to characterise a recently discovered pockmark field of over 200 pockmarks in ca. 600km2 on the Malin Shelf, northwest Ireland. Swath sonar bathymetry reveals sub-conical depressions on the seabed surface, between 140 and 180m, interpreted as gas pockmarks. Shallow seismic profiling has shown gas related features (such as masking, disturbed sediment and enhanced reflectors) in association with these pockmarks and throughout the study area. Typically, the pockmarks are discrete, 50-350m in diameter and 0.5-5m deep. Two larger pockmarks are composite, with dimensions up to 650m in diameter and 8m depth.
Pockmark distribution is observed to correlate with the main structural trends of the region, suggesting a thermogenic origin for fluid flow and structural constraints as a primary control on pockmark distribution. A higher density of pockmarks, with increased dimensions, are observed in the central part of the basin in association with local igneous intrusions. Amplitude analysis of multibeam and singlebeam echosounder data indicates that variations in near surface sediment properties plays a secondary role in the morphology and size of pockmarks. The multibeam backscatter response at 95 kHz in the region is generally very low, without apparent variation between the craters and the surrounding seafloor, suggesting the surface sediment drape is uninterrupted. These results are in concordance with the sediment samples taken from the area. Pockmark maximum length is strongly positively correlated to pockmark depth (r=0.8), and moderately positive correlated to water depth (pressure; r=0.31).