Near seabed geology in a shallow environment in the Malin Sea (Ireland) using a geophysical multidisciplinary approach

Abstract

Background and Aims: During the Irish National Seabed Survey (INSS) in 2003, a gas related pockmark field was discovered and extensively mapped in the Malin Shelf region (NW Ireland). In summer 2006 additional complementary data were acquired in specific locations in the area. Objective: To study near-seabed composition, pathways and presence of gas by using an approach on the basis of shallow EM data, high frequency acoustics and short sediment cores in a known shallow gas bearing area. The combination of these various methods have enabled us correlate the main geophysical signatures and some geological properties of the seabed, providing a unique tool for geohazard identification, multibeam seabed characterization, fluid flow migration paths and sediment porosity. Methods: The multidisciplinary approach consisted in core sample analysis, multibeam and single-beam backscatter classification, and a marine controlled-source electromagnetic survey. Results: The results from the EM survey have been plotted in terms of conductivity and have been integrated in GIS software with other geological and geochemical information for interpretation. The maps of conductivities of the upper 20 m of the sub- seafloor show a high sensitivity to the presence of gas in sediments. At a regional scale, areas with gas charged sediments show a decrease in conductivity, this correlates well with gas related echo-facies (SBP). Although there are areas in which gas echo-facies correlates with high conductivities and we have associated this to the presence of microbial activity feeding on methane. The crossings of the EM instrument over the pockmarks show a very distinctive pattern that shows clearly how most gas escapes along the walls of the pockmarks. The analysis in terms of porosities of the conductivity data (using Archie¿s law) shows well-connected, lower-permeability fluid distribution sediments for the central area of the Malin basin. Conclusions: Good correlation of EM anomalies and gas echo-facies. Increase of conductivity towards the center of the basin, with good layering, corresponding to changes in sediment grain. Areas of positive conductivity anomalies, enhanced reflectivity in SBES and an increase of sandy fraction have been interpreted as indicators of presence of microbial activity, which cause an increase of redox reactions with a release of anions and cations, and therefore an increase of electrical conductivity. Pockmarks have a distinct signature, with a drop of conductivity in the center (gas) and a sharp increase on the walls, interpreted as favored expulsion paths and increased presence of microbial activity.