HYDRO 2016 Paper 2A3

Spatial and temporal analysis of gas seep activity in Eckernförde Bay and assessment of its linkage to pockmark morphology and sub-bottom strata using marine acoustic methods

Arne Lohrberg


Highly elevated methane concentrations in Eckernförde Bay bottom waters during campaign AL447 raised attention. Earlier studies focused on pockmarks and groundwater seepage to be the main factor controlling methane concentration in the water column. This thesis presents high-resolution bathymetry data for three pockmark clusters, high-frequency sub-bottom profiles for methane-rich sediments, a spatial activity distribution grid of gas seepage and a time series of in situ monitored gas seepage events. It aims to (1) analyse pockmark morphology, (2) estimate the spatial distribution of shallow gas accumulations, (3) examine the spatial and temporal activity of gas seepage, (4) find possible trigger mechanism and (5) estimate a gaseous methane flux to the water column of Eckernförde Bay.

High-resolution bathymetry data indicate the formation of micro-scale structures on the pockmark floor of 20 to 50 cm depth and less than 5 m in diameter. Comparison with bathymetric data acquired eight years earlier suggests a stability of the pockmark rims over decades.

Shallow sub-bottom data suggest the presence of methane accumulations in the sediments at water depths exceeding approximately 20 m surrounding the biggest pockmark close to shoal Mittelgrund. Surface sediment methane concentrations appear to be higher at pockmark rims and the pockmark floor and lower in the central part of the bay.

Single beam data show gas seepage to occur in wide areas of the bay, especially in the southwestern extent of the survey area. Gaseous methane flux estimations using gas bubble ebullition rates derived from in situ monitoring and radius estimations derived from single-beam echo sounding suggest a significant contribution to methane concentrations in the water column.

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