Short-duration events associated with active seabed methane venting: scanner pockmark, North Sea

Bayrakci, Gaye, Bull, Jonathan M., Minshull, Timothy A., Robinson, Adam H., Lavayssiere, Aude, Illsley-Kemp, Finnigan, Henstock, Timothy J., MacDonald, Calum and Chapman, Mark (2024) Short-duration events associated with active seabed methane venting: scanner pockmark, North Sea. In, Bayrakci, Gaye and Klingelhoefer, Frauke (eds.) Noisy Oceans: Monitoring Seismic and Acoustic Signals in the Marine Environment. (Geophysical Monograph Series, 284) 1 ed. John Wiley & Sons, pp. 143-159. (doi:10.1002/9781119750925.ch09).

Abstract

The process by which methane propagates through near-surface sediments and is vented into the ocean is uncertain. Here, we describe seismic events (short-duration events [SDEs]) recorded on ocean-bottom seismometers (OBSs) close to an actively venting pockmark at a water depth of 150–165 m. Analysis of 120 hours of data recorded by 10 OBSs showed natural events with consistent characteristics: monochromatic, with a peak frequency of 4 Hz and relatively short duration (less than 6 seconds); an absence of clear body- or surface-wave arrivals; spectral content that decreases in frequency over the event duration; and larger amplitudes on horizontal components than on the vertical component, indicating a shallow subsurface origin. Only the SDEs with the largest geophone amplitudes are recorded on the hydrophones, suggesting that the events originate in the seabed rather than in the water column. In the pockmark, clusters of events are recorded simultaneously on two OBSs 20 m apart; but more generally, events are not recorded on multiple instruments, indicating the very local nature of the events. The rate of occurrence of the SDEs is negatively correlated with tidal height and total water depth, indicating a hydrostatic pressure control.

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Contributors

Author: Jonathan M. Bull

Author: Timothy A. Minshull

Author: Adam H. Robinson

Author: Finnigan Illsley-Kemp

Author: Timothy J. Henstock

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