Nonseismic Signals in the Ocean: Indicators of Deep Sea and Seafloor Processes on Ocean‐Bottom Seismometer Data
|Author(s)||Batsi Evangelia1, Tsang‐hin‐sun Eve1, Klingelhoefer Frauke1, Bayrakci Gaye2, Chang Emmy T.Y.3, Lin Jing‐yi4, 5, Dellong David1, Monteil Clément1, Géli Louis1|
|Affiliation(s)||1 : Ifremer, Marine Geosciences CS Plouzané, France
2 : University of Southampton, Ocean and Earth Science, National Oceanography Centre Southampton Southampton, UK
3 : Institute of Oceanography, National Taiwan University Taipei ,Taiwan
4 : Department of Earth SciencesNational Central University Taoyuan City ,Taiwan
5 : Center for Environmental Studies, National Central University Taoyuan City ,Taiwan
|Source||Geochemistry Geophysics Geosystems (1525-2027) (American Geophysical Union (AGU)), 2019-08 , Vol. 20 , N. 8 , P. 3882-3900|
|Keyword(s)||marine seismology, gas and fluid migration, Sea of Marmara, Chilean subduction|
Ocean bottom seismometers (OBS) commonly record short duration events (SDEs), that could be described by all of these characteristics: (i) duration < 1 s, (ii) one single‐wave train with no identified P‐ nor S‐wave arrivals and (iii) a dominant frequency usually between 4 Hz and 30 Hz. In many areas, SDEs have been associated with gas or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from 2 contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the EMSO‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs’ waveforms by injecting air or water in a tank filled by sand and sea‐water and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea‐state related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth and the dominant frequency.
Short duration high amplitude events routinely recorded on marine seismometers are not created by fish bumping into the geophone.
In laboratory experiments, the waveforms of these events are well reproduced by fluid migration in the sediments.
We propose 3 simple source parameters to discriminate gas‐related short duration events in ocean bottom seismometer data.