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| Titel |
Seafloor ground rotation observations: potential for improving signal-to-noise ratio on horizontal OBS components |
| VerfasserIn |
Fabian Lindner, Joachim Wassermann, Mechita Schmidt-Aursch, Ulrich Schreiber, Heiner Igel |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106237
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| Publikation (Nr.) |
 EGU/EGU2015-5897.pdf |
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| Zusammenfassung |
| It is well known that the horizontal components of ocean bottom seismometer (OBS) records
have a very poor signal-to-noise (S/N) ratio compared to the vertical components, the
difference substantially exceeding that of terrestrial records. This is unfortunate
as 1) OBS experiments are expensive and the main possibility to gather data in
offshore areas, and 2) today we are more and more interested in modelling complete
waveforms including all three components aiming at optimally constraining geophysical
parameters for inverse problems for Earth’s structure and seismic sources. Despite
the fact that it is expected that tilting is the major cause of this high S/N – to our
knowledge – this effect has never been directly observed. The reason is that (standard)
instruments for the measurement of uncontaminated rotational ground motions
with the required sensitivity still do not exist. Here, we report observations from an
experiment we carried out in the North Sea, close to the island of Helgoland in
the summer of 2014. A commercial fibre-optic gyro (usually used for navigation
purposes) recording ground rotation rate with a sensitivity of approx. 10-7 rad/s was
mounted on an OBS system together with a broadband seismometer. The system
was lowered to the seafloor for about a week. To investigate a potential connection
between rotational ground motions around the two horizontal axes (i.e., tilting)
we calculate the coherence between the corresponding motion components (e.g.,
rotations around x-axis and translational motions along y-axis, and vice versa).
We find very high correlations, on average exceeding 0.73 in the period interval
7-13 seconds. Correlations seem to increase with noise amplitude. Rotation rate
amplitudes are in the range of 10-6 -10-5 rad/s. This clearly indicates that the
horizontal translational components are severely contaminated by rotations around the
horizontal axes. The ground rotation observations allow correcting for this effect thereby
considerably improving the S/N on the horizontal translational components. The relatively
high-amplitude rotational motions are expected to originate from water motion, seafloor
interface waves or both. An added benefit of seafloor ground rotation observations
using fibre-optic systems is that the orientation of the recording system is known
to within a degree, as the optical system uses the Earth’s rotation as a reference. |
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