Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments |
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Authors: | Gerald Klein Thomas Bohlen Friedrich Theilen Simone Kugler Thomas Forbriger |
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Institution: | 1. Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Dienstgeb?ude Ostufer, Wischhofstr. 1-3, D-24148, Kiel, Germany 2. Institute of Geosciences, University Kiel, Kiel, Germany 3. BFO-Schiltach, University Karlsruhe, Karlsruhe, Germany
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Abstract: | Two types of dispersive seismic waves have been acquired in different geological settings to investigate the potential to
reveal the elastic parameters of the shallow marine subsurface. Scholte waves as well as acoustic guided waves are excited
by a near-surface towed airgun, and recorded using two acquisition methods: (1) the towed-acquisition system using a hydrophone streamer towed close to the sea floor, and (2) the stationary-receiver method using Ocean-Bottom Seismometers and/or Hydrophones (OBS/OBH). Our diverse data sets reveal that the spatial sampling
of the wavefield required to avoid aliasing may vary significantly for different geological settings. Scholte waves are characterised
by a few distinct modes observed at low frequencies and low phase velocities. Their dispersion is mainly controlled by the
depth profile of the shear-wave velocity. Acoustic guided waves show profound amplitude variations of numerous higher modes
over a broad frequency range. These are sensitive to shear-wave velocity, but more sensitive to compressional-wave velocity
than Scholte waves are. To avoid the identification of distinct modes we infer 1-D models of elastic parameters of the subsurface
from the inversion of the full wavefield spectra of acoustic guided waves. In the Siberian Laptev Sea we infer the presence
of a soft sediment layer (8–10 m) with a well resolved strong S-velocity gradient (150–450 m/s). In the Baltic Sea a low P-velocity
layer with a strong vertical gradient (1250–1440 m/s) corresponding to a post-glacial gassy mud layer could be resolved, which
agrees well with the sediment stratigraphy derived from a gravity core. |
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Keywords: | Acoustic guided waves Scholte waves dispersion analysis wavefield inversion shallow marine seismics waveguides |
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