The Whisper of Deep Basins: Observation & Modelling
Abstract
Free oscillations of Earth have been used for a long time to retrieve information about the deep Earth's interior. On a much smaller scale, standing waves develop in deep sedimentary basins and can possibly be used in a similar way. The sensitivity of these waves to subsurface properties makes them a potential source of information about the deep basin characteristics. We investigated the sequence of two-dimensional resonance modes occurring in Rhône Valley, a strongly over-deepened, glacially carved basin with a sediment fill reaching up to 900 m thickness. We obtained two synchronous linear-array recordings of ambient vibrations and analysed them with two different processing techniques. First, both 2D resonance frequencies and their corresponding modal shapes were identified by frequency-domain decomposition of the signal cross-spectral density matrix. Second, time-frequency polarization analysis was applied to support the addressing of the modes and to determine the relative contributions of the vertical and horizontal components of the fundamental in-plane mode. Simplified 2-D finite element models were then used to support the interpretation of the observations. We were able to identify several resonance modes including previously unmeasured higher modes at all investigated locations in the valley. Good agreement was found between results of our study and previous studies, between the two processing techniques and between observed and modelled results. Finally, a parametric study was performed to qualitatively assess the sensitivity of the mode's order, shape and frequency to subsurface properties like bedrock geometry, Poisson's ratio and shear wave velocity of the sediments. We concluded that the sequence of modes as they appear by frequency depends strongly on subsurface properties. Therefore addressing of the higher modes can be done reliably only with prior information on the sediment structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S11A2301B
- Keywords:
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- 7212 SEISMOLOGY Earthquake ground motions and engineering seismology;
- 7255 SEISMOLOGY Surface waves and free oscillations