Two-dimensional resonances in Alpine valleys identified from ambient vibration wavefields

Abstract : Although numerical simulations have for long shown the importance of 2-D resonances in site effect estimations of sediment-filled valleys, this phenomenon is usually not taken into account by current hazard assessment techniques. We present an approach to identify the resonance behaviour of a typical Alpine valley by analysis of ambient noise recorded simultaneously on a dense array. The applicability of the method is evaluated further using synthetic ambient noise acquired with current 3-D numerical simulation techniques. Resonance frequencies of the fundamental mode SV and the fundamental and first higher mode of SH are identified from measured data with the reference station method, verifying results of previous studies. Patterns of spectral amplitude and phase behaviour obtained from observed and synthetic noise correlate well with properties expected for 2-D resonance. Application of a frequency-wavenumber technique shows that the noise wavefield is dominated by standing waves at low frequencies (0.25 to 0.50 Hz). The different 2-D resonance modes are creating prominent peaks in horizontal-to-vertical spectral ratios, which can not be interpreted in terms of 1-D resonance. We conclude that ambient noise records measured simultaneously on a linear array perpendicular to the valley axis may be used for identification of resonance modes in sediment-filled valleys.