Structure of an ambient vibration wavefield in the frequency range of engineering interest ([0.5,20] Hz): insights from numerical modelling

The expected structure of an ambient vibration wavefield at the top of a shallow soft layer overlying a rigid bedrock is explored by applying a full wavefield physical model, under the hypothesis that ambient vibrations are the effect of a uniform distribution of random independent point-like harmonic sources at the surface of a flat, weakly dissipative Earth. The comparison of the results provided by this model with those deduced on the assumption that surface waves dominate the wavefield allows evaluation of the respective roles of body and surface waves (Love and Rayleigh) in their fundamental and higher modes. This analysis reveals that the structure of the ambient vibration wavefield strongly depends on the subsoil structure (P- and S-wave velocity profiles and thickness of the uppermost soft sedimentary layer) and on the distribution of ambient vibration sources around the receiver. This dependence also changes along with the frequency range of interest. In this regard, three frequency domains are identified, each showing a different sensitivity to the relevant parameters: below the fundamental resonance frequency for S-waves f(S), above the frequency f(u) min {2f(S), f(P)}, where f(P) is the resonance frequency for P-waves and in-between. A consequence that emerges is that a number of possible combinations of body and surface waves are possible, which could account for the heterogeneous results obtained from experimental studies. These findings also indicate constraints on the use of simplified models based on the assumption that surface waves dominate the ambient vibration wavefield, as is currently the case in most engineering applications.