Higher modes in the near surface: the influence of gradients
DOI:
https://doi.org/10.26443/seismica.v5i2.2404Keywords:
surface waves, higher modes, dispersion, gradientsAbstract
Higher modes of surface waves are more sensitive to the details of the wavespeed distribution with depth than the fundamental modes, and in particular to the wavespeed gradients within layers and the transition between layers. The character and dispersion of the various modal branches are readily envisaged with the aid of calculations of frequency-phase speed spectra for surface sources. Colour-coding of the three-component response is helpful in assessing the nature of the modes and the way that they interact with recording systems. The near-surface can have very variable wavespeed gradients, from slight in the presence of recent sediments to quite strong under compaction. In the presence of gradients the dispersion properties of higher modes are not well represented by models with just a few uniform layers even though this may be adequate for the fundamental mode. Where gradients are expected, direct parametrisation is beneficial, even if implemented with sequences of uniform layers.
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