Higher mode surface wave interferometry. Geologica Ultraiectina (336)

Abstract

In this thesis, we investigate the feasability of using higher modes in seismic interferometry. This is done by a theoretical and computation analysis of the approximations involved, development of a method of stacking noise traces, and analysis of a small array in the northern part of the Netherlands. Higher modes are unambiguously present in ambient noise, and can thus be used to improve vertical resolution in the subsequent inversion procedure. The practical issue in their use, remains the problem of distinguishing them from spurious events/noise, and even distinghuising higher modes from the fundamental mode, since at high frequencies they can be in close proximity in a time-frequency diagram. Higher modes can furthermore be lost in the data due to: -their small amplitude compared to the fundamental mode. -intermodal cross-terms that occur with imperfect distributions, but occur even in the case of a perfectly surrounding integration surface. However, if noise is generated in a large band-like distribution (or very far away) the cross-terms should pose no problem. - source-source correlation terms. The signal to noise ratio is much smaller for higher modes. The exact instantaneous phase takes much longer to converge to the true solution than the envelope, implying larger uncertainty for phase velocity measurements. -incoherent noise not associated to wave propagation, or local effects. Directional noise shows much faster convergence for higher modes, again at least for station paths inline in this direction. Also in our data, the noise is strongly directional. It can be argued that the higher mode information would not be retrieved if this were not the case. The higher mode has a different origin from the dominant secondary microseism.The TF-phase weighted stack is a powerful tool to denoise the retrieved seismograms. It is possible to measure the group velocity from the phase stack directly. To retrieve higher modes, the phase stack provides an easy to use, additional tool. Most information remains in relative few TF-stacks from long displacement traces. Verification with additional information was essential for a robust estimate of the 1D S-velocity structure beneath Annerveen, The Netherlands. As an average for the region, the higher mode information gives additional constraint on the deeper sedimentary layers. The tomographic images seem to correlate with the main geological features, but for the higher modes the amount of interstation paths that can be reliably measured is not large enough. This is due to interference with the fundamental mode in the group velocity images and the limited directionality. The choice of a substantial spread in interstation distances led to a broad frequency range of analysis, but also limited the number of measurements per frequency. A recommendation for future studies is to use a strict 1-D configuration along the prevalent noise direction. The advantages are the large possible frequency bandwidth, and the possibility to use and invert the higher modes.