Dispersion curves extracted from the PL waves by the Frequency-Bessel Transform method

Abstract:

Surface wave dispersion curve inversion is one of the most important methods of geophysical imaging. In practice, most of the dispersion curves are extracted from Rayleigh waves and Love waves. In addition to Rayleigh waves and Love waves, PL waves also have dispersion characteristics. In 1960, Oliver and Major observed the PL phases. After a series of studies, PL waves can be well explained by leaking mode. However, so far, the dispersion characteristics of PL waves are rarely used in geophysical imaging.

Considering the success of the Frequency-Bessel Transform (F-J) method in extracting high-mode dispersion curves, we believe that it can effectively extract the dispersion curves of PL waves. Therefore, we intercepted the PL waves, and successfully extracted the multi-order dispersion curves with the phase velocities greater than maximum possible shear velocity through the F-J method. In addition, we synthesized theoretical seismograms using local reference models, and performed the same dispersion curve extraction test. The results are in good agreement with actual data.

When interpreting the observed dispersion curve of the PL wave, we found that in addition to the leaking modes, there is another set of curves which shows a “twisted” coupling with the leaking mode. We believe that these curves are related to the stratification of the P-wave structure, which can be explained by acoustic mode. On this basis, we calculated the corresponding theoretical dispersion curves and successfully corresponded to these curves. Furthermore, we analyzed the relationship between the leaking mode and the acoustic mode under different models, and successfully separated them from the actual observed dispersion curves. We believe that this work will play an important role in imaging through these two types of dispersion curves in the future.