The influence of local irregularities on the propagation of Love waves

Abstract

The propagation of Love waves in models with local irregularities (a canyon or an alluvial valley) related to general waste deposit problems in environmental geophysics is numerically investigated. The investigation is based on a direct boundary element method for calculating wave fields and on a Green's function technique for evaluating transmission, reflection and ‘diffraction’ coefficients for Love waves, incident on a local surface or subsurface irregularity in layered media. The present model studies reveal that the influence of local irregularities on the propagation of Love waves depends both on the geometry and material properties of the local irregularities and on the dynamic parameters of the incident Love waves. Local irregularities lead to strong scattering of Love waves whose wavelengths are comparable to the characteristic sizes of the irregularities. For Love waves having wavelengths which are very large compared to the characteristic sizes of irregularities the influence of these irregularities are negligible. In addition, Love waves incident on a local irregularity always undergo mode conversion. Part of the energy of the incident Love wave will covert to other Love wave modes and body waves. With respect to the application of surface waves in environmental geophysics, two results may be stressed: (i) Though the conversion of Love waves to body waves has been qualitatively anticipated in the past, its enormous strength quantified by the diffraction coefficients is interesting; (ii) Love waves are more sensitive to local velocity changes than to local density changes. For in situ measurements, this result leads to the conclusion that density is a less relevant quantity, although it may better characterize the material inside a waste deposit than velocity.