Reflection of surface waves by fault structures in granular media

Abstract

Laser vibrometry has previously been studied as a new non-invasive imaging method to acoustically image physical analogue models. This study aims to characterize the interaction between acoustic waves and deformation zones in granular media to study the viability of using laser vibrometry as an imaging method. Physical and numerical models are used to study the effect of deformation zone properties and geometry on the wave behaviour. The results from these experiments show that there is no observable interaction with the body wave. The surface waves do show significant interaction with deformation zones. The properties and geometry of the deformation zone affect the reflection coefficient of surface waves. With an increase in thickness of the deformation zone, and an increase in density contrast between the deformation zone and the surrounding material, the reflection coefficient increases. The geometry of the deformation zone affects the waves mainly based on the dip direction. If the deformation zone dips away from the source, the surface wave has a very low reflection rate and passes through the zone with little reflection. When the deformation zone dips towards the source the refraction rate is high, little of the wave passes through the deformation zone and it partially reflects back. Thus, while the laser vibrometry method could be used to study the properties and geometry of faults at the surface, improvements are needed to be able to use it for imaging the internal structure of a granular material.