Multi-dimensional shear wave velocity imaging of the granitic rock weathering zone along Chile’s climate gradient

The weathering zone, where rock is transformed into regolith through chemical and physical processes, plays a crucial role in various environmental and geological processes. This thesis tests the hypothesis that the production of regolith from bedrock highly depends on the climate conditions, especially the rate of precipitation. Multidisciplinary studies of the weathering zone for different climate conditions are performed within the joint EarthShape project that set up four study sites along the Chilean continental margin. This thesis investigates the weathering zone structure for different climate conditions by applying different seismic methods to image the subsurface, allowing the demonstration of seismic surface wave methods capabilities in the multi-dimensional imaging (1D, 2D, and 3D) of the weathering zone. Multi-channel Analysis of Surface Waves (MASW) and Horizontal-to-Vertical Spectral Ratio methods were used to characterise the weathering zone. Application of active (MASW) and passive (HVSR) surface wave methods display robustness in modelling the weathering zone structure based on shear-wave velocities (Vs). In addition, the employed trans-dimensional hierarchical Bayesian inversion approach offers a significant improvement to surface wave inversion and provides additional insights into the weathering zone structure and lithology. The produced Vs model shows that a completely unweathered granite bedrock can be identified with a Vs larger than 2.50 km/s while the fractured bedrock layer has a velocity between 1.20 and 2.50 km/s. On the other hand, the saprolite layer can be identified with its low Vs characteristic lower than 1.20 km/s. These findings allow a comprehensive characterization of the weathering zone structure based on the Vs values which is less explored in previous weathering zone studies. A comparison of the seismic characteristics of the weathering zone structures across the Chilean climate gradient in this study reveals a visible trend of deepening of the weathering zone with increasing precipitation. Furthermore, not only the thickness of the weathering zone structure varies at the different sites, and thus across the different climate zones, but also the seismic characteristics of the layers. The difference between the saprolite’s Vs characteristics in the semi-arid and Mediterranean climates indicates different weathering intensities between the Santa Gracia and La Campana sites. This suggests that in addition to the thickness of the layers, characterisation of the Vs value is also an important aspect to be considered as it is related to the site-specific weathering intensity. Integration of the seismic model with geochemical and core analysis data further enhances the understanding of the weathering zone structure and its related processes. With the 2D geophysical data, the horizontal and vertical extent of the weathering zone can be modelled which reveals a potential geological structure that can directly affect the advance of the weathering zone. This is shown by the results in the Santa Gracia site where the combination of the seismic data with Chemical Depletion Factor (CDF) data highlights a potential fault structure that can allow fluid infiltration to the deep weathering zone. Additionally, the 3D imaging results from Pan de Azúcar reveal a dike structure which can trigger hydrothermal alteration of the minerals in the weathering zone. Furthermore, the seismic velocity models also help in understanding the relationships between the advance of the weathering front and the effects of topography. This is especially visible in Santa Gracia where more bedrock fractures are found in topographic lows compared to in topographic highs. Therefore, the multi-dimensionality of this study further emphasises the importance of geophysical modelling for weathering zone studies, especially as it enables the extension of the information retrieved from the borehole location. The findings of this thesis demonstrate the efficacy of seismic surface wave methods, combined with trans-dimensional Bayesian inversion, for weathering zone characterization. This approach provides a robust, data-driven tool for investigating the subsurface structure and lithology of weathering zones, particularly in arid to semi-arid environments. The study also emphasises the importance of integrating multidisciplinary data sources, such as geochemistry and core analysis, for comprehensive weathering zone assessment. Further research is warranted to explore the potential of this approach in other climate regimes and to elucidate the complex relationship between weathering zone structure and changing climate.