Advanced joint inversion techniques for electrical and seismic tomography in environmental applications

Geophysical tomographic techniques have gained significant relevance worldwide for environmental, hydraulic and civil applications, due to their ability to rapidly and non-invasively provide high-resolution on subsurface engineering targets. For instance, the EU Water Framework Directive (2000/60/EC) mandates integrated approaches to water resource management, that benefit from high-resolution geophysical investigations. Similarly, the Eurocode 8 (with recent updates adopted in Italy through the national NTC 2018) highlights the critical role of subsurface characterization in seismic risk assessment and infrastructure resilience, and the Directive 2014/52/EU (Environmental Impact Assessment) emphasizes the need for precise geophysical methods in evaluating project impacts. On a broader scale, the United Nations Sustainable Development Goals (e.g., SDG 6 and 9) promote the use of innovative geophysical methods for water resource management and resilient infrastructure, while the International Energy Agency (IEA) guidelines advocate for integrated geophysical approaches to optimize resource exploration and environmental monitoring. On a national level, the Italian Legislative Decree 152/2006 (Consolidated Environmental Protection Act or “Environmental Code”) establishes strict requirements for groundwater protection and contamination assessment, underscoring the need for advanced subsurface imaging techniques for environmental protection and sustainable land use. Consequently, there is an emerging demand for more accurate subsurface reconstruction and the integration of diverse datasets, driven by technological advances that continually improve the cost-effectiveness of field campaigns. Effective data fusion is therefore expected to enhance decision-making in civil and environmental monitoring, as well as in groundwater management. Traditionally, multi-parameter geophysical tomographic methods were interpreted separately, through visual comparisons of anomalies or layers that exhibited similar geometries (in terms of shape, size and depth) in the inverted models. However, because each independent method is sensitive to a specific physical property, this approach can lead to interpretative ambiguities or misinterpretations. In recent decades, joint inversion techniques have become increasingly prominent due to their ability to overcome inherent resolution limits and provide more accurate and reliable geophysical models. Within this context, this thesis introduces new elements advancing the non-invasive quantification of environmental, civil and hydraulic engineering parameters through the joint inversion of complementary datasets for a more accurate subsurface characterization, in line with current regulatory frameworks. To this aim, our research focuses on electrical and seismic tomography, two techniques that respond to different physical properties of the subsurface. Electrical resistivity, for example, primarily reflects variations in water content, porosity, and clay fraction, making it a valuable tool for characterizing hydrogeological and lithological heterogeneities. In contrast, seismic wave velocities are governed by the elastic properties of the medium, including density, porosity, and saturation, thus providing insights into the mechanical behavior of subsurface materials. Additionally, the induced polarization method complements electrical methods by providing sensitivity to electrochemical properties, such as grain surface conductivity and polarization effects, which are linked to the presence of clays, metallic minerals, and pore-fluid chemistry. By integrating these techniques, a more comprehensive subsurface characterization is achieved, offering complementary information on hydraulic, mechanical, and electrochemical properties, thereby reducing ambiguities inherent in single-method approaches.