Resistivity and Surface Wave Seismic Surveys in Geotechnical Site Investigations

University dissertation from Engineering Geology, Lund University

Abstract: The adaptation of geophysical methods for civil engineering purposes represents an important contribution to the development of geotechnical site investigation methodology. The term geotechnical site investigation here refers to all investigations performed prior to or during construction; i.e. investigations to support and refine a conceptual geological model as well as to provide a model of geotechnical design parameters. At any stage in the site investigation process, geophysical methods provide information to facilitate the interpolation of geological, geotechnical and hydro-geological structures between positions where detailed information, e.g. from drilling, are available. Geophysical methods have the potential to provide information that describes sections, areas or volumes; such information that would not be readily available from any other investigation method.

Common to almost all geophysical methods is the need for inverse modelling of the observed data. The modelling result can be interpreted directly in terms of the physical properties that it describes.

DC resistivity and surface wave seismics are two methods that perform well in geotechnical site investigations. This thesis focuses on the use of these two methods and different approaches for inverse modelling; the thesis illustrates and comments on the value of these approaches, e.g. through field studies.

- 2D smooth inversion, the commonly used technique for inversion of profiling resistivity data, is a robust technique also for data from complicated geological environments. However, this method is unable to produce sharp layer interfaces, which sometimes makes the resulting models difficult to interpret.

- 3D smooth inversion of resistivity data results in improved models in environments with prominent three-dimensional structures.

- The recently developed laterally constrained inversion of resistivity data provides a few-layer model together with estimates of the uncertainty of model parameters. When this technique is used together with 2D smooth inversion the interpretability of the results is improved.

- The laterally constrained inversion of dispersion curves from surface wave seismic data for a layered shear wave velocity model was developed within this thesis work. It provides a more stable inversion process compared to individual inversion of the dispersion curves.

- The new concept of mutually constrained inversion is implemented for the first time for combined inversion of resistivity and surface wave seismic data. It produces a better model estimate than separate inversion of the two data types and still allows for differences in geometry between the shear wave velocity and the resistivity models.

- By constraining the model geometry with a priori information, the effects from problems with hidden or suppressed layers, non-uniqueness and equivalence in the inversion can be reduced. The laterally constrained inversion allows the inclusion of a priori information on the model so that the uncertainties of the geophysical model parameters are reduced and the final geophysical model is improved.

These methods for measurement and inversion of geophysical data provide cost-effective, fast and robust tools for describing geological units. If they are used to complement the traditional geotechnical methods, an improved material model is achieved. This in turn leads to a safer design and at the end most probably a reduction of the construction costs.