Laterally constrained inversion of ground roll from seismic reflection records

Seismic reflection data contain surface waves that can be processed and interpreted to supply shear-wave velocity models along seismic reflection lines. The coverage of seismic reflection data allows the use of automated multifold processing to extract high-quality dispersion curves and experimental uncertainties in amoving spatial window. The dispersion curves are then inverted using a deterministic, laterally constrained inversion to obtain a pseudo-2D model of the shear-wave velocity. A Monte Carlo global search inversion algorithm optimizes the parameterization. When the strategy is used with synthetic and field data, consistent final models ith smooth lateral variations are successfully retrieved. This method constitutes an improvement over the individual inversion of single dispersion curve

Electric resistivity and seismic refraction tomography: a challenging joint underwater survey at Äspö Hard Rock Laboratory

Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Äspö Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100 m below and 20 m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450 m underwater section of the 700 m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model

Resistivity and Surface Wave Seismic Surveys in Geotechnical Site Investigations

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

Laterally and mutually constrained inversion of surface wave seismic data and resistivity data

The laterally and mutually constrained inversion (LCI and MCI) techniques allow for the combined inversion of multiple geophysical datasets and provide a sensitivity analysis of all model parameters. The LCI and MCI work with few-layered models, and are restricted to quasi-layered geological environments. LCI is used successfully for inversion of surface wave (SW) seismic data and MCI for combined inversion of SW data and continuous vertical electrical sounding (CVES) data. The primary model parameters are resistivity or shear wave velocity and thickness, and depth to layer interfaces is included as a secondary model parameter. The advantages and limitations of LCI and MCI are evaluated on synthetic SW data. The main conclusions are: Depth to a high velocity halfspace is generally well-resolved even if thicknesses of overlaying layers and the velocity of the halfspace are unresolved; Applying lateral constraints (LCI) between individual SW soundings improves model resolution, particularly for velocities and depths, and; Adding mutual constraints (MCI) to resistivity data improves model resolution of all parameters in the shear wave velocity model. When applied to field data, model resolution improves significantly when LCI or MCI is used, and resistivity and velocity models correlate structurally with better correlation to lithological interfaces identified in drill logs

Combination of ID laterally constrained inversion and smooth inversion of resistivity data with a priori data from boreholes

Resistivity imaging in combination with borehole information is a powerful tool for site investigation. We show that the combination of 1D laterally constrained inversion (1D-LCI) with the use of a priori information from borehole data and 2D smooth inversion adds significant value to the interpretation of continuous vertical electrical sounding (CVES) data. The ID-LCI offers an analysis of the resolution of the model parameters. This is helpful when evaluating the integrity of the model. Furthermore, with the 1D-LCI it is possible to constrain model parameters with a priori information, e.g. depth-to-layer interfaces, based on borehole information. We show that 2D smooth inversion resolves lateral changes well, while 1D-LCI results in well-defined horizontal layer interfaces. In geological environments where the lateral variations are not too pronounced, the 1D-LCI contributes to a geological interpretation of the resistivity measurements. Depths to layers can be interpreted with greater certainty than if using results from 2D smooth inversion only. The inclusion of a priori information in the inversion reveals further details and enhances the geological interpretation significantly

CVES Resistivity Investigations for Optimizing Groundwater Protection at Highway Construction

CVES resistivity has been used to achieve a continuous image that reflects the hydraulic properties of the ground along a road stretch. The purpose was to map the vulnerability for pollutants to penetrate into the precious groundwater magazine under the Kristianstad plains. The investigation shows that a large part of the area is covered by impermeable layers in the form of a thick clay till. Large parts of the area is covered with sandy till over clay till, and this sequence has been determined to have a permeability that gives sufficient protection of the aquifer below. In parts of the area, close to the Linderöd horst where soil cover is thin or non-existent and at the glaciofluvial deposits at and around Helgeåsen it is motivated to construct the road so that the ground water is protected. In about 10 % of the investigated area it will be necessary to perform further investigations to define the need of groundwater protection as part of the road design