Roger Wisén

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 1D-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.

3D Effects on 2D Resistivity Imaging – Modelling and Field Surveying Results

Today resistivity surveying plays an important role in many large-scale area investigations. The existence of 2D effects on 1D resistivity modelling is a well known problem; however, former studies show that 3D effects in 2D surveying are less evident. The results presented here show that, nevertheless, there is an advantage in performing 3D inversion. A comparison between 2D inversion and 3D inversion has been made with analysis of data from two different synthetic models and three field datasets. From the synthetic study it is clear that 3D inversion give higher contrast, less inversion artefacts and better model recognition than 2D inversion. From the field studies it is also evident that 3D inversion gives models with higher contrast. With only limited ground truth data it is not always possible to determine which model is closest to the true one; however, where ground truth data is available it is clear that the 3D inversion gives a better result. In addition we show that the choice of array configuration have a significant influence on the result, with gradient array generally giving better results than the other options.

Application of 2D laterally constrained inversion and 2D smooth inversion of CVES resistivity data in a slope stability investigation

In this paper we present how a 2D least squares algorithm, Res2Dinv (Loke and Dahlin, 2002), and the 2D Lateral Constrained Inversion (2D-LCI) algorithm (Christiansen et al., 2003) can be applied in geotechnical site investigations. The 2D-LCI algorithm performs a parameterized 2D Laterally Constrained Inversion (Auken and Christiansen, 2003) on resistivity data.

2D and 3D Resistivity Imaging in an Investigation of Boulder Occurrence and Soil Depth in Glacial Till

The uncertainty regarding depths to bedrock and occurrence of boulders in glacial till can make the planning of a major road construction more difficult. Despite the fact that geophysics is frequently used in many applications and also occurs as a natural method to solve problems in geotechnical engineering, it has often a subordinate role as a site investigation method prior to road construction in Sweden. This project aims at evaluating 2D and 3D resistivity imaging to determine soil depths and track boulders in glacial till. Hopefully, the extent of the excavations preceding a road construction will thus be better estimated in the future, as the geotechnical investigations are suitably complemented. When compared with existing geotechnical data and brought together, both the 2D and 3D resistivity methods are regarded as good producers of reliable soil depth models. However reference data from alternative methods must always be used for validation and calibration of results. It is shown that a 3D resistivity dataset, consisting of a number of parallel CVES profiles, in some cases can give significantly improved resistivity models.

Geophysical pre-investigation for a Stockholm tunnel project : Joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment

Underground constructions for public traffic purposes are becoming increasingly important for urban areas in order to use the limited space more efficiently. Several electric resistivity tomography and seismic refraction tomography measurements were performed crossing a water passage near Stockholm during the pre-investigation phase of a tunnel building project. The objective was to determine the bedrock interface and qualitatively assess the rock quality. The scope of this study is to present a field case in an urban environment and show improvements of geophysical results due to additional model constraints by a joint inversion. Results of individual inversions show a large transition zone below the seabed from electric resistivity tomography. Some parts of the seismic refraction tomography have a low model resolution, due to gas-bearing sediments with a low velocity together with a high noise level, which leads to insufficient investigation depth that makes it difficult to determine the bedrock interface. However, the bedrock interface could be reconstructed in the resistivity model by performing a joint inversion, using the seismic velocity model to constrain the electric resistivity tomography model and vice versa. Adjacent geotechnical soundings support the joint inversion results. A vertical low resistive zone could be identified as a dominant fracture zone in the southern part of the investigated area. In general, the joint inversion approach significantly improved the electric resistivity tomography results and provided a more reliable bedrock estimation. (Less)

Urban underwater ERT for site investigation in lake Mälaren, Sweden

An underwater ERT survey was made along a planned road tunnel for The Stockholm Bypass Project (Forbifart Stockholm). The aim was to assess the ability to identify variations in the depth of the bottom sediments, as well as variations in rock quality and the possible presence of weak zones in the rock. Reference data from a seismic survey and geotechnical drilling was used for evaluation the result. An underwater layout of 121 underwater and 37 land based electrodes at a separation of 5 m was used. Repeated measurements were made to assess the temporal variation in urban noise, which lead to doing the actual survey during the quiet night hours. The resistivity data are of good quality and the inversion results in low residuals. The invested section shows a low resistivity layer corresponding to sediments on top of bedrock with varying resistivity, where the interpreted depth to bedrock is in good accordance with the reference data. A major low resistive zone corresponds well with poor quality rock according to drilling and a low velocity zone.