Asgeir Kydland Lysdahl

Comparison between 2D and 3D ERT inversion for engineering site investigations – a case study from Oslo Harbour

Excavation and piling works related to seafront development in Oslo’s historic harbour area need to mitigate the risk of damaging buried archaeological objects. In the Bjorvika harbour in Oslo, Norway, electrical resistivity tomography was performed to detect structures with potential archaeological value. A 2.5 dataset consisting of four equally spaced parallel lines was collected, trimmed, and systematically processed with both 2D and 3D inversion routines. The results were in good agreement with known underground features, and for the present dataset, an iteratively reweighted least squares 2D inversion was clearly preferable over a 3D inversion. This conclusion is based on differences in model resolution, data processing costs, and the value of the final product for engineering decision-making.

Helicopter Electromagnetic Scanning as a First Step in Regional Quick Clay Mapping

Identification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects in Norway. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution airborne electromagnetics (AEM) collected from helicopter. However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.

From Manual to Automatic AEM Bedrock Mapping

ABSTRACT An extensive airborne electromagnetic (AEM) survey was carried out in Norway with the primary purpose to obtain information of depth to bedrock in areas with little or no prior geotechnical knowledge. We present different approaches to extract a bedrock model from the high-resolution time-domain AEM data, including both automated and manual procedures. It was found that in the area of investigation a user-driven approach of manual bedrock picking was the most suitable, taking into account the strongest vertical resistivity gradient and geological information as additional information. A semi-automatic, statistical method, called Localized Smart Interpretation (LSI), is also presented and discussed. This method, while not included in the original bedrock model for the entire area, showed promising results while using less time compared to the fully manual approach. It is recommended that LSI be considered in future projects of similar scope.

ERT INVERSION INDUSTRY STANDARD VERSUS CUTTING EDGE DEVELOPMENTS, TIME FOR A CHANGE?

ERT (Electrical Resistivity Tomography) is a versatile and widely used method in terms of applications for surface geophysics. For processing and inverting resistivity data engineering geophysics consultancies around the globe largely use the software package Res2Dinv as an industry standard. On the other hand, universities and other research groups have made numerous attempts towards different and maybe better solutions to derive an earth model from resistivity data. We have investigated the variability of ERT results based on three different inversion packages and based on datasets with ground truth information. Here we present two of our studied cases. Our work illustrates advantages and shortcomings of academic, semi-academic and commercial software to the user and interpreter. As one may expect, there is no clear winner or loser, yet the results indicate the potential benefit for interpretation when different results from the same dataset are combined to one geological model with minimum uncertainty.

Black Shale Mapping by AEM for Geotechnical Applications

Airborne Electromagnetics (AEM) has been used to map the extent of environmentally harmful black shale in two geotechnical projects in Norway, supported by electrical resistivity ground surveys and laboratory measurements. The work demonstrates that resistivity is a suitable parameter for both mapping and characterizing black shales, since their high sulfide content in general means a more harmful shale and in addition gives very low resistivity (0.1-1 Ωm). Various interpretation procedures to define a black shale volume have been explored, and some of the data show vertical resolution sufficient for geotechnical applications.

Resistivity and chargeability survey for tunnel investigation: a case study on toxic black shale in Norway

In the past few years, the focus on Alum shale hazards and the need for efficient mapping tools have increased in Norway. Alum shale is highly toxic and poses a substantial obstacle to infrastructure development such as tunnel projects. We present an evaluation of the ground-based electrical resistivity tomography, induced polarisation, and airborne electromagnetic methods for mapping purposes using a recent case study. This evaluation is done in combination with resistivity and chargeability laboratory measurements applied on drill cores. The aim of the geophysical survey was to improve the knowledge of Alum shale occurrence to assist a tunnel project in Gran, southeast Norway. Resistivity and chargeability models derived from an electrical resistivity tomography/induced polarisation survey enabled us to map the presence of Alum shale during the tunnel investigation. The resistivity models point to geological layers that are in agreement with the rock types observed from early drillings together with subsequent geological logging during tunnelling. The time-domain chargeability models are imperfect but nonetheless reveal the presence of polarisable minerals. These are likely due to the high levels of sulphides contained in black shale. An airborne electromagnetic survey was done close to the area of interest, which enabled us to fly some sparse lines across the tunnel alignment as a piggyback survey. Although the airborne electromagnetic resolution is lower than electrical resistivity tomography, the successful test flight lines illustrate the potential of airborne electromagnetic surveys for Alum shale mapping in Norway and affirm the promise of airborne electromagnetic in the early stages of project exploration.

Probability of Sensitive Clay from AEM Data

Summary Identification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution Airborne Electromagnetics (AEM). However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.

Shear-wave Reflection-seismic and ERT Investigation for Slope Stability

As part of a civil engineering project for line of communication extension, slope stability analysis was required. The ground model, based on geotechnical soundings alone failed to resolve the required parameters. Therefore, a more extensive study combining Electrical Resistivity Tomography and S-wave reflection seismic was conducted. The resulting geological model properly address the need for a comprehensive slope stability analysis. The integration of structural information from the SH-wave reflection seismic, the resistivity model and the geotechnical soundings provides a sounded geological model down to the bedrock and a clear image of the overlaying moraine, marine clay and fluvial deposits.

Resistivity/Induced Polarization/Self-Potential Methods and Applications

Modern multielectrode and multichannel resistivity systems have made it relatively easy and rapid to collect time domain induced polarization (IP) data in near surface surveys. This paper will examine a wide variety of applications through case studies in a variety of geological settings in Western Canada. Case studies will show various applications and complementary features of IP surveys including distinguishing salt water from conductive clays, identifying faults, locating deeply buried structures underneath active facilities, and distinguishing landfilled debris from leachate. IP data sets will be correlated with other data sets including resistivity, seismic reflection, and borehole geophysical parameters.

On the Use of ERT and AEM Resistivities for a Tunnel Pre-investigation

We have investigated the applicability of resistivity derived either from surface electrical resistivity tomography (ERT) or airborne electromagnetic (AEM) surveys to a tunnel pre-investigation. We have exploited resistivity models to map the extent of toxic black shale along a tunnel alignment in central Norway. The resistivity models acquired before excavation and supported by laboratory measurements enabled us to map geological layers that are in agreement with the rock types subsequently observed from drillings and geological logging during tunneling. Our results highlight the potential of AEM surveys for such tunnel pre-investigations.