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

Abstract

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 to 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. logical model was available. However, the complexity of the project required a supplementary study to gather continuous information between the boreholes and, thus, decrease the overall project risk in terms of finances and scheduling. The first objective of this study was to characterize the rock mass and its overburden using resistivity derived either from groundsurface electrical resistivity tomography (ERT) or airborne electromagnetic (AEM) surveys.Experience shows that ERT usually gives good results for tunnel investigations (Danielsen and Dahlin 2009), and it is a time and cost-effective method compared with other groundbased geophysical methods. Conversely, the use of AEM for tunnel pre-investigations is only starting for economic reasons,

3D Simulation of Semi-airborne Magnetometric Resistivity Data for the Monitoring of Water-rich Fracture Zones

“…AEM is also employed for mapping toxic black shale, demonstrating its potential during the initial phases of project exploration. [4]. Like airborne geophysics, semi-airborne methods primarily measure electromagnetic data by drones in the air, but the transmitter source is on the ground.…”