Addressing anthropogenic noise and electrical anisotropy hints in magnetotelluric data from the Annecy area (French Pre-Alps)

Abstract

The magnetotelluric (MT) method is a widely used tool for geothermal reservoir exploration due to its capability of imaging fluids in the Earth’s crust. However, in urbanised environments, its application is often challenged by anthropogenic noise and low signal-to-noise ratios. In this study, we investigate the potential of MT data to image a low-enthalpy hydrothermal reservoir located at 1.5–2 km depth beneath the urban region of Annecy (France), a geologically complex area marked by the active Vuache Fault systems. A total of 46 broadband MT stations were deployed over a 100 km $$^{2}$$ area. We aimed to: (1) Enhance MT signal quality in an urban setting using a processing based on multivariate linear regression and eigenvalue-based selection criteria performed on long-duration recordings; and (2) Assess the potential impact of electrical anisotropy on data interpretation through dimensionality analysis, isotropic and anisotropic inversions. Results show that the processing significantly improved the signal-to-noise ratio and MT responses, especially from 1 to 0.1 Hz when long duration (1 week) recordings are performed. Dimensionality analysis reveals a consistent geoelectrical strike direction and anisotropic hints, yet with spatial variability suggesting a structurally complex medium. A 3D isotropic inversion imaged the main local geological units and structures that are expected in the Annecy area: a clear resistivity contrast associated with the Vuache Fault, high resistivity associated with Jurassic and Cretaceous limestones and a shallow conductive area in the Molasse units. An outstanding conductive area (10–20 $$\Omega$$ m) is found around the Vuache Fault trace, close to a known thermal spring. An axial anisotropic impedance inversion provided a better fit and the calculation of the horizontal anisotropy index showed remarkable consistency with seismicity distribution. The anisotropy index indicates a higher conductivity along the Vuache Fault, in the 1996’s M5.3 earthquake aftershock zone. We discuss the computed anisotropy and propose a simple test to identify the possible presence of spurious anisotropy related to resistivity and topography contrasts. Our findings demonstrate the feasibility of MT for urban geothermal exploration when advanced processing techniques are applied, and highlights the importance and challenges of considering electrical anisotropy in MT data.