Abstract
ABSTRACTIn the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground‐based long‐offset transient‐electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large‐scale survey resulted in an extensive dataset acquired with multiple high‐power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi‐dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step‐on transients are well fitted using a conventional one‐dimensional inversion. Due to superimposed induced polarization effects, the transient step‐off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long‐offset transient‐electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one‐dimensional inversion models of frequency‐domain data show strong multi‐dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency‐domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2‐D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.
Highlights
Since strategic important minerals often exhibit high electrical conductivities (e.g. Airo, 2015; Spagnoli et al, 2016), electromagnetic methods are an important tool for mineral exploration
As a validation area for the Deep Electromagnetic Soundings for Mineral Exploration (DESMEX) semi-airborne exploration concept, a former antimony mining area located in the Thuringian Slate mountains was selected
To validate a novel semi-airborne controlled source electromagnetic methods (CSEM) exploration concept for deep mineral exploration, a large-scale long-offset transientelectromagnetic (LOTEM) field survey was conducted in a former mining area in Schleiz, Germany
Summary
Since strategic important minerals often exhibit high electrical conductivities (e.g. Airo, 2015; Spagnoli et al, 2016), electromagnetic methods are an important tool for mineral exploration. Additional land-based long-offset time-domain EM measurements were carried out to provide a validation model for the semi-airborne concept. Land-based controlled source electromagnetic methods (CSEM) exhibit a high-resolution as well as a high-penetration depth and are an important tool to gain insights to deep geological formations. By measuring the transient response of the electric and magnetic field components at offsets up to several times the target depth, the method is typically applied to retrieve information about deep structures. LOTEM data are traditionally interpreted in time and not in frequency domain. Multi-dimensional time-domain modelling algorithms have been presented by, for example Martin (2009), Commer (2003) and Oldenburg et al (2012), open source inversion tools for time-domain land-based CSEM are still not commonly available and not routinely utilized. We present a 2.5D inversion of the data in frequency domain, including a geological description of the shallow subsurface structures as verification
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