2-D joint inversion of semi-airborne CSEM and LOTEM data in eastern Thuringia, Germany

Abstract

Summary Various electromagnetic (EM) techniques have been developed for exploring natural resources. The novel frequency-domain semi-airborne controlled source electromagnetic (semi-AEM) method takes advantages of both ground and airborne techniques. It combines ground-based high-power electrical dipole sources with large scale and spatially densely covered magnetic fields measured via airborne receivers. The method can survey the subsurface down to approximately 1000 m and is particularly sensitive towards conductive bodies (e.g. mineralized bodies) in a more resistive host environment. However, the signal-to-noise ratio of semi-AEM is lower than that of ground-based methods such as long-offset transient electromagnetics (LOTEM), mainly due to the limited stacking time and motion induced noise. As a result, the semi-AEM often has reduced depth of investigation in comparison to LOTEM. One solution to overcome these flaws is to analyse and interpret semi-AEM data together with information from other EM methods using a joint inversion. Since our study shows that LOTEM and semi-AEM data have complementary subsurface resolution capabilities, we present a 2-D joint inversion algorithm to simultaneously interpret frequency-domain semi-AEM data and transient electric fields using extended dipole sources. The algorithm has been applied to the field data acquired in a former mining area in eastern Thuringia, Germany. The 2-D joint inversion combines the complementary information and provides a meaningful 2-D resistivity model. Nevertheless, obvious discrepancies appear between the individual and joint inversion results. Consequent synthetic modelling studies illustrate that the discrepancies occur because of i) differences in lateral and depth resolution between the semi-AEM and LOTEM data caused by different measuring configurations, ii) different measured EM components, and iii) differences in the error weighting of the individual datasets. Additionally, our synthetic study suggests that more flexible land-based configurations with sparse receiver locations are possible in combination with semi-AEM without a significant loss of target resolution, which is promising for accelerating data acquisition and for survey planning and logistics, particularly when measuring in inaccessible areas.

Three-Dimensional Inversion of Long-Offset Transient Electromagnetic Method over Topography

“…We also introduce 5% Gaussian noise to the synthetic data. Since we use the weighting matrix W d , which is based on the observed data error, there are no additional weighting parameters or matrices introduced between E x and dH z /dt [32]. In this study, the initial regularization parameter for inversion is set to 1.0.…”

“…However, some scholars have been aware of the topographic effects when using the 1D inversion method for LOTEM data processing [30]. Therefore, the study into how topography affects the LOTEM signal has become a focal point in the field of LOTEM research [18,25,31,32]. Particularly for 2D and 3D LOTEM inversion, we expect to obtain a more accurate geological interpretation by considering the influence of topography.…”

“…For example, some scholars analyzed LOTEM data from the Merapi Volcano region in Indonesia, where they considered topographic effects and successfully identified the resistivity properties of the volcanic structure, subsequently yielding valuable new insights into the geological environments of the region [2]. Furthermore, the 2D joint inversion of semi-airborne controlled source electromagnetic (semi-AEM) and LOTEM data over topography was also applied to perform geological interpretation in eastern Thuringia, Germany [32]. However, 3D LOTEM inversion algorithms are currently in the exploratory and developmental stages.…”