Modeling of airborne electromagnetic anomalies related to fractured bedrock and overburden

Responses of a multifrequency, multicoil airborne electromagnetic (AEM) system were modeled using numerical techniques. Special emphasis was given to poor, three-dimensional electrical conductors embedded both in the bedrock and in the overburden. The results cover vertical coaxial and horizontal coplanar configurations and three frequencies: 888 Hz, 7837 Hz and 51250 Hz. The models studied are signal conductors in free space, and single or multiple conductors embedded in a host rock of high but finite resistivity (5000 Wm) and overlain by a layer of overburden with finite resistivity and thickness. Two different types of computer software were used in the modelling: the free-space PLATE code, and the EM3D set of codes. Modeling results are given both as profiles and as charasteristic diagrams for the various coil configuration - conductor-model combinations. On the basis of the modeling results, limits of detectability for poor conductors have been determined. The study is a part of the preliminary site investigations for the radioactive waste disposal in Finnish bedrock