Electric study of fracture anisotropy at Falkenberg, Germany

Abstract

Electric and electromagnetic methods have been applied for mapping subsurface fractures and the directional dependence of in‐situ electric parameters at the hot dry rock site at Falkenberg, Germany. This study includes the determination of several anisotropy parameters like the mean, longitudinal and transverse resistivity components ([Formula: see text], [Formula: see text] and [Formula: see text], respectively), the anisotropy coefficient λ, and the strike angle Θ. Terrain conductivity measurements using the technique of frequency‐domain electromagnetic induction reveal a dominant anomaly strike of east‐south‐east— west‐north‐west, nearly parallel to the fracturing strike of N110°. With increasing distance from the central borehole, the mise à la masse potential differences exhibit a transition from a direct to a paradoxical relationship to the resistivity anisotropy induced by the fracturing. These observations are explained using a model for an ellipsoidal fracture. The qualitative interpretation of the sounding data of Schlumberger and crossed‐square arrays clearly shows the anisotropy paradox related to the N110° strike. The crossed‐square method applied over a range of electrode spacings of 2.1–447 m yields apparent anisotropy values of [Formula: see text], [Formula: see text], [Formula: see text], λ = 1.15–1.34 and Θ ≈ 110°. The quantitative interpretation of the sounding data shows a conductive fracture zone (ρ ≈ 300–700 ω ⋅ m) embedded in the resistive granitic basement (ρ > 1200 ω ⋅ m) at a depth of 55–85 m. The anisotropy parameters for the fracture zone are ρm = 700 ω ⋅ m, [Formula: see text], ρt = 1225 ω ⋅ m and λ = 1.75. The fact that the anisotropy coefficient of the conductive zone is higher than the apparent λ is in agreement with the conclusion that the anisotropy at the study site is related mainly to fracturing.