Conductive Fracture Zones Research Articles

An airborne tensor VLF system. From concept to realization1

AbstractRadio signals from very low frequency (VLF) transmitters distributed world‐wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so‐called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.

Electric study of fracture anisotropy at Falkenberg, Germany

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.

Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock : Olsson, O; Falk, L; Forslund, O; Lundmark, L; Sandberg, E Geophys ProspectV40, N2, Feb 1992, P109–142

Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock : Olsson, O; Falk, L; Forslund, O; Lundmark, L; Sandberg, E Geophys ProspectV40, N2, Feb 1992, P109–142

Hydrochemical investigations at Finnsjön, Sweden

Ground water collected from within and around a highly conductive low-angle fracture zone in crystaline rock has been studied in relation to drilling and sampling methods, water quality and hydrogeochemical interpretation. Air-flush percussion drilling was demonstrated to be efficient down to 200–250 m; at greater depths, large amounts of drilling debris and increasing groundwater contamination from air dissolution become apparent. A stepwise drilling/sampling procedure using water-flush rotary drilling, in combination with water analysis carried out on site using a mobile laboratory, is recommended for future studies. The chemistry of the Finnsjön ground waters shows a sharp contact between saline and non-saline compositions, this contact corresponding to a highly conductive low-angle fracture zone (Zone 2). This zone is therefore a horizon along which ground waters of contrasting age and chemistry come into contact and partially mix. This mixing has produced a highly supersaturated water resulting in the precipitation of calcite. This is most evident in fractures within the more conductive, upper part of Zone 2, thus partly forming a seal between the two groundwater environments. Comparison of the Finnsjön saline waters with other saline environments, particularly in the Fennoscandian Shield, has shown that the ground waters are dominantly marine in origin, but with clear modifications resulting from water/rock interaction. Apparent radiocarbon ages range from 9000 to 15000 years in those waters containing tritum levels below detection limits. These ages are considered too young and are thought to have resulted from mainly carbon-14 dilution caused by groundwater mixing during the Holocene, when large quantities of glacial melt water were available. This mixing process is also supported by the oxygen- 18 data.

A Novel Conceptual Model for the Flow and Transport in Fractured Rock

ABSTRACTIn crystalline rock groundwater flows predominantly in fractures and fissures. Strongly varying fracture aperture guides the flow preferentially in some parts of a fracture plane, in so called channels. In our hydraulic model the degree of channeling together with the aperture variation along a channel is included as a factor which is the ratio of the aperture from transmissivity measurements and the aperture from the tracer tests.The developed transport model takes into account the coupling of molecular diffusion and advection in a velocity field varying linearly over a characteristic width. Various flow velocities in different parts of a channel cause a transient phase with non-Fickian behavior of dispersion. This might erroneously be attributed to other processes e.g. matrix diffusion when not taken into account in the migration modeling of tracers. Molecular diffusion across the flow field, however, tends to smooth out the transport time differences. With time the dispersion diminishes and becomes more symmetric in confined channels.The concept and models have been applied to predict and interpret field experiments aimed to investigate transport over long distances in highly conductive fracture zones. The analyzed experiments have been performed at the Finnsjön research area in Sweden and they belong to the test case 5 of the INTRAVAL project.

Radioactive Tracer Study Performed in a Dipole Geometry in a Highly Conductive Fracture Zone

ABSTRACTA radioactive tracer experiment has been performed in a highly conductive fracture zone, using a dipole geometry. Anions (131I− and 82Br−) and complexed metal ions (51Cr-EDTA, 58Co-EDTA, 111In-EDTA, 140La-DOTA, 160Tb-EDTA, 169Yb-EDTA and 177Lu-DOTA) have been injected and their properties as non-sorbing tracers were evaluated. Besides, studies of slightlysorbing cations (24Na+, 58Co2+, 86Rb+ and 201T1+) were performed. 99mTcO4 and its chemical analogue 186ReO4 were also injected to study the behaviour of Tc at low redox-potentials. Breakthrough was obtained for Re and for all metal complexes and anions. Some differences in the recovery yields could be seen. No transport of cations, except for Na, could be measured. No breakthrough of Tc could be observed. This indicates that TcO4− was reduced and sorbed.