- 1 - Use of Dual Laterolog at the determination of fracture parameters in hard rock aquifers by Vilmos Vasvári Geoteam GmbH. Consultants for Hydrogeology, Geothermics and Environment, Austria A-8200 Gleisdorf, Weizerstr. 19, Tel.: +43-3112-6515; Fax: +43-3112-6830 e-mail: geoteam@aon.at Abstract At the present time geophysical measurement and evaluation methods developed in the 1980ies and 1990ies in the petroleum industry for fracture detection are used routinely for deep groundwater exploration. In hard rocks methods for quantifying fracture zones are essential in order to describe their hydraulic properties. In the present paper the estimation of fracture porosity and aperture using Dual Laterolog is presented. The separation between deep and shallow laterolog allows the estimation of fracture porosity and in high-contrast formation the calculation of fracture aperture if mud conductivity is also known. The experiences in application of the methods in practice is presented by examples of different, both carbonate and crystalline aquifers. The calculated fracture apertures of horizontal fractures are compared to those using parallel plate model and cubic law. 1. Introduction Within the scope of a research project the geophysical borehole measurement methods were investigated and selected which are suitable for the characterization of fracture parameters and determination of hydraulic properties in fractured aquifers. New imaging technologies, such as the Formation MicroScanner (FMS) and Formation MicroImager (FMI), can yield data to determine fracture porosity, aperture and dip of fractures (LUTHI & SOUHAITÉ, 1990; HORNBY & LUTHI, 1992), but these measurements and their evaluation are expensive in comparison to others. Furthermore, the data evaluation requires special knowledge and specific hardware and software. Therefore, the Dual Laterolog tool (DLL) was chosen which is used in deep thermal water exploration routinely and allows the estimation of fracture parameters in hard rocks quantitatively as well. By using the Dual Laterolog, based on resistivity anomalies and separation between shallow (LLs) and deep laterolog (LLd) and on mud conductivity, fracture zones can be detected; in addition the fracture porosity and the fracture aperture of horizontal and vertical fractures can be estimated easily. The applicability of the calculation methods was tested in carbonate and crystalline aquifers. 2. Dual Laterolog resistivity measurement The Dual Laterolog (DLL) provides two resistivity measurements with different depths of investigation: deep (LLd) and shallow (LLs). In both devices, a current beam 2 ft-thick is forced horizontally into the formation by using focusing currents. Two monitoring electrodes are part of a loop that adjusts the focusing currents so that in the section of the borehole between the two electrodes no current flow occurs. For the deep measurement, both measure and focusing currents return to a remote electrode on the surface. Thus the depth of investigation is greatly improved, and the effect of borehole conductivity and of adjacent formations is reduced. In the shallow laterolog, in contrast, the return electrodes which measure the bucking currents are located on the sonde. Therefore, the current sheet retains focus over a shorter distance than the deep laterolog.