HYDROGEOPHYSICAL CHARACTERIZATION OF GROUNDWATER FORMATIONS BASED ON WELL LOGS: CASE STUDY ON CENOZOIC CLASTIC AQUIFERS IN EAST HUNGARY

Ground geophysical surveys can be effectively used for detecting and delineating shallow groundwater structures. For calculating the freshwater reserves, well-logging measurements need to be made in prospecting wells. In this paper, groundwater formations are evaluated using electric and nuclear logging data to extract the petrophysical and hydraulic parameters of aquifers and aquitards. To quantify the relative fractions of rock constituents, the effective porosity, shale content, water saturation and rock-matrix volumes should be estimated. The effective layer-thicknesses of permeable beds are of importance in locating the depth-intervals of water exploitation. The above parameters cannot be calculated reliably when the zone parameters such as cementation exponent, tortuosity factor, pore-water density and resistivity are not certain. With accurate petrophysical and zone parameters, an estimate can be given to hydraulic conductivity, which is one of the most important hydraulic rock properties in solving hydrogeophysical problems. First, a comprehensive interpretation method proposed by Professor János Csókás (1918-2000), former head of the Department of Geophysics, University of Miskolc, is used to give an estimate of hydraulic conductivity and critical velocity of flow without the need for grain-size data. Then, shale volume and hydraulic conductivity are determined separately by statistical factor analysis of well logs. Effective porosity, specific surface of grains and water saturation can be derived by well-known deterministic equations. Those of core measurements confirm the results of well log analysis. A set of detailed regression analyses is performed to specify the local regression relationships between the estimated parameters. It is also shown that there is a strong correlation between shale volume and hydraulic conductivity (and other quantities) and that the independent interpretation results are consistent. The advantage of the Csókás and factor analysis-based approaches is that instead of using a single well log, they utilize all types of well logs sensitive to the relevant petrophysical/hydraulic parameters for a more reliable hydrogeophysical characterization of aquifers