Abstract
In this study, the practical usefulness and fundamental applicability of a self-potential (SP) method for identifying the hydraulically-active fractures were evaluated by a comparison of SP methods with other geophysical logging methods and hydraulic tests. Potentially permeable fractures intersecting a shallow, 10-m borehole were first identified using conventional geophysical methods such as acoustic borehole televiewer imaging and temperature, electrical conductivity and gamma-gamma logs. These results were then compared to results of analysis using the SP method. Constant pressure injection and recovery tests were conducted for verification of the hydraulic properties of the fractures identified by various logging methods. The acoustic borehole televiewer and gamma-gamma logs detected the openings or weathering zones associated with fractures intersecting the borehole, but they could not prove that groundwater was flowing through the detected fractures. The temperature and fluid-column electrical conductivity logs were unable to detect the fractured zones where groundwater in the borehole flows out to the surrounding rock aquifers. Comparison of results from the different methods tested showed that the most effective correlation between logs and the distribution of hydraulic conductivity was given by the SP signals, and that SP logging can accurately indicate the location of hydraulically-active permeable fractures. Based on these results, the SP method is recommended for determining the location of hydraulically-active fractures rather than other conventional geophysical logs. This self-potential method can be effectively applied in the initial stage of a site investigation to select the optimal site location or to evaluate the hydrogeological properties of fractures in underground exploration studies, such as those related geothermal reservoir evaluation and radioactive waste disposal.
Highlights
Fractured rock aquifers are currently regarded as of particular interest and an important issue because of their usefulness related to underground water supply, energy storage, and radioactive waste disposal
It is difficult to determine the hydraulic activity of the fractures detected using only borehole televiewer (BHTV) image logging without the verification provided by precise hydraulic tests
After identifying possibly permeable fractures with various logging methods, the results of precise hydraulic tests were used to verify the hydraulic properties of the various fractures detected by the BHTV, temperature, fluid electrical conductivity, gamma-gamma, and self-potential logs
Summary
Fractured rock aquifers are currently regarded as of particular interest and an important issue because of their usefulness related to underground water supply, energy storage, and radioactive waste disposal They are generally characterised by strong heterogeneity due to the inherently large contrasts in hydraulic properties between the highly permeable fractures and the surrounding rock matrix of generally negligible permeability. There are many conventional geophysical methods for the detection of individual fractures in situ, such as temperature, calliper, gamma-gamma and fluid electrical conductivity logs (Tittman, 1965; Malcolm, 1984; Hearst et al, 2000) These borehole-logging methods, which are generally used for identifying a relatively large-scale open fractured or weathered zone, have limitations for identifying the small-scale permeable fractures serving as pathways for groundwater flow. The acoustic borehole televiewer, a more visually-advanced logging technique, provides high-resolution images of the location and dip of fractures within a borehole (Keys, 1990), but still cannot provide any information with regard to the hydraulic activity of the fractures
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