Characterisation of hydraulically-active fractures in a fractured granite aquifer

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.

What Do P-Wave Velocities Tell Us About the Critical Zone?

“…In most cases, depending on how the sample was obtained, the lab sample will have a lower hydraulic conductivity because the pump test is an average of high and low hydraulic conductivity fractures along the entire length of the borehole (Clauser, 1992) (equivalent to the large yellow area in Figure 8). It is not uncommon in fractured rock aquifers to have one or two highly productive fractures while others do not contribute to flow (Sausse and Genter, 2005;Lee and Kim, 2015).…”

Hydraulic characteristics of a fractured crystalline basement aquifer in Nzhelele area, Limpopo Province, South Africa

“…Due to the low intrinsic primary permeability and porosity of the bedrock, crystalline aquifers differ from other aquifer types, and demand-specific knowledge and techniques if groundwater is to be extracted and managed efficiently (Holland, 2011). In addition, Lee and Kim (2015) reported that sustainable use of groundwater in fractured rock aquifers requires detailed knowledge of their hydraulic properties (transmissivity, storativity and hydraulic conductivity).…”

Fracture flow characterization with low-noise SP logging

“…This potential vanishes as the borehole waters are homogenized by diffusion and advection. For mature wells, the SP signals are mainly from electrokinetic origin, nevertheless achieving much lower amplitudes up to 10 mV (Revil et al, 2006;Mendonça et al, 2012;Lee and Kim, 2015). The electrokinetic potential is caused by the electrical double layer (EDL, Fig.…”