Correlation between geological logs and geophysical logs to estimate aquifer positions in the crystalline basement rocks of the birimian system, Ghana

Abstract

This study has enabled the lithological units intersected by a well, locate contacts of adjacent beds, locate depth to groundwater and determine the thickness of the units. Records of three geophysical logs parameters; resistivity, gamma ray and self-potential (SP) measured in five boreholes which were drilled as part of a Rural Water Supply Project Phase II in the Juabeso –Bia District were correlated with the geological logs on site from the drill cuttings of the bore hole. The boreholes depths were between 45-52m and were drilled over hornblende, phyllites and granites which constitute part of the various rock types of the Birimian system. Deflections of the SP logs established the presence of permeable rock formations. Alternatively, the long-short- normal logs distinguished between permeable and impermeable beds by recording lower or higher apparent resistivity signals respectively on the log sheet. These anomalies are identified mostly in the weathered zones and also within the fresh rocks. In almost all the boreholes the gamma ray logs indicated low clay content which may be due to the absence of intensely weathered rocks or low concentration of radioactive rocks within the survey area. The application of gamma ray logs rely on the fact that the concentrations of radioactive isotopes varies from one rock type to another and that acidic rocks are more radioactive than the basaltic rocks hence most of the rocks in the project area is more basaltic. Though the SP deflection was able to indicate permeable formations there was no guarantee that the rock formation could contain water. However, the combined interpretation of the three other logs shows that deflections of SP close to the basement rocks and lower resistivity values on the long short-normal logs generally indicated the presence of water bearing formations. Three lithological units were identified over the various rock types: a top layer predominantly clay of thickness between 0-25m overlying a weathered zone about 16-45m thick above the fresh rocks of the various types