Application of Electromagnetic and Electrical Resistivity Methods for Borehole Siting in Low-Grade Metavolcanic and Metasedimentary Rocks, Sunyani West, Ghana

Abstract

This paper employs the electromagnetic (EM) and the electrical resistivity methods for siting boreholes for communities in the Sunyani-West district of Ghana, a terrane comprising a mixture of granites and low-grade metavolcanic and metasedimentary rocks. These communities which depend on groundwater for livelihood have over the past few decades been hit by drastic seasonal irregularities in rainfall, making it imperative to seek an alternative source of potable water (preferably groundwater). Measurements to locate sites for boreholes were made by employing the Geonics EM-34 conductivity meter. In addition, co-located measurements were made with the ABEM SAS 1000C Terrameter in the Schlumberger electrode array configuration. Results of the measurements showed that each community could have at least two borehole sites with a high potential for groundwater extraction. The overburden of selected sites varied for the communities and ranged from 40 to 67 m. However, recommended drilling depths at these sites were 60 m for Adwinsa community, 70 m for Akurakan community and 60 m for Nana Ameyaw community. Test boreholes sunk at respective depths of 45 m, 55 m and 68 m in Adwinsa, Nana Ameyaw and Akurakan had yields of 12 m 3 /h (200 l/min), 24 m 3 /h (400 l/min) and 1.14 m 3 /h (19 l/min) respectively. In all three communities the aquifer was intercepted in phyllite bedrocks.