Application Of Electromagnetic Methods To Locate Shallow Water Well Sites In A Fractured Aquifer

Low permeability fractured bedrock makes it difficult to select productive locations<br>for shallow water wells in the Atebubu district of Ghana, West Africa. Drilling has<br>demonstrated that productive and nonproductive sites are frequently separated by less than<br>50 meters. The drilling success rate is approximately 25 percent when experienced<br>hydrogeologists use only surface visual observations to select drilling sites. A combination<br>of remote sensing and geophysics was used to improve the success rate. SPOT satellite<br>imagery and stereo aerial photography were used to identify areas where suspected<br>fractures cross drainages. These areas were typically 100 to 150 meters in diameter. The<br>frequency domain electromagnetic method was used to locate zones in these areas where<br>the electrical conductivity increased with depth. These were interpreted as zones of deeper<br>weathering and fractured bedrock, and were selected as favorable drill sites. Use of this<br>exploration strategy increased the success rate 35 f 10 percent when the best site within<br>2 kilometers of the target community was selected. This doubled the success rate and<br>reduced the number of holes drilled to provide wells to these community by more than a<br>factor of two. When SPOT satellite imagery and aerial photography were used without the<br>frequency domain electromagnetics, the success rate only increased 8 f 8 percent. The<br>methods are cost effective when they are implemented by Ghanaian personnel and in areas<br>that have a low drilling success rate.