Mapping the Subsurface Structure of the Bosumtwe Impact Crater Using Ground Penetrating Radar (GPR)

The Bosumtwe crater in Ghana, is a unique structure and ecosystem on earth. It is one of the 19 impact craters known in Africa. The crater is located 30 km southwest of the city of Kumasi in the Ashanti Region of Ghana. The Bosumtwe crater is 1.07 million years old and is almost completed filled with a lake which is 8.5 km in diameter. This has made the Bosumtwe crater a location of prime importance to the scientific community. Other reasons for interest in the Bosumtwe crater are; the visits of over a 100,000 tourists to the area each year leading to the development of numerous tourists resorts, the presence of over 24 communities within the crater whose sustenance depends on the sustainability of the crater, and the presence of known gold deposits. The challenge is to use modern scientific methods to solve and prevent future problems such as landslides, collapse of buildings and contamination of the lake that may arise. One of the effective set of tools for studying the subsurface is near surface geophysics. Ground penetrating radar (GPR) was used in this case because of its higher resolution and the ability to show more detail. In recent years, this method has evolved into the fastest growing geophysical method worldwide. This study had the objective of delineating subsurface structures such as the depth to bedrock; the water table; the fracture pattern, distribution and orientation within the crater; and also find out if fractures serve as potential fluid migration paths for contaminants to enter the lake. Radar methods use the reflections of short bursts of electromagnetic energy (chirps) spanning a range of frequencies from about 50% below to 50% above some specified central frequency. The Mala GPR system usinf 25 MHz and 50 MHz RTA was used for conducting the geophysical survey. 20 GPR profiles were taken across the northern section of the inner morphological crater. The digital data gathered was processed using Sandmeier’s Reflex-Win software and iv three migration algorithms were used namely; diffraction stack, f-k (Stolt) and Kirchoff. Various subsurface features were delineated. These include unconsolidated breccia layer, shattered bedrock, fractures, duplex or lens shaped bodies, and a coherent water table. The depth to bedrock varies between 5 m to 20 m. The depth to water table also varies between 5 m to 20 m. Within the inner morphological crater, multiple fractures with different orientations occur. The water table within the inner morphological crater is at a similar elevation as the surface of the lake. Movement of groundwater into the lake is negligible. The water table within the crater is largely influenced by the water level within the lake. There is a high possibility of contamination of the lake from agrochemicals used for cocoa farming within the crater migrating through the fractures into the lake. The resort towns of Abonu and Obo have a high occurrence of fractures and this information is relevant for engineering sustainable resorts in these towns. The diffraction stack algorithm successfully migrated hyperbolas and the subsurface features to their correct depths.

Source: KNUST