CHARACTERIZATION OF FAULT SYSTEMS USING GEOPHYSICS IN THE SOUTHWESTERN PARTS OF THE AKUAPEM – TOGO RANGE, SOUTHEAST GHANA

ABSTRACT The Akwapim Fault zone is one of the two major fault zones in the southeastern part of Ghana. Its junction with the Coastal Boundary Fault happens to be epicenter for most of the earthquakes that occur in Ghana. The earliest work done in this region was by the Gold Coast Geological Survey on a regional scale in mapping the area to determine the geological structures. With most of geological deformational features being inferred, it is prudent to carry out more research to identify more features to substantiate or disprove the inferences that have been made through different modes of investigation. In this study, regional resolution magnetic data, time and frequency domain electromagnetic data and gravity data are some geophysical data that are used to help define the structural setting of the Akwapim fault zone. Ground geophysical survey in addition to geological field mapping is carried out in the area to augment the information given by the aero geophysical data. The electrical resistivity methods comprising the Square array, the Schlumberger vertical electrical sounding and the Wenner azimuthal sounding were the geophysical survey methods used. The data obtained from the surveys were processed and interpreted. The data from the geological field mapping is plotted and analyzed to identify the attitudes of the geological structures associated with deformational terrain. From the aero geophysical data processed, grids are made to distinguish features relating to the deformation occurring within the study area and also identify a relation between the results of the surveys carried out. The occurrence of relatively high amounts of uranium from the radiometric concentration grid produced indicates the occurrence of tectonism within the area. The various results obtained from the ground geophysical survey were compared to see which array or combination of arrays best describes the subsurface. Plots from the various geo-electric arrays produced results to indicate a consistent layer of very low apparent resistivity for all the survey points. The Square array and the azimuthal array were used to identify fractures and their properties while the Square array and the Schlumberger array were used to locate the bedrock at the various sample point. The square array had a higher penetration effect than the schlumberger array. Data from the azimuthal array was used to plot possible fault lines on a geological map of the area indicating that the faulting is not limited to just one geological domain. The square array tends to require less space than the two other arrays used and as such can be singularly used to do both depth soundings and azimuthal surveys to locate fractures at depth.