Application of Seismic Refraction Survey Methods in Foundation Evaluation in Parts of Legon-Adenta Areas, Greater Accra Region

Abstract

This dissertation sought to find out the use of seismic refraction to determine the depth of the bedrock beneath unconsolidated sediments at Legon in the Accra Metropolitan Area of the Greater Accra Region of Ghana. Seismic refraction which is a traditional geophysical method commonly used to determine the depth to bedrock, competence of bedrock, depth to water table or depth to other seismic velocity boundaries was used to carry out the survey. Since the early 1980,s, seismic refraction method has been increasingly used in shallow environmental and engineering site characterization works. This work is based on generation of direct compressive wave using a near surface impulsive energy which propagates through the soil media and it is refracted along stratigraphic boundaries. The signal generated undergoes refraction based on the interface boundary encountered media through which it is propagated. The refracted signal return to the ground surface ant time of arrival is recorded by geophones. Four major tasks were carried out in determining the depth of the bedrock beneath unconsolidated sediments. The first part was using arrival time at geophones and the distance from shot points to geophones to plot time – distance graph to obtain intercept on time axis. The second was inferring two different layers due to the two different velocities obtained as the waves propagate through the soil. Also models were generated to observe the two layers as well as the depth of the second layer and thickness of the first layer. Finally the velocities in the layers were used to infer the nature of the layers. These major tasks are carried out and interpreted to produce results that will determine the depth and ability of the subsurface to support foundations of structures.