Geophysical methods of investigating Seismic refraction at dam sites in Nigeria

Abstract

Ajele Badmus Ernest

Seismic refraction study has been carried out on Gurara Dam Phase II proposed axis with the aim of determining its feasibility for the founding of a dam. The field techniques involved off-end/reverse and split spread shootings on five profiles. The study site is situated on the Precambrian Crystalline Basement Complex rocks of North Central Nigeria. Velocity values obtained from the shots vary between 183 ms-1 in unconsolidated soil materials and 6522 ms-1 in the fresh bedrock. The overburden is presumably characterized by low to moderately high velocity varying between 183 and 1235 ms-1 . Fractured/partially weathered basement rocks within the study area are presumably defined by velocity ranging from 543 to 1667 ms-1 . The fresh bedrock is presumably characterized by velocity in excess of 2000 ms-1 . The results of the seismic refraction study conducted on the proposed dam axis showed that some portions of the western flank of the axis present noticeable seepage characteristics within the bedrock especially on profiles 1 and 2. Profile 4 on the eastern bank of Gurara River present a downward displacement of the bedrock. Profile 5 also on the eastern bank is characterized by significant fracturing of the bedrock with possibilities for seepage. The general characteristics of the materials within the investigated sections of Gurara Phase II dam study area indicate that they are competent and suitable. However, the existence of the delineated features susceptible to seepage requires some form of remediation to prevent seepage from the area proposed for reservoir through fractures and buried stream channels.

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