Seepage Investigation on an Existing Dam Using Very Low Frequency Electromagnetic (VLF-EM) Methods: A Case Study of Shagari Earth Dam, Sokoto, North Western Nigeria

摘要

Very low frequency (VLF-EM) was used to assess variations in overburden composition, bedrock lithology and the hidden Sedimentary structures within the foundation of Shagari Dam of the sedimentary basin of Northwestern Nigeria. Four VLF-electromagnetic (EM) traverses were occupied at 10 m in-tervals. The VLF normal and filtered real component irregularities identify major geological interfaces suspected to be faults/fractured zones. The points of crossover between the real and imaginary components delineate the fractured zones, which were identified as areas of possible seepage (sloughing and piping). The fractured zones are suspected to be present at all traverses. In total, 38 fractured zones were identified along the dam embankment and canal site, while 17 major fractures occurrence dippers along the Traverses at a point, Traverse 1 (F3, F4, F5, F7, F10, F11), Traverse 2 (F12, F13, F21, F22), Traverse 3 (F23, F24, F29) and Traverse 4 (F32, F33, F34, F38), and coincide with cross over point at 4 traverses. These seepage zones cause heterogeneity in the sub-surface structure that could be prime to dam failure which in turn leads to the flooding, decreases in irrigations activities of the peoples leaving around the dam and loss of several hundreds of life when care not taken. The result of the study suggests that VLF is suitable for observing seepages in embankment dams.