标题：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, bedrocklithology and the hidden Sedimentary structures within the foundation ofShagari Dam of the sedimentary basin of Northwestern Nigeria. FourVLF-electromagnetic (EM) traverses were occupied at 10 m in-tervals. The VLFnormal and filtered real component irregularities identify major geologicalinterfaces suspected to be faults/fractured zones. The points of crossoverbetween the real and imaginary components delineate the fractured zones, whichwere identified as areas of possible seepage (sloughing and piping). The fracturedzones are suspected to be present at all traverses. In total, 38 fracturedzones were identified along the dam embankment and canal site, while 17 majorfractures 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 overpoint at 4 traverses. These seepage zones cause heterogeneity in the sub-surfacestructure that could be prime to dam failure which in turn leads to theflooding, decreases in irrigations activities of the peoples leaving around thedam and loss of several hundreds of life when care not taken. The result of thestudy suggests that VLF is suitable for observing seepages in embankment dams.
