Geoelectric investigation of Awba earth dam embankment, University of Ibadan, Ibadan, Southwestern Nigeria, for Anomalous seepages.

Abstract

A geoelectric investigation involving the Vertical Electrical Sounding (VES), Dipole-Dipole Electrical Horizontal Profiling and Total Field Self Potential (SP) method was carried out along the embankment of Awba Earth Dam constructed within the Precambrian Basement Complex terrain of Southwestern Nigeria. The study was aimed at assessing possible seepage zone(s) under or within the dam embankment. Thirty two VES stations at 10 m interval, were occupied along two 170 m long N-S trending traverses established on the crest of the dam embankment. The VES data were quantitatively interpreted using the partial curve matching technique and 1-D forward modelling with WinResist 1.0 version software. The dipole-dipole data were inverted into 2-D resistivity images using the DIPPRO™ 4.0 inversion software. The SP data were presented as a profile and interpreted qualitatively. Three geoelectric layers were delineated along the two traverses. These include the clay/sandy clay/laterite (cap rock) with layer resistivity and thickness values ranging from 59 - 675 ohm-m and 0.3 3.2 m respectively; a clay/sandy clay core/weathered layer with layer resistivity and thickness range of 8 - 166 ohm-m and 3.2 53.3 m respectively and a third layer of basement bedrock with layer resistivity values of between 498 and 100000 ohm-m. The 2-D resistivity structure revealed that the core of the dam embankment is characterized by relatively low resistivity zones typical of seepage paths at stations 2 - 3 (20 - 30 m) at about 8 m depth; 4 - 6 (40 - 60 m) and 9 - 11 (90 - 110 m) at a depth range of between 4 and 10 m within the dam embankment along traverse 1. The low resistivity zones were delineated between stations 4 and 6 (40 60 m) and 9 and 11 (90 110 m) along traverse 2 at depth range of between 0 and 5 m. A major peak negative SP anomaly with amplitudes of about -80 mV and -104 mV on traverses 1 and 2 respectively was delineated between stations 2 and 4 (20 40 m). This anomaly is located on a metallic bleeding pipe concealed between the spillway and the dam embankment. The study concluded that the dam embankment contain three suspected seepage zones within / beneath the core.

Keywords: Geoelectric Investigation, Dam Embankment, Anomalous Seepage, University of Ibadan, Nigeria.