Geoelectrical Characterization of Rock Formations Underlying Idonyi River, Amaeke-Abam, Southeastern Nigeria

Abstract

The geoelectrical characterization of soils underlying the Idonyi River Dam site Amaeke-Abam, Southeastern Nigeria has been carried out with a view to determining the presence or otherwise of deleterious matter along the dam axis down to a depth of 37m below the surface; evaluate the geo-structural setting of the concealed bedrock, the fracture pattern and possible dam seepage along the dam axis and its banks as well as giving suggestions on any other measures/studies that may be needed for a proper engineering design. The study area is around the river located between latitudes 5 o 34'N-5 o 37'N and longitudes 7 o 39'E-7 o 42'E. Heights above sea level in the area range from 68 to 91 meters. The survey was carried out with Abem Terrameter SAS 1000C. The sounding stations were laid out in three (3) profiles; eight (8) VES on each profile. A total of twenty-four (24) VES data were conducted. The Schlumberger electrode configuration was adopted to a total spread length of 110 meters (AB/2 of 55 meters). Results showed that resistivity values of between 182 and 2637Ωm at the eastern part of the River and another range of 6.2 to 848Ωm in the western end suggest overriding sandy substratum to east and overriding shaly characteristics to the west. These results show clearly that the area around Idonyi is porous with relatively higher sandy fractions.The derived parameters on both sides of the river agreed with the standard parameters required for a dam construction. The failure of a previous dam near the river is thus attributable to much seepage due to overriding sandy substratum. To avoid the seepage, the base of the dam should be blanketed with an impervious material such as shale. The area, being the most fertile area (phosphatic Ameki units) with CRIN, NRRI, FRIN, RCRIN having research institutes near the river, a dam is necessary here and this study has shown that it is possible to have a dam with solid foundation. However, we suggest a baseline data survey of water quality before any construction so as to monitor any contamination trend due to impoundment.