Investigation of seepage paths from dam embankments using magnetic methods: case study of Jebba earth dam, Nigeria

Abstract

This study aims to detect possible seepages paths from the dam embankment and describe structural defects such as faults, joints resulting from fracturing, and their antagonistic effect on the dam mbankment. The land magnetic survey was conducted to identify the chief structure and the depth to the basement along the embankment part of the Jebba dam. Data were acquired with a proton precession magnetometer G-857 along three parallel profiles oriented NS with a length of 1.21 km along the dam embankment at an interval of 10 m apart. The Analytical Signal map against the total magnetic intensity (TMI) map indicated a fault at 48 m distance from the beginning of profile 1 closest to the dam axis, and an intrusion at 34 m distance. Faults were detected around horizontal distance 12 m, 38 m and there is an intrusion at 4 m and 54 m distance along with profile 2. No faults were observed but an intrusion was observed at a 95 m distance along with profile 3. Reduction to Pole map was produced to show the correct position of the magnetic susceptibility over profile distance. The correlation of the upward modeling maps and first derivative maps shows that the geological structures are continuous from shallow depth to a deeper depth. Thus, this study shows that beneath the embankment (i) there exist fractures and intrusions (ii) the geological structures are continuous from shallow depth to the deeper depth, (iii) there is enough evidence to suspect seepage in the dam area. Therefore, the magnetic investigation revealed the present condition of the Jebba dam.