Geophysical Assessment of Dam Foundation Integrity at Challawa Gorge Dam, Kano State, North West Nigeria

Challawa Gorge Dam in Kano State is a major water supply reservoir serving Kano metropolis, and concerns about potential foundation seepage and structural anomalies in the embankment require systematic geophysical investigation to support dam safety management. This professional study employs electrical resistivity tomography, self-potential survey, and seismic refraction to assess the integrity of the dam foundation, abutments, and embankment materials at Challawa Gorge. ERT profiles totalling 3,000 metres were acquired along the dam crest, abutment flanks, and downstream area using the Wenner-Schlumberger array with a 10-metre electrode spacing. Self-potential measurements were obtained along 8 profiles covering the embankment and downstream apron, with the fixed-base electrode configuration. Seismic refraction traverses of 120 metres were acquired at 6 locations in the foundation zone to characterise rock quality and estimate dynamic elastic moduli. Anomalously low self-potential values of between -45 and -80 millivolts on the downstream left abutment are interpreted as indicators of preferential seepage through a fractured zone in the abutment rock. ERT models confirm the presence of a low-resistivity corridor at depth consistent with water-saturated fractured granite underlying the left abutment. Seismic velocities of 1,200 to 2,500 metres per second in the foundation zone indicate moderately weathered to fresh rock suitable for the existing gravity dam loads, except in two anomalous sections. The study provides a quantified risk assessment and prioritised remediation recommendations for the dam authority. Keywords: dam integrity, seepage, ERT, self-potential, Challawa Gorge