Evaluation of Geophysical Methods for Water-Table Monitoring in the Sahel Zone of Yobe State

This study evaluates the suitability and performance of geophysical methods for monitoring temporal variations in water table depth in the Sahel zone of Yobe State, North East Nigeria. The Sahel zone's seasonal and inter-annual variability in groundwater recharge creates critical management challenges for community water supplies dependent on shallow aquifers. Conventional monitoring using observation wells is prohibitively expensive for the spatial densities needed across the region, and geophysical methods that can detect water table fluctuations cost-effectively would significantly improve monitoring capability. This study tests three geophysical approaches for water table monitoring at two instrumented sites in Nguru and Potiskum local government areas where reference observation wells with continuous data loggers are installed: time-lapse VES surveys acquired at monthly intervals over one hydrological year, time-domain EM surveys (TEM47) at quarterly intervals, and frequency-domain EM (EM34) profiling at fixed monitoring stations monthly. Geophysical water table depth estimates are compared with direct observation well measurements at each acquisition date. Findings reveal that time-lapse VES is the most accurate method, with mean absolute error in water table depth estimation of 0.8 metres over the monitoring period. TEM achieves comparable accuracy at 1.2 metres mean error with faster acquisition. EM34 shows greater positional uncertainty for thin, shallow water table targets. All three methods detect the major seasonal water table decline of 3.1 metres observed in the reference wells. The study concludes that time-lapse VES and TEM are both feasible for regional water table monitoring in the Sahel zone and recommends a quarterly TEM monitoring network as the most cost-effective monitoring approach.

Keywords: water table monitoring, time-lapse geophysics, Sahel zone, Yobe State, TEM.