Shallow seismic investigation for location and evaluation of groundwater reserves in the weathered mantles of the Basement Complex in southwestern Nigeria

Abstract

This study revealed that the groundwater zones in the weathered mantles in the Basement Complex of southwestern Nigeria can be reliably located by the seismic refraction method. The water table and the weathering front were each detected as a distinct refracting horizon. A seismic velocity range of 820–1060 m/s was observed in the unsaturated weathered material above the water table. The zone of saturation has a velocity range of 1550–1740 m/s, while in the underlying weathering country-rocks the range is from 2840 to 3160 m/s. The depth to water table, the saturated zone thickness and the absolute depth of weathering were seismically determined with a high level of precision. The data acquired were statistically analyzed to establish the major geohydrological characteristics of the regoliths and to evaluate the groundwater storage in the overburden. The outstanding geohydrological features detected in the two study areas are: 1. (1) A widespread zone of saturation in the weathered mantle which implies a high success ratio for well-sinking programmes and which provides the evidence that baseflow from the regolith aquifers is a component of the main rivers' discharge. The basin-anddome weathering pattern, however, gives rise to locales of exceptionally thick ground-water zone. 2. (2) A direct relationship between saturated zone thickness and weathering depth which implies that water wells should penetrate the entire weathering profile to ensure maximum and perennial yields. 3. (3) Depth to water table significantly increases from the main river channels to the major water divides, a finding that conforms with an established geohydrological principle and which implies that the depths needed for wells at specific sites in the studied catchments can be reliably predicted in advance of the actual digging or drilling operations. 4. (4) A vast quantity of groundwater resources is stored in the regolith and as much as about 55% of the total storage is recoverable as well-water, baseflow in the main rivers, and as springwater.