Integrated geophysical methods and techniques for siting productive boreholes in basement complex terrain of southwestern Nigeria

Abstract

Electrical resistivity survey involving 2D Dipole-Dipole subsurface imaging and 1D Vertical Electrical Sounding (VES) and an integrated magnetic and electrical resistivity survey involving magnetic profiling and 1D VES/2D electrical imaging techniques were carried out at two sites underlain by grey gneiss and pegmatised schist, respectively, within the Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. The surveys were carried out with a view to delineating the subsurface layers and determining the geoelectrical characteristics; identify geological structures such as faults, fractured, jointed and sheared basement zones that are favourable to groundwater accumulation and transmission and estimate hydrogeologically significant borehole drill depth in an area with previous experience of failed borehole drilling attempts. At the two sites (A&B) investigated, the 2D resistivity images identified suspected fractured basement zones as low resistivity vertical discontinuities within high resistivity fresh basement host rock. The magnetic profiles at site B displayed typical magnetic anomaly of a thick dyke (two magnetic lows with central high in low magnetic latitude) over each vertical resistivity discontinuity. The follow-up VES delineated maximum of five subsurface layers which include the topsoil/laterite, weathered layer, partly weathered/fresh basement, fractured basement and the fresh basement. The weathered/fractured basement constitutes the main aquifer unit with thicknesses varying from 17.7-125 m. Two boreholes, one per site, were drilled to 80 m at site A and 70 m at site B. Both boreholes encountered the predicted fractured basement zones and the borehole logs correlated well with the interpretation models of the drilled VES locations. The boreholes were very productive (yields of about 1.5 l/s and 2.2 l/s at sites A & B respectively). The study demonstrated the effectiveness of integrated geophysical methods and techniques in providing information on the subsurface sequence and the structural disposition required for a successful groundwater development through borehole drilling, in a borehole-failure-prone basement complex terrain.

Hydrochemical characterization of shallow and deep groundwater in Basement Complex areas of southern Kebbi State, Sokoto Basin, Nigeria

“…Hydrogeological studies of the pre-Cretaceous Basement Complex Formations of Nigeria (Akinluyi et al 2018;Anderson and Ogilbee 1973;Olorunfemi and Oni 2019), revealed an average borehole yield of 14.67 gallons per sec (gps) from a mean depths of 37.49 m. Though, few boreholes in the unweathered rock, typically granite or gneiss produced more than a meager amount of water; in many wells, no water was found (Anderson and Ogilbee 1973). In contrast, wells tapping weathered granite and gneiss, where fully saturated, are found to yield the highest quantity of water (51.67 gps); the average yield for boreholes tapping battered rock was estimated to be around 23.33 gps per borehole (Anderson and Ogilbee 1973).…”

“…3 The geological cross section of Sokoto Basin and the normal depth to water stand at about 6 m (Anderson and Ogilbee 1973). Generally, groundwater condition in the basement section of Sokoto Basin is typical of the Nigerian Basement Complex described by (Akinluyi et al 2018;Olorunfemi and Oni 2019).…”

“…This kind of poor yields is also characteristic of Basement Complex Formation of Nigeria. Such low yield zone results from aquifers of weathered or fractured materials, with limited areal scope, such that during pumping or well development the spreading cone of depression hits the highly indurated frontier rocks in a short time (Akinluyi et al 2018;Anderson and Ogilbee 1973;Olorunfemi and Oni 2019). The water level, or the drawdown, drops sharply and steadily decrease the yield per individual boreholes.…”

“…High yields can be attained in some areas in the Basement Complex, contingent on the tectonics in those areas. Despite the poor yields, boreholes and hand-dug wells in Basement Complex zones are sufficient to sustain hand pumps, in rural water supply programs (Akinluyi et al 2018;Anderson and Ogilbee 1973;Olorunfemi and Oni 2019). Figure 4 illustrates a typical lithologic section of the study area.…”

“…The fissures are usually most open above a depth of 91 m but even so, yields to boreholes are relatively low and cause high drawdown (Anderson and Ogilbee 1973;Offodile 2002). While boreholes are widely used in the study area as sources for improved water supply, shallow groundwater remained the most reliable source of drinking water especially in rural areas (Mohammed et al 2007;Adelana et al 2003;Aleke and Nwachukwu 2018;Olorunfemi and Oni 2019). However, the geological settings in addition to changes in land use combined with rock mineral influence groundwater composition (Dehnavi et al 2011; Khatri and Tyagi 2014;Redwan and Abdel Moneim 2016;Venkatramanan et al 2017;Jebreen et al 2018;Mukate et al 2019).…”