Groundwater Exploration in a Heterogeneous Basement Complex Environment Using Integrated Geophysical Methods Sola O. Ayantola * , John O. Amigun Department of Applied Geophysics, Federal University of Technology, Akure 340110, Nigeria Corresponding Author Email: ayantolaso@futa.edu.ng https://doi.org/10.18280/eesrj.100103 ABSTRACT Received: 15 July 2022 Accepted: 15 December 2022 The integrated very low frequency of electromagnetic (VLF-EM) and vertical electrical sounding technique (VES) of electrical method were carried out to explore for groundwater resource in a very complex terrain within Akure metropolis. The VLF data was acquired at 10m interval along five traverses (TR 1 to 5). The conductive zones mapped on EM profiles and 2-D pseudo-section suspected to be fracture or localized features were further probed using a VES technique. A total number of 12 VES points were acquired to ascertain the fractured or contact zones. Relatively high conductivity is observed around the portion marked as F1, F2 and F3. VES conducted on these conductive zones acknowledged the suspected fractured or geologic contacts zone observed on the EM profiles. VES 1, 7 and 11 acquired on the conductive zones delineates both weathered and fractured layers with the resistivity values ranging from 80 to 490Ωm and 228 to 935Ωm respectively. These layers are considered to favour groundwater accumulation. VES acquired on the high resistive zones of EM profiles also showed high resistivity which may be not be prolific for groundwater development. The outcomes of this research may serve as a guide for sustainable water supply and water management plan. Keywords: electromagnetic method, fracture, geologic contact, vertical electrical sounding, low frequency, aquifer, water 1. INTRODUCTION Water is one of the most essential resources throughout the world, without which man cannot survive for more than a week [1]. Water is important for the existence of a man nonetheless, its use for domestic and industrial purposes cannot be overemphasized [2]. According to the UNICEF article released in November, 2020, it was reported that the average amount of water required by a person daily is 9 litres while the minimum admissible range by National standards is between 12 and 16 litres per day [3]. Undesirably, a large number of people in some communities have to walk long distances to fetch water from lakes, streams and rivers most especially during the dry season. A report of survey, released by Nigeria’s Federal Ministry of Water Resources and UNICEF in 2019 revealed that about 33.3% of Nigerian population drink contaminated water and about 46 million still continue practicing open defecation thereby contaminating the surface water such as stream and lake [3]. Vulnerability of natural sources of water such as lake, ocean and river, to pollutants and contaminants has made them not suitable for human consumption. As inaccessibility to safe and clean water continue to pose a serious danger to those living in a very complex terrain, groundwater provides a better clean water due to its natural microbiological and chemical qualities [4]. Groundwater is the water found beneath the ground surface most especially within the pore spaces or voids in the rock. The appreciably porous zone in the subsurface provides a better accommodation for groundwater. This zone is not usually easy to find in the basement environment due to the geologic settings and heterogeneous nature of crystalline rock. In some environment, rocks that are extremely weathered or fractured would have to be delineated before groundwater could be successfully exploited. In order to delineate those weathered or fractured zone believed to be porous, a detailed geophysical investigation has to be carried out to understand the hydrogeologic settings of such complex basement terrain before citing a well or borehole. This deals with deployment of geophysical methods to determine structures, thickness, properties of subsurface layers etc. Acquisition of geophysical data help to effectively identify an aquifer and characterise them into hydro-geologically potential zones which is significant in the groundwater exploitation and decision making in groundwater development [5]. Very low frequency of electromagnetic (VLF-EM) and electrical resistivity (ER) methods are efficient tools for groundwater investigations [5]. Some authors have described VLF-EM as a reconnaissance tool for mapping fractures in the crystalline basement environment [5-8]. They succeeded in delineating some water bearing fractures and recommended those points for drilling. However, they are some deep seated fractures in a basement complex environment or an environment with conductive thick overburden that VLF-EM may not capture due to its shallow depth of investigation. It is necessary to integrate the EM method with other geophysical methods particularly ER method, to increase the success rate of exploration [8]. ER method has been adjudged efficient and has been effectively applied to environmental and hydrogeological related problems [9]. Braga et al. [10] 2006 applied ER method to solve aquifer vulnerability. They were able to delineate two types of aquifers namely confined and unconfined but we found out that they did not have challenge with groundwater Environmental and Earth Sciences Research Journal Vol. 10, No. 1, March, 2023, pp. 18-25 Journal homepage: http://iieta.org/journals/eesrj 18