FRACTURE ZONE DETECTION USING VERY LOW FREQUENCY ELECTROMAGNETIC METHOD (VLF-EM) OF GEOPHYSICAL PROSPECTING FOR GROUNDWATER IN ORU-IJEBU SOUTH-WEST NIGERIA

Abstract

Very low frequency (VLF) electromagnetic method was conducted to study fracture zone detection as part geophysical assessment and preliminary exploration for groundwater studies were carried out in Oru-Ijebu; locations which lies within the Crystalline Basement Complex rock of Southwestern Nigeria with the aim of delineating subsurface structures associated with groundwater occurrence by determining the depth and lateral extent of conductive zones inferred as major aquifers in the study area. A total of twelve (12) profiles were covered during VLF field data acquisition with 10m sample interval along each profile with spread length ranging between 100m and 200m using Geonics EM-16GL instrument. The acquired VLF data were processed and interpreted using Karous-Hjelt software to generate the 2-D conductivity image of the subsurface with emphasis placed on the Fraser filtered plot which transformed genuine cross-over points in peaks and pseudosection. The results of the study showed the presence of fracture zones and are prominently oriented in the NE-SW and NW-SE direction. The result also revealed that most of the fractured zones were located at a depth range of 0 to 30m within the subsurface. The presence and interconnectivity of the fractured zones show that the study area has good prospects for groundwater. The study therefore recommends the drilling for prospective and suitable boreholes at locations VLF-ORU01, VLF-ORU02, VLF-ORU04, VLF-ORU05, VLF-ORU07 and VLF-ORU012 between 50-75m, 120-165m, 30-50m, 165-200m, 170-190m, and 83-100m respectively along the profile. The most conductive zone was found at VLF-ORU03 at a lateral extent of 70-105m, while the least conductive zone was identified at VLF-ORU01 at a lateral extent of 125-165m. Therefore, on the basis of the investigation, the drilling of productive boreholes suitable for groundwater exploration are recommended at the traverses identified above.