Geophysical investigation of the Orle River fracture system in Igarra Township, southwestern Nigeria

Abstract

The subsurface structure and propagation geometry of the fracture system controlling the Orle River Channel in the Igarra Township, Southwestern Nigeria, have been investigated via a multi-method geophysical survey. The goal was to delineate the nature, distribution, and spatial propagation geometry of the fracture system and evaluate its potential to serve as storage and distribution features for groundwater within adjoining areas. 2D electrical resistivity, total field magnetic, and co-planar loop conductivity measurements were collected along four traverses using an ABEM1000 Terrameter unit, a GEMS Magnetometer, and an EM34 Co-planar loop electromagnetic system. Traverses were established to run across and parallel to the river channel. The observed electrical resistivity field data were inverted for subsurface 2D resistivity structure using a commercially available 2.5D finite element modelling inversion software. Magnetic field intensity data and ground conductivity data were presented against station positions. Three subsurface layers were delineated at the survey site; (1) surficial humus-rich and wet top-soil, (2) a thin poorly developed weathering layer, and (3) the fresh bedrock which occurs as relatively shallow levels and often outcrops. The River channel is controlled by multiple fractures usually located at or near the contracts between contrasting rock types. Fracture dip is usually in the northerly direction but conjugates, dipping southwards also occur in the most northerly extremes of the imaged fracture system. Upstream the fracture path is wider and along with the imaged overburden, is
juxtaposed northwards of the channel axis suggesting a much broader river channel in the geologic past. Low aperture fractures imaged tangential to channel axis likely serve to funnel surface and groundwater from the channels to the surrounding areas. Where such low aperture fractures can be delineated, they offer the best chances of groundwater abstraction within adjoining areas, particularly during the dry season.