Abstract
Characterization of fracture anisotropy in Aiyegunle area of Igarra Southwestern Nigeria was done by geologic mapping of exposed outcrops and azimuthal resistivity survey (ARS) at three vertical electrical sounding (VES) stations. Rose diagrams, stereographic projections, and resistivity anisotropy polygons were used to delineate and integrate surface and subsurface structural trend. Dominant joints orientation is North-west-South-east (NW-SE) and North- South (N-S). The orientation of fracture in the subsurface is N-S with anisotropy prominent in the NW-SE direction. The coefficients of anisotropy (λ) at depth are less than 2.2, with the degree of fracturing opening and closing with depth. The diversity of fracture attitude implies they were products of different tectonic episodes corroborating evidence of multiphase deformation. The fracture directions suggest that surface structures are hard-linked and produced by similar tectonic event relative to subsurface fractures of Pre-Pan African orogenies. Key words: Fractures, orientation, surface, subsurface, anisotropy.
Highlights
Fracture anisotropy is associated with electrical and hydraulic variation resulting from the preferred orientation of fracture sets (Slater et al, 2006)
Results from this study indicates that the schists belts of the Igarra area may not be Proterozoic in age but connected to earlier Pan African events in order to AB/2 1 2 3 4 6 6 9 12 15 15 20 25 32 40 40 50 65 80
Azimuthal resistivity surveys and geologic field mapping conducted at Aiyegunle area was aimed at characterizing fractures in the study area
Summary
Fracture anisotropy is associated with electrical and hydraulic variation resulting from the preferred orientation of fracture sets (Slater et al, 2006). Azimuthal resistivity survey is a technique used for determining the principal directions of electrical anisotropy and intensity; a change in apparent resistivity with azimuth is indicative of fracture anisotropy (Slater et al, 2006; Boadu et al, 2005; Skyernaa and Jorgensen, 1993). The architecture of fractures is space and time related, the intensity of fracturing in an area can be observed on different scales, from megascopic (seismic section, satellite images, and aeromagnetic maps), macroscopic (joints, faults and veins in outcrop) to microscopic scale in thin sections (Omosanya et al, 2012 a,b). Depth-related fractures observed in macroscopic scale are deep seated, hard-linked and apparently connected to regional tectonic events or trend. The structural information of the entire basement rocks was derived or up-scaled from studies of the belts. The Igarra schist belt is characterized by a complex geological framework made of different structures and rocks
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