Application of cross-square array and resistivity anisotropy for fracture detection in crystalline bedrock
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ORIGINAL PAPER
Application of cross-square array and resistivity anisotropy for fracture detection in crystalline bedrock Olateju Olateju Bayewu 1 & Moroof Olasunbo Oloruntola 2 & Ganiyu Omotola Mosuro 1 & Temitope Ayodeji Laniyan 1 & Julius Ogunmola Fatoba 3 & Ismail Oluwaseye Folorunso 4 & Ajibola Udochukwu Kolawole 4 & Boladale Tosin Bada 1
Received: 7 January 2015 / Accepted: 4 January 2016 # Saudi Society for Geosciences 2016
Abstract Azimuthal cross-square array resistivity soundings were used to detect fractures and investigate the anisotropic properties of crystalline basement rocks in some parts of Eruwa, southwestern Nigeria. Resistivity measurements were taken at ten locations using an Ohmega terrameter. The technique involves the rotation of the electrode array in four different azimuths, namely, 0°, 45°, 90°, and 135°, at electrode spacing of A(2)1/2. The measured apparent resistivity were used to calculate azimuthal resistivity, fracture strike, bedrock anisotropy, coefficient of anisotropy, secondary porosity, and azimuthal inhomogeneity ratio (AIR). The results showed that the measured apparent resistivity changed with the orientation of the array implying fracture anisotropy in the subsurface. The calculated azimuthal resistivity shows that values increase with electrode spacing with polar plots indicating varied fracture directions. Primary fractures in the study area are oriented dominantly in N-S, NW-SE, and E-W directions which contrast with NE-SW and E-W trends of their secondary counterparts. Few tertiary fractures are aligned in E-W direction. Coefficient of anisotropy (λ) ranges between 1.05 and 3.68, while bedrock anisotropy (N) varies from 1.002 to 2.483; calculated secondary porosity (fracture porosity) ranges from 0.07
* Olateju Olateju Bayewu [email protected]
1
Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Nigeria
2
Department of Geosciences, University of Lagos, Lagos, Nigeria
3
Department of Geosciences, Federal University, Oye Ekiti, Nigeria
4
Department of Geology and Mineral Sciences, University of Ilorin, Ilorin, Nigeria
to 0.99. The comparison of the fracture porosity and mean resistivity revealed four major fracture zones in the study area. Interpreted AIR (azimuthal resistivity ratio) values of −5.09 to 0.97 show that anisotropy inhomogeneity of the study area is low. This work therefore gives insight to anisotropic properties as well as the fracture characteristics in the study area. Keywords Azimuth . Cross-square array . Fracture . Anisotropy . Porosity
Introduction Electrical anisotropy (or inhomogeneity) of crystalline basement is often attributed to structural elements like foliations, joints, and fractures (Billings 1972; Olasehinde and Bayewu 2011; Bayewu et al. 2014; Malik et al. 1983). It has been shown by Odeyemi et al. (1985), Beeson and Jones (1988), Okereke and Esu (1994), and Edet and Essien (1994) that the well yield and its direction flow in fractured rocks are directly related to the density, frequency, orientation
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