Application of cross-square array and resistivity anisotropy for fracture detection in crystalline bedrock
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Date
2016
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Journal ISSN
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Publisher
Springer
Abstract
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 from0.07 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
Description
Keywords
Azimuth . Cross-square array . Fracture. Anisotropy . Porosity