Measurement and Characterisation of Fractures in Parts of the Togo Structural Units (TSU) and Dahomeyan Gneissic Complex (DGC), SE Ghana

Abstract

Fracture characterisation in fractured rocks is a critical step in the generation of discrete fracture network (DFN) modelling and the evaluation of the hydraulic and mechanical properties of the rockmass. In this study, an integrated approach of using structural geological mapping, linear and circular scanline mappings, and laboratory investigation of rock samples were undertaken at two selected sites, Site 1, within the Togo structural units (TSU) near Ablekuma settlement, and Site 2 within the Dahomeyan Gneissic Complex (DGC), near Danfa settlement to derive information on fracture characteristics in the Study Area. A total of 1128 fractures were surveyed along a total length of 238 m of scanline at Site 1, and 629 fractures along a total of 156.0 m at Site 2. Fourteen and thirteen circular scanlines were surveyed at Site 1 and Site 2 respectively Linear fracture density or intensity (P₁₀) from the 7 scanlines at Site 1 ranged from 3.50 fractures/m to 6.26 fractures/m, compared to fractures at Site 2, which have fracture linear density ranging from 2.34 fractures/m to 5.04 fractures/m. The mean linear fracture intensity for Site 1 was computed as 4.739 fractures/m from 1128 fractures along 238.0 m scanline length, and 4.030 fractures/m from 629 fractures along a total of scanline of 156.0 m for Site 2. The DIPS software was used to analyse the orientation data, while the EasyFit used for modelling fracture spacing distribution. The best fit model for fracture spacing at Site 1 was a negative 2-parameter exponential distribution function (pdf), while that for Site 2 was a negative exponential distribution. There appear to be a lithological and/or stratigraphic thickness control on fracture density, as fracturing was more intense in the medium grained less thick quartzite beds of the Site 1, within TSU compared to the coarse grain much thicker stratigraphic units of gneiss of Site 2 within the DF.

Keywords: Fracture characterisation, Togo Structural Units, Dahomeyan Gneissic Complex.