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Method for Measurement and Determination of Density and Intensity of Fractures in Layered Rock of Kornati Islands

Characterizations of fractures have an important role in describing the rock masses. Some geometrical characteristics of fractures which are evaluated in this research are intensity and density (measures of abundance). These characteristics/measures are important and commonly used for characterizations of fractures in rock masses (BANDPEY et al. 2019). Furthermore, according to KAMALI et al. (2016) these characteristics are beneficial and important for estimating the elastic properties of the rock masses, porosity of fractures, rock mass description for determining its hydraulic and mechanical behavior, prediction of In-situ Blok Size Distribution (IBSD), etc. Determination of the fracture abundance is also necessary for Discrete Fracture Network (DFN) modeling and verification (KAMALI et al. 2016). In this research the above characteristics are estimated using three methods: scanline sampling, window sampling, and circular sampling methods. Sampling methods, such as scanline sampling (linear sampling methods), window sampling, and circular sampling (areal sampling methods) are used to estimate the correlation of fracture geometrical characteristics with their position in larger geological structures. To provide a high availability of data required for density and intensity measurements in the layered carbonate rocks, three conditions are required: (1) areas of slightly deformed geological structures ; (2) well defined lithology, and (3) surface area not covered by vegetation. According to the mentioned conditions, the area of Kornati islands was found as suitable area for investigation and three locations were selected for detailed research there. Kornati islands region (northern Dalmatia, Croatia) are predominantly built from Upper Cretaceous to Eocene pre-orogenic layered carbonates rocks (KORBAR et al., 2010) with well- defined regional and local geological structural settings. Surface of the terrain on all three locations (Magazinova Škrila, Tureta and south part of island Levernaka) is formed by layers of very high persistence, without vegetation, with very pronounced natural cuts providing high availability of fracture traces on which measurements of density and intensity analysis were performed. Magazinova Škrila and Tureta are built of middle to thick layered rock, belonging to Gornji Humac Formation (GHN ; K22-5), and south part of island Levernaka belongs to unit with thin layered carbonates rock named Milna Formation (MF ; K21) (BRČIĆ et al., 2019). The preliminary field research included: geological and engineering geological mapping, standard measurement methods, and digital photogrammetry methods which are divided into (1) terrestrial photogrammetry – ShapeMetrix3D (ShapeMetriX3D ; 3G Software & Measurement GmbH, 2007) and (2) aerial photogrammetry – Unmanned Aerial System. Fracture abundance is defined in 1, 2 and 3-dimensions. It is difficult to directly measure all the fractures and their properties in 3-D space (ZHENG et al., 2017). Therefore, it is widely accepted to infer fracture properties from one-dimensional (linear sampling methods) and two-dimensional (areal sampling methods) measurements in order to estimate three-dimensional properties of rock mass(ZHANG, 2017). Linear (scanline sampling) and areal (window and circular sampling) methods vary in their application, and the parameters they provide have advantages and limitations (ZEEB et al., 2013). Orientation, truncation, censoring, and size bias, among others, can cause significant errors in statistical parameters estimation and thus can potentially affect the characterization of fracture networks (ZHANG & EINSTEIN, 1998). The scanline and window sampling method is affected by orientation, truncation and censoring bias, except size bias, which affects only the scanline sampling method. The circular sampling method is not subject to sampling bias. This method does not provide information on important parameters such as fracture orientation, length distribution, or width. If possible, all three methods should be combined. Geostatistical analyses will quantify the spatial continuity of fractures and will define spatial distributions of fracture density and intensity in layered carbonate rocks. Along with geostatistical analyses, directional statistics (circular and spherical statistical analyses) will be carried out with aim to quantify all the geometrical features of the fracture. Lithology and thickness of the layers will be correlated with the intensity and density of the fractures in layered carbonate rocks. This research will also examine the hypothesis that thickness of layers, their physical - mechanical properties and their position in larger geological structures have influence on the density and intensity of the fractures in layered carbonate rocks.

Fracture ; Linear and Areal sampling ; Geomechanical characteristics ; Statistical analyses

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