Fracture System and Rock-Mass Characterization by Borehole
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Article Fracture System and Rock‐Mass Characterization by Borehole Camera Surveying: Application in Dimension Stone Investigations in Geologically Complex Structures Ivica Pavičić 1,*, Ivo Galić 1, Mišo Kucelj 2 and Ivan Dragičević 1 1 Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia; [email protected] (I.G.); [email protected] (I.D.) 2 HGE‐Solutions j.d.o.o. Radnička 2c, 10315 Novoselec, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +38‐59‐9805‐1725 Featured Application: The presented methodology is a relatively fast and low‐cost method that gives solid input into the state of the investigated rock mass, bedding orientation, degree of joint‐ ing, and preliminary block size estimation. All these parameters are very important for decision‐ making in the initial phase of quarry investment since these factors control the potential of the location for dimension stone deposit and type of excavation. The methodology is also applicable to hydrogeological (fractured aquifers), geotechnical, civil engineering, and engineering geology research (rockfalls, construction of roads, viaducts, railways, bridges, tunnels, etc.) where knowledge about fracture systems in the rock mass is crucial for further works. Citation: Pavičić, I.; Galić, I.; Kucelj, M.; Abstract: The successful exploration of dimension stone mainly depends on the quality, size, and Dragičević, I. Fracture System and shape of extractable blocks of dimension stone. The investigated area is in the Pelješac Peninsula Rock‐Mass Characterization by (Croatia), in the External Dinarides orogeny, built from thick carbonate succession, characterized Borehole Camera Surveying: by relatively small deposits of high‐quality dimension stone. These conditions demand challenging Application in Dimension Stone geological investigations in the “pre‐quarry” phase to find optimal quarry location. The size and Investigations in Geologically shape of dimension stone blocks are mainly controlled by fracture pattern systems. In the rugged, Complex Structures. Appl. Sci. 2021, covered terrains, it is very hard to obtain a satisfactory amount of fracture data from the surface, so 11, 764. https://doi.org/10.3390/ it is necessary to collect them from the underground. Borehole camera technology can visualize the app11020764 inner part of the rock mass and measure the fracture characteristics. The main conclusions are as follows: (1) the digital borehole camera technology provides a quick, effective, and low‐cost geolog‐ Received: 8 December 2020 ical survey of fractured rock mass; (2) statistical fracture distribution parameters, P10, fracture spac‐ Accepted: 9 January 2021 Published: 14 January 2021 ing, Volumetric Joint Count (Jv) based on borehole wall survey can reflect the integrity of rock mass, providing a solid decision‐making base for further investment plans and dimension stone excava‐ Publisher’s Note: MDPI stays neu‐ tion method. tral with regard to jurisdictional claims in published maps and insti‐ Keywords: borehole camera; fracture patterns; overturned anticline; dimension stone; External tutional affiliations. Dinarides Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Dimension stone and crushed rocks are considered key natural resources that enable This article is an open access article the sustainable functioning of the worldwide economy. Global demand for dimension distributed under the terms and con‐ ditions of the Creative Commons At‐ stone, due to its aesthetic and petrophysical properties being used in the construction in‐ tribution (CC BY) license (http://cre‐ dustry, including indoor and outdoor spaces, is related to the development of the econ‐ ativecommons.org/licenses/by/4.0/). omy [1]. A global increase in demand for dimension stone increased research activities worldwide, and thus in the Dinarides in Croatia and Bosnia and Herzegovina. The Dinar‐ ides are a fold‐and‐thrust orogenic belt that is geotectonically positioned along the NE Appl. Sci. 2021, 11, 764. https://doi.org/10.3390/app11020764 www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 764 2 of 17 margin of the Adriatic Sea. The Dinarides have great potential for dimension stone of sed‐ imentary origin, but locations and the lateral extent of the potential areas are primarily controlled by regional tectonic settings. The External Dinarides are represented by a thick succession of platform carbonates, which are very potential for dimension stone, proven by a dozen smaller quarries along the Adriatic coast. Since Dinarides are orogeny created by strong compressional tectonic forces, the rocks are often very faulted, folded, and frac‐ tured. The result of all these natural factors is that dimension stone deposits are usually of high quality but relatively small (on a global scale). These conditions put more challenges on geological research to define the optimal position for quarry opening. The block size distribution is a crucial parameter for the profitable production of dimension stone within the deposit. The distribution of blocks and their sizes and shapes are mainly controlled by three‐dimensional discontinuity systems (i.e., bedding planes, faults, fracture systems) [2]. In the research of dimension stone deposits, the particular emphasis should be on the dis‐ tribution of discontinuities and their characteristics because fracture patterns mainly con‐ trol block size distribution, type (surface pit or underground exploration), and exploita‐ tion success. In the initial phase of research, for the potential quarry, discontinuities are rarely available in a satisfactory amount for the spatial distribution analysis and statistics, due to terrain morphology, vegetation, karst features on the surface, and so forth. Every serious “pre‐quarry” investigation includes several drill holes with cores, but it is not al‐ ways possible to obtain discontinuity orientation from drill hole cores. Although all these factors are unfavorable, it is crucial to define some fracture pattern parameters, for pre‐ liminary Volumetric Joint Count estimation, as a key factor for reserve estimation and later profitability of the surface or underground exploration investment. Challenged by this situation, we have developed the method of defining fracture orientation parameters from the borehole camera video logging technique. Recently, borehole video logging tech‐ nology (borehole camera, optical and acoustic televiewer) is widely used in geological and hydrogeological and nuclear waste research [3–10], civil engineering, and geotechnical engineering [8,9,11–16]. Geological characteristics of the rock mass such as lithology, frac‐ ture orientation, and morphology, karst phenomena (caves), rock weathering, and so forth can be detected by acquiring high‐quality drill hole wall images [11,13]. In this paper, statistical analysis of the fracture pattern data obtained by borehole camera technology was examined to estimate expected block size and fracture set orientations. This is a rela‐ tively low‐cost method for obtaining reliable preliminary fracture orientation data. Data were analyzed, and fracture sets with spatial parameters were defined: fracture sets, bed‐ ding, fracture set orientations, P10 linear fracture intensity, fracture spacing, and Volumet‐ ric Joint Count (Jv). The proposed approach is a relatively low‐cost methodology for frac‐ ture pattern data collection and analysis to obtain key parameters in their areas with a low amount of surface data. This is especially important in the initial phase of dimension stone quarry research. The presented methodology is not applicable only in the dimension and crushed rock research but also in the hydrogeological (fractured aquifers), geotechnical, civil engineering, and engineering geology research (rockfalls, construction of roads, via‐ ducts, railways, bridges, tunnels, etc.). 2. Geological Settings The investigated area is located on the E side of the Pelješac Peninsula, which is part of the External Dinarides, an approximately 600 km long fold‐and‐thrust orogenic belt [17–24]. The External Dinarides are geotectonically positioned along the NE margin of the Adriatic Sea, which corresponds with the NE part of the Adria Microplate [18,20,23–25]. The Dinarides orogenic belt was uplifted during the Late Eocene to Oligocene, due to the Adria Microplate–European Plate collision [18,19]. Most of the sediments in the External Dinarides were deposited during Mesozoic on the Adriatic Carbonate Platform from lower Jurassic to Upper Cretaceous [21,22]. The total thickness of the sediments is esti‐ mated to a few kilometers (in places up to 8 km) [21]. During their geological evolution, Appl. Sci. 2021, 11, 764 3 of 17 Dinarides had a polyphase uplift, due to multiple compressional tectonic regimes that formed present tectonic settings of the area [17,20,21]. The wider research area in the Pelješac Peninsula is composed of three stratigraphical and depositional units: Cretaceous carbonates, Paleogene carbonate, and clastic sediments and quaternary clastic sediments [26–31] (Figure 1). Cretaceous carbonates are the domi‐ nant geological unit in the Pelješac Peninsula, and they are represented by limestones and dolomites subdivided into four chronostratigraphic units from Hauterivian to Maastricht‐ ian age (Figure 1). Since the research area is composed of carbonates of Coniacian to Maas‐ trichtian (K23‐6), they will be described in more detail. Upper Cretaceous