Study of the fractured carbonate aquifers of the Calcari con Selce formation in the Lagonegro Units integrating classical methods with modern digital techniques

A study of the well-bedded carbonates of the Calcari con Selce formation (CCSf), exposed in the Agri valley (Basilicata, Italy), has been focussed on the control of large-scale folds and faults on the geometry of the fracture network. The CCSf is a 300-500 m thick late Triassic succession, consisting of pelagic carbonates, which were deposited within the Mesozoic Lagonegro basin. These carbonates represent excellent aquifers exploited for civil uses both in Basilicata and in the neighbouring regions of Southern Italy, therefore playing a very important role from a structural-geological and hydrogeological perspective. In particular, in the Agri valley a large number of springs are sourced from fractured carbonate rocks belonging to Apennine Platform and the Lagonegro Units. The High Agri Valley is a NW-SE oriented tectonic depression in the central sector of the Southern Apennines. The latter is a thrust and fold belt formed following the tectonic collision between the African and European plates during the since the early Miocene.

The CCSf is a multi-layered succession with carbonate layers containing chert levels and nodules, rare marly layers, and clayey intercalations. The selection of outcrop for the analyses has been performed taking into account distal and proximal basinal environment facies within the CCSf, and the presence of large-scale structures such as folds and faults. In each study area faults are characterized by different orientations and frequency, and folds display different geometry.

The goal was therefore to start from the study of orientation, density and intensity of fractures allowing to derive the specific porosity and permeability parameters. In each area the attributes of each set of faults, stratabound and non-stratabound fractures, veins and pressure solution cleavage were measured. The method used was to acquire data with linear scanline and circular windows using the classic field methods and integrating these measurements with drone-UAV acquisition of images to obtain digital outcrop models. The 3D model allowed the extraction of orthophotos which were digitized with a graphic software to identify the different structures that were processed in FracPaQ, to obtain qualitative and quantitative results for portions of the outcrop.

The geometry of the fracture network in each area has been compared with the geometry and kinematics large-scale structures, indicating a control of the major faults in the study area on the formation of the studied fracture networks. Moreover, we observed the strengths and weaknesses of the adopted measurement methods. The measurement of the data in the field allowed us to increase the accuracy in the measurement and to select the outcrops with the best exposure conditions. On the other hand, detailed fieldwork requires longer acquisition time and difficulty in reaching some outcrops can be encountered. The use of a UAV partly overcomes these problems, making it possible to study larger portions of outcrops in a shorter time. The integration of the different approaches and the advancement of digital techniques could be exploited or improved for future studies.