Improvement of structural data by means of FracPaQ software to implement groundwater flow model in karst settings

Carbonate rock formations are inherently extremely anisotropic rock masses, due to the simultaneous presence of well-defined stratabounds discontinuities and wide-spread fracturing. When karst processes occur, they can obliterate or widen the aperture of the original discontinuity networks, adding further complexity to the system. In karst territories the carbonate rock masses host important freshwater resources, which are often the only available water supply for local communities. In order to protect karst groundwater, it is imperative to properly evaluate the underground flow dynamics. To do so, we need to build detailed datasets of the three-dimensional (3D) spatial distribution of discontinuity networks, which serve as primary input for numerical simulation of fluid movement in the underground water reservoirs. Field structural-geological surveys are common means for obtaining the necessary information regarding the spatial distribution of the discontinuity networks. However, this approach is highly time-consuming and struggles to provide sufficient data to build robust statistics.

In recent years, the combination of new technologies for data acquisition (e.g. drones and high precision cameras) and new freely-available softwares, such as DigiFract (Hardebol and Bertotti, 2013), FracPaQ (Helay et al., 2017), and NetworkGT (Nyberg et al., 2018) are bridging the gap between fast and reliable fracture data acquisition and analyses. Through the use of these techniques, we are now able to directly work on digital images taken from the outcrops as input, and to produce outputs which provide robust statistics about the discontinuities within the analysed medium.

In this contribution, we present research aiming at full characterization of the rock mass discontinuities within a study area in Apulia Region (Southern Italy). Specifically, we studied the Canale di Pirro polje where the deepest Apulian cave, named Inghiottitoio di Masseria Rotolo, is located. The cave opens at 300 m a.s.l. and reaches the water table at about -260m depth below the topographic surface. By means of combining detailed photogrammetric survey and the use of the FracPaQ software toolbox, we were able to analyse in details the discontinuity network exposed at the outcrops, and consequently to use this information for evaluating how the network influences the underground flow direction and its velocity.  The statistical and spatial analysis of the discontinuity network, together with data derived from both the surface and underground, with specific surveys performed within the cave, allowed to present the first considerations about the groundwater flow in the surroundings of the karst system, useful to implement a numerical model heavily based upon direct observations from surface and underground karst areas.

 

 

REFERENCES

Hardebol, N. J., & Bertotti, G. (2013). DigiFract: A software and data model implementation for flexible acquisition and processing of fracture data from outcrops. Computers & Geosciences, 54, 326-336.

Healy D., Rizzo R.E., Cornwell D.G., Farrell N.J.C., Watkins H., Timms N.E, Gomez- Rivas E. and Smith M. (2017). FracPaQ: A MATLAB^TM toolbox for the quantification of fracture patterns. J. Structural Geology, 95, 1-16. http://dx.doi.org/10.1016/j.jsg.2016.12.003.

Nyberg, B., Nixon, C. W., & Sanderson, D. J. (2018). NetworkGT: A GIS tool for geometric and topological analysis of two-dimensional fracture networks. Geosphere, 14(4), 1618-1634.