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Fracture networks
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A discontinuity in a rock mass can be defined as surfaces across which a material has lost its cohesion, thus including faults, foliations, stylolites, compaction or deformation bands and fractures. Fractures can be additionally subdivided into shear and tensional fractures, the latter further divided in joints when empty or veins when filled by minerals (Twiss and Moores, 2007; Davis et al., 2012). Discontinuities that have the same formation age, kinematics and orientation can be grouped into families or sets and multiple sets form a fracture network (Davis et al., 2012). Fundamental for its impact on the mechanical and hydraulic properties of fractured rock mass, is the connectivity of a fracture network (Gueguen et al., 1991), that will be characterized by intersection lines where pairs of fractures show crosscutting or abutting relationships (Hancock, 1985).

Although three dimensional by nature, most of the times discontinuities appear as 2D lineaments or traces, i.e. the intersection of discontinuity surfaces with a secondary surface that reveals the internal structure of the rock mass, such as the outcrop or topographic surface (but also a surface cut in a sample or a borehole).

Expanding on these basis, fracture networks can be considered as the composition of three main components:

+ **Fractures**: In this case, any type of discontinuity in a rock volume (so faults, foliations, stylolites etc..).

+ **Boundaries**: The observational limits that the interpreter has on an outcrop i.e. areas of exposed rock where fractures can be measured. These limits can be defined as :term:`interpretational boundaries` and also internal areas in which interpretation cannot be carried out can be present (:term:`interpretational holes`)

+ **Nodes**: The intersection points between two fractures (pertaining to the same or different set) or between a fracture and the boundary/hole


.. figure:: ../images/frac_net.png
   :align: center

   Subdivision of a simple fracture network