Quantitative hydrogeological assessment, in particular hydraulic conductivity estimation, is a key point in many engineering-geological activities, such as underground excavations, hydrocarbon reservoirs and geothermal projects. Understanding geomechanical and thermo-physical features and their relationships with groundwater flow is particularly important in geothermics, especially when facing non-homogeneous rock masses (flysches, calcschists or faulted hard rock lithologies) since joints setting and related parameters can significantly improve the heat transfer mechanism (Chicco et al. 2019). Particularly in open geothermal systems, where the water is exploited along permeable fractures at depth (Rybach and Muffler 1981), regional scale groundwater models need to predict: i) groundwater paths, ii) properties of the rock medium iii) potential water propagation via discontinuities.
Guidelines to define groundwater models at regional scale were developed by many authors, suggesting appropriate methodologies and procedures for fault zone parameterization and representation (Turnadge et al. 2018; Underschultz et al. 2018). A general model conceptualization (Bense et al. 2013) considers the total width of a fault zone made up of two components: the fault core (FC) and the damage zone (DZ). The FC represents the location of the greatest strain and displacement; the DZ surrounds the fault core and a smaller amount of strain is here attended. Rawling et al. (2001) extended the conceptualization of faults to include a core of low hydraulic conductivity flanked by mixed zones. The main outcome is that faults can broadly exhibit three behaviours in a fluid flow context (Fig. 1): a) a barrier to both along- and across-fault flow, b) a conduit to both along- and across-fault flow and c) a conduit to both along-fault flow and a barrier to across-fault flow.
The hydraulic conductivity estimation in FC and DZ is however complex due to the heterogeneity and anisotropy of rock masses (Piscopo et al. 2018). Many equipment and methodologies have been developed for direct in situ(Zhu and Yeh 2006 and reference herein) or laboratory(Weydt et al. 2021 and references herein) determination. Laboratory measurements are commonly performed on intact or ad-hoc fractured (i.e. with saw-cutting discontinuities) rock samples. Results of these measurements are strongly affected by scale effects, which can lead to large misestimations (even some orders of magnitude) with respect to the real in-situ conditions. Moreover, the reduced sample dimensions are not always representative of the whole volume of investigation. Even if direct in situ tests are the most reliable for hydraulic characterization of rock masses, they require big economic and logistic efforts, especially for a preliminary characterization stage, and can be challenging in complex sites conditions.
Consequently, preliminary hydrogeological analyses can be performed using empirical correlations and can be divided into two main groups: a) those that evaluate hydraulic conductivity as a function of depth (Snow 1969; Louis 1972; Burgess 1977; Carlssn et al. 1983; Black 1987; Wei et al. 1995) and b) those that evaluate hydraulic conductivity as a function of discontinuity features (Hsu et al. 2011) and rock mechanics classifications (Gates 1997). Since stress increases with depth while discontinuity frequency and aperture decreases (Shahbazi et al. 2020), hydraulic conductivity follows a hyperbolic function with depth. Recent studies focus the attention to a 3D modelling of fracture paths however, the higher is the problem complexity, the higher is understanding the sensitiveness of involved parameters (Roe 2017). Simplified models were developed for the calculation of hydraulic conductivity by considering discontinuities orientation. For instance, for a planar and parallel discontinuity set (Snow 1969), conductivity can be estimated by using Richard’s equation. For conduits dispersed in orientation, frequency measurements (e.g volumetric joint count, Jv) and joint orientation allow the evaluation of the contribution of each joint on the conductivity tensor. These approaches are suited for outcrops and surficial flows: it is difficult to use them to reconstruct hydraulic conductivity models at depth, such as for geothermal purposes or deep tunnels.
Hydro-geomechanical classifications were developed to identify bedrock groundwater through the fracture network. The Hydro-Potential (HP) value technique (Gates 1997), based on the modification of the rock mass quality designation (Q-system), estimates the rock mass potential to hydraulically transmit groundwater. Hsu et al. (2011) proposed the Hydraulic Conductivity classification (HC) by including rock quality designation (RQD), depth index (DI), gouge content designation (GCD) and lithology permeability index (LPI).
A comprehensive multiscale characterization allows the definition of a) the main discontinuity characteristics (orientation, opening and persistence) of the rock mass, b) the discontinuity interconnections at the outcrop scale, c) forecasting discontinuity evolution at depth and d) the possible discontinuity interference with groundwater flow. This kind of information is usually obtained through traditional geomechanical surveys (ISRM 2015). Nowadays, they can be also coupled with non-contact investigation techniques, such as UAV and photogrammetric surveys or laser scanner acquisitions.
Geophysical surveys can be also used to investigate the shallow evidence of geological structures and fractures networks in depth. In recent years, near-surface geophysical prospecting has become a standard tool for the study of faults in a variety of geological and tectonic contexts (e.g. Demanet et al. 2001; Vanneste et al. 2008). When determining the thickness and variability of weathered horizons as well as localized deep fracture zones, Electrical Resistivity Tomography (ERT) in particular was shown to be effective (Chaudhuri et al. 2013; Belle et al. 2019). Many research projects have successfully used ERT to model the petrophysical properties of carbonate aquifers (e.g. Whitman and Yeboah-Forson, 2015) and hard rock aquifers (Descloitres et al. 2008). Other studies have also provided additional quantitative information on the aquifer properties, such as storage properties and recharge processes (Mézquita González et al. 2021; Singh and Sharma 2022). However, due to their complexity and lack of data on petrophysical model input parameters, only a small number of studies have evaluated the relationship between geophysical measures and hydraulic properties in hard rock aquifers (Leopold et al. 2013; Flinchum et al. 2018).
In this paper, geomechanic and geophysical techniques, applied at the outcrop scale, were therefore used to estimate the hydraulic conductivity of two fractured rock masses in the Los Humeros geothermal area (Trans-Mexican Volcanic Belt – TMVB - Mexico). Traditional and non-contact geomechanic surveys and high-resolution ERT have been used to investigate two test site. The combined use of geomechanical and geophysical information allows a first estimation of the hydraulic behaviour of the outcropping rock masses, useful for modelling hydraulic circulation within faulted area and, more in general, heterogeneous rock masses. The preliminary results were validated by using independent laboratory measurements performed by other researcher involved in the same project on the same rock types collected in the surrounding of test sites. These measurements demonstrate the promising nature of the proposed methodology.